Hello there, and welcome to this first lesson from the "Hidden Forces" unit, which is "Gravitational Force and Weight." In the lesson, you're gonna find out about what the weight of an object is and how it relates to the mass of an object and to gravitational fields.

By the end of this lesson, you're going to be able to describe what the weight of an object is and how it's connected to mass and gravitational fields.

You are also gonna be able to calculate the weight based upon the mass and the gravitational field strength.

So, when you're ready, let's begin.

These are the keywords that'll help you understand the lesson.

The first is gravitational force, and you've encountered that before.

It's the force that pulls objects together.

The second is mass.

Mass is a property of an object that's affected by gravitational fields.

Gravitational field strength, which tells you how strong gravity is in a particular area; newton, which is the unit of force; and newtons per kilogramme, which is a measure of the gravitational field strength I mentioned earlier.

And this is a set of definitions of those keywords.

You can return to this slide at any point in the lesson to refresh your memory.

There are three parts to this lesson.

The first part's about gravitational forces and what they are, and it's a bit of a refresher of work that you've done in previous lessons.

The second part is all about the mass of an object and what that is, and the final part brings those two things together, how the mass is related to gravitational forces and how to calculate weight.

So, let's get on with the first part, which is all about gravitational forces.

Now, you probably already remember a bit about gravitational forces and what they do, but we'll do a brief recap just in case you've forgot some things.

Now, gravitational force is a force that pulls objects down towards the centre of the Earth when you're on it, so if you're on the ground in a van, then the gravitational force will pull you down to the centre of the Earth, but you don't have to be in contact with the Earth for that to happen.

You can be in a plane, and the gravitational force will still pull you downwards.

Gravitational forces always act towards the centre of the Earth when you're near it.

The reason objects experience gravitational forces near the Earth is because the Earth produces something called a gravitational field, and that gravitational field surrounds the Earth, and it's an invisible zone where any object inside will experience a gravitational force.

If I place objects near the Earth, they'll all be pulled towards its centre with gravitational forces.

That gravitational field is weaker when you're further away from the Earth and stronger when you're close to it, so if you place an object near the surface of the Earth, it'll experience a stronger force than it would if it was further away, but that gravitational field does reach out a huge distance into space and reaches all the way to the Moon, so the Earth is pulling on the Moon, even all the way to the Sun, and Earth is pulling on the Sun.

So that gravitational force, although it's weaker further away, it doesn't disappear completely.

Although the gravitational field gets weaker when you're far away from the Earth, it's fairly constant near the surface.

If you are in an aeroplane and there's a gravitational force pulling you down and you go a bit lower, the gravitational field doesn't really change much, so the force on you is about the same.

If you go higher, it doesn't change much either.

In fact, you've got to travel a very large distance for the gravitational field to get noticeably weaker.

When you're on the ground, again, it's about the same, so we say the gravitational field is constant near the surface of the Earth.

Anything up to tens of kilometres, you wouldn't notice any difference at all, so I've got a first check for ya here.

The gravitational force on a ball resting on the ground is five newtons.

What size is the gravitational force on the ball once it's been kicked up into the air? So pause the video.

Make your selection.

And then restart when you're ready.

Okay, the answer to that one was five newtons.

Although you kick the ball into the air, the gravitational force doesn't change.

You'd have to kick the ball hundreds of kilometres into the air for there to be any change that you could measure at all.

When an object's in a gravitational field near the surface of the Earth, there's a force pulling it downwards, that gravitational force.

And so you're going to need a force to prevent it from falling, to hold it still, or to lift it up.

That force could be a support force from your hand or a spring or something like that.

The force that you need to support it's gonna be equal to the force pulling it down, and we call that force the weight of the object.

The weight of the object is the force needed to hold it still or lift it inside that gravitational field, so the weight of the object is the gravitational force acting on it or is the same as it.

Okay, let's check if you understood that idea.

I've got a large bag of feathers and a set of metal balls hung on two newton metres, and both the newton metres show the same reading, so you can see the situation shown here.

There's a support force acting on both of them.

So what do you think about each of these statements? "The same-size force is needed to lift both objects." "The metal is harder to lift than the feathers." "More gravity is pulling on the feathers." Or "the feathers and the same".

So what d'you think about each of these four statements? I'd like you to select from those four options so statement A, "the same force is needed to lift both"; B, "the metal is harder to lift than the feathers"; C, "more gravity is pulling on the feathers"; or D, "the feathers weigh the same as the metal." So pause the video, and select the options there.

And then restart.

Okay, for each statement, this is what you should've put.

For statement A, you should've been sure that that was correct.

Statement B is definitely wrong.

Statement C is definitely wrong, as well, and statement D, that's correct.

The feathers weigh the same as the metal.

Okay, it's time to find out what you know about weight, so I've got pupils discussing the weight of a heavy box.

I've got Lucas, who says, "Its weight is measured in kilogrammes"; Alex, who says, "Its weight is the force it presses down with"; Sam, who says, "Its weight is the amount of stuff it's made from"; and Izzy, who says, "The weight is the force needed to lift it up." And what I'd like you to do is to decide which of those ideas are correct and then write your own description of weight that includes all the correct ideas and any ideas of your own as well, so pause your.

Pause the video.

Think about those two questions.

Come up with your answer.

And then restart, and we'll go through it.

Okay, let's identify the two correct ideas.

The first of them was from Alex: "The weight is the force that the box presses down with." The second of them was the weight is also the force needed to lift that box up, so both of those are correct ideas about weight, and when you wrote your description, you should've come up with something like this: "Weight is equal to force needed to lift something or hold it up.

It's the same as the force that pushes it down on the surface." "Weight is measured in newtons" you could have included, and weight depends on the strength of gravity, so well done if you came up with any of those.

Okay, we're ready to move on to the second part of the lesson, which is all about mass.

Now, you should already know that objects are made up of lots and lots of tiny particles.

I've drawn a small object here, and I've tried to draw some of the particles inside it there.

Now, those particles are atoms. In reality, the atoms are very much smaller than I can draw on a diagram like this, but you get the general idea.

A larger object will contain more of the particles, so there'll be more atoms in a large object, and an even larger object will contain many more particles again so many more atoms. So the larger the object is the more particles it's gonna contain.

Every atom inside an object will have a tiny bit of a property called mass, and the mass of the particles is the property that is affected by gravitational fields, and that causes each tiny atom to be pulled downwards with a small gravitational force, so if you look at this small mass, each of those atoms is gonna be pulled downwards with a tiny, tiny gravitational force.

I've drawn it on like this.

And so if I do all of them, they're all gonna be pulled down.

And so you get an overall-downwards gravitational force.

Now, the object with the larger mass has got more particles in it.

And so there's gonna be more of those tiny forces pulling down, so eventually, you get a slightly larger gravitational force, and the larger mass is gonna have even more forces because there's more atoms in it, and that gives the largest gravitational force.

So objects with more mass will experience a larger gravitational force acting on them, so bigger objects, more massive objects, have stronger gravitational forces.

The number of atoms inside an object doesn't change when you move it from one place to somewhere else, and that means that the mass of the object doesn't change either, because the mass is basically just a measure of how many of those particles there are.

So if I've got a small object on the Earth and I moved it to the Moon, then its mass doesn't change, because it's still got the identical number of atoms inside it, so the mass is unchanging there.

If I've got a larger object on the Earth, it's got a larger mass on Earth, and it's still got a larger mass if I move off into deep space or somewhere like that, because there's still the same number of atoms in it.

So as long as you don't change the object by chopping up or something like that, its mass will stay the same wherever it is.

The weight of an object can change when it's moved, because the weight of the object is based upon the gravitational field that the object's in, so if I've got a small object on Earth, each of the atoms is pulled downwards with a certain force, and if I move that to the Moon, then each of the atoms is pulled downwards with a slightly less force, so the weight overall is less because there's a smaller downwards force acting on it when it's on the Moon.

If I move that object to the Sun, where the gravitational field is much stronger, then I'd have a greater weight because each of the atoms is pulled down with a larger force, so that gives a larger force overall.

And if I move that object into deep space, where it's outside of all gravitational fields so far, far away of any planet or stars, then there'd be no force acting on each of the atoms. And so I get no weight at all.

The object would be weightless.

Right, I'd like to check your understanding of mass, so I'd like you to look at those three objects, metal weight, large cardboard box, and bottle of water, and decide which of them has the largest mass, and give me a reason for your decision.

So pause the video.

Have a think about that, and restart when you've come up with an explanation.

Okay, welcome back.

Your explanation could've included some of these ideas.

So, first of all, it's not possible to decide.

You might've looked those pictures and thought, "Well, it depends on how big that water bottle is.

It might have more mass than that one-kilogram metal weight." The large cardboard box, though, that's pretty empty, so it's unlikely to have the largest mass, because it contains air and air doesn't contain many atoms per cubic metre, so it's probably not the cardboard box.

The metal weight is designed to be heavy, so it's probably got quite a lot of mass in it, and as I mentioned, that's a one-kilogram sample of metal there, and finally, it's not possible to judge them very well, because the scale's not the same for each of the figures, so that water bottle could be quite small or quite large.

And so it could contain a lot of mass or a small amount of mass, so well done if you came up with ideas like that.

In physics, we use the kilogramme as the standard unit of mass, so we measure things in kilogrammes.

You may've seen one of these around the laboratory.

It's a one-kilogram metal mass, but you might've also been to the shops and seen a one-kilogram bag of sugar, so both of those are one kilogramme.

They've got the same amount of mass in each.

If you need to measure smaller amounts of mass, we use the gramme, and one gramme is a much smaller amount.

It's 1/1,000 of a kilogramme, so one kilogramme is 1,000 grammes.

Well, five grammes of sugar is much, much smaller than one kilogramme of sugar.

We often need to convert between kilogrammes and grammes or the other way around, so I'll show you how to do that here.

One kilogramme is 1,000 grammes, so in order to convert from kilogrammes to grammes, I just need to multiply by 1,000, so for example, 2.

5 kilogrammes, that's 2.

5 times 1,000.

That's 2,500 grammes, and the conversion the other way is just as simple.

1,000 grammes is equal to one kilogramme, so to convert from grammes to kilogrammes, you just have to divide by 1,000.

So, another example, 600 grammes, to convert that to kilogrammes, it's 600 divided by 1,000, and that's 0.

6 kilogrammes.

Okay, let's check if we can do those conversions together, so I'll do the first one.

What's 5,400 grammes in kilogrammes? So, what I need to do to convert to grammes is to divide by 1,000.

So, 5,400 grammes divided by 1,000 is 5.

4 kilogrammes.

I'd like you to try this one.

I'd like you to try this conversion, please.

What is 0.

76 kilogrammes in grammes? So pause the video, and work out your solution.

And then restart.

Okay, to convert to kilogrammes, you need to multiply by 1,000, so what ya shoulda done is this: 0.

76 kilogrammes times 1,000 is 760 grammes.

So well done if you've got that.

Okay, it's time for you to complete a task now.

What I'd like you to do is to fill in the gaps in these sentences, please, using only the words mass and weight, and you'll obviously have to use each of those words more than once to fill in all those gaps.

So, pause the video.

Read through the sentences, and fill in the gaps.

And then restart when you've made all your selections.

Okay, welcome back.

Let's read through this together, see what the answers are: "John stands on his bathroom scales.

He's pushing down with a force of 430 newtons.

This means he has a blank of 430 N." Well, N's a force, so this answer must be a force, so it must be his weight he's pushing down with.

"The scales tell him he has a mass of 43 kilogrammes," 'kay? So that was kilogrammes, so it must be a mass.

"John visits the Moon.

On his way, he floats inside a spaceship because he has no." Well, his mass is still the same, so this must be his weight.

He still has that mass because all the particles, all the atoms, in his body are still there.

"John likes space food.

He eats a lot.

His body is now made of more material, so he has a bigger." Well, he's got more atoms in him now, so he's got a bigger mass.

"On the Moon, he can be lifted with just 90 newtons because he has a smaller weight." His weight is much less when it's on the Moon.

He's still got all those particles in him, so it's only his weight's changed, not his mass.

Well done if you've got all those answers.

Okay, we're onto the final part of the lesson now, and this is all about calculating weight, so you may need a calculator to help you with some of the maths.

Every kilogramme on Earth is pulled downwards with a force of about 10 newtons, and it doesn't matter what that kilogram's shape is.

Any object that's one kilogramme will be pulled down with a force of 10 newtons when it's near the surface of the planet Earth, but the strength of gravity on different planets is difference, so for example, on Venus, the strength of gravity is a little bit weaker, so every kilogramme on Venus is only pulled down with a force of nine newtons.

The force on every kilogramme is called the gravitational field strength, and the gravitational field strength of different planets is different; for example, the gravitational field strength on the Moon is much smaller than the gravitational field strength of the Earth, and the gravitational field strength at the Sun is much higher than the gravitational field strength of the Earth.

Let's have a quick check of that idea: "An astronaut visits the Moon, and they hold a hammer, and it takes a force of 20 newtons to lift that hammer on the Earth.

What force would be needed to lift the hammer while on the Moon?" So I'd like you to pause video, have a think about that, and select your answer.

And then restart.

Okay, the answer to that was three newtons.

The first answer there, answer A, zero newtons, that would mean there's no gravity on the Moon, and that's not true.

There is a gravitational force on the Moon.

It's just a bit less than that on the Earth.

Answer C, well, that's the same as the gravitational field on the Earth, and we know that the Moon's got a weaker gravitational field.

And answer D, well, that's higher, and obviously, that's wrong.

So well done if you chose answer B.

So what was the reason that the hammer takes a smaller force to lift on the Moon? Is it because there's no gravity on the Moon? The Moon has a lower gravitational field strength? The hammer always has the same weight? Or there's no atmosphere on the Moon? Which of those options is correct? Pause the video, and make that selection.

And then restart.

Okay, you should've remembered: the reason that the hammer's got a lower weight on the Moon is because it's got a lower gravitational field strength, so well done if you chose that option.

We're gonna learn how to calculate the weight of an object based upon its mass and how strong gravity is, the gravitational field strength, so we have an equation that links those two things together.

The weight is equal to the mass times the gravitational field strength.

Now, that's quite a long thing to write out.

Normally, we use symbols for these sorts of calculations, so we're gonna be using the symbols W for weight, m for mass, and g for gravitational field strength, just like this.

The weight is measured in newtons.

The mass is measured in kilogrammes, and the gravitational field strength is measured in newtons per kilogramme.

So, I'll do an example first.

And then you can try one after me.

My question is this: "A table has a mass of six kilogrammes.

What's its weight?" And we're gonna use g, gravitational field strength, as 10 newtons per kilogramme.

That's the strength of gravity on Earth, so the first thing I'd do is I write out the equation, like this: W = m X g.

I find the values and substitute them in so 6.

0 for m, the mass, and 10 for g, the gravitational field strength.

And finally, I do a brief calculation: W = 60 newtons.

So the weight's 60 newtons here.

Okay, your turn.

I'd like you to answer this question: "A bowling ball has a mass of 5.

5 kilogrammes.

What's its weight?" And again, use g = 10 newtons per kilogramme.

So, pause the video.

Do your calculation.

And then restart once you've got an answer.

Okay, what you should've done was write W = m X g, weight is mass times gravitational field strength, put in the two values from the question there.

Mass is 5.

5 kilogrammes, and the gravitational field strength is 10, and that gives the weight of 55 newtons.

Well done if you've got that answer.

I'm gonna do a second calculation now.

I'm gonna use the same equation, but this time, I'm gonna try and find out what the mass of an object is.

So, a bucket of water weighs 54 newtons.

What's its mass? And gravitational field strength is 10 newtons per kilogramme.

So as before, I'll write out the equation.

And then I substitute in the two values from the question.

I've got a weight of 54 there and gravitational field strength of 10.

I rearrange the equation slightly, moving the variables around to get mass on one side, so 54 divided by 10 is equal to the mass, and that gives me a final answer of 5.

4 kilogrammes.

The mass of the bucket of water was 5.

4 kilogrammes.

And now it's your turn to do the same sorta procedure as I've just done.

I'd like you to answer this question, please, so pause the video.

Go through the same steps, and come up with an answer.

And then restart.

Welcome back.

Let's have a look at the calculation.

So, you should've written out the equation, substituted in the values there, then rearranged it slightly to get m on its own on one side, and that gives a final answer of a mass of 1,270 kilogrammes.

Well done if you got that.

Okay, the third and final type of calculation you need to be able to do is to work out the gravitational field strength, so I'll do an example, and then you can do one.

So, I've got a space probe of mass 500 kilogrammes landing on the Moon, and on the Moon, it weighs 800 newtons.

What is the gravitational field strength on the Moon? And I'm gonna refer to that as g with a little Moon underneath it, a subscript there.

So as before, I write out the equation, W = m X g, and I substitute in the values that I've got in the question.

So the weight is 800, and the mass, 500, and g-Moon, there for the gravitational field strength of the Moon.

I rearrange it so I've got g-Moon on its own, and finally, I do the calculation 800 divided by 500 is 1.

6, so the gravitational field strength on the Moon is 1.

6 newtons per kilogramme.

So what I'd like you to do is to find out the gravitational field strength on Mars for me, please.

An identical probe lands on Mars, and it weighs 1,850 newtons.

What's the gravitational field strength on Mars? So pause the video.

Find your answer, and then restart.

Okay, the solution here, write out the equation.

And then substitute in the two values, and the key thing you needed to notice here is the identical probe, so it's got a mass of 500 kilogrammes.

Rearrange slightly.

And then finally, do the calculation, and that gives you a gravitational field strength on Mars of 3.

7 newtons per kilogramme.

Well done if you got that.

That was a fairly tricky one.

Right, we're at the final task now, so what I'd like ya to do is to answer these questions, please.

Got aliens building a space probe that has a mass of 6,000 kilogrammes.

On their home world, it has a weight of 96,000 newtons.

It then travels through deep space, finally landing on Earth, and what I'd like you to do is calculate the weight of the probe when it's reached Earth, calculate the gravitational field strength of the aliens' home world, and then describe what happens to the mass and to the weight as that probe goes through its journey, travelling from the aliens' world all the way to Earth.

So pause the video.

Try and answer those three questions.

And then restart, and I'll go through the answers with you.

Okay, welcome back.

Let's go through those answers: "Calculate the weight of the probe on Earth." So your solution should look something like this, just multiplying the mass times the gravitational field strength on Earth, and that gives 60,000 newtons.

Part two, your calculation should look a bit like this, the same procedures we carried out a few minutes ago: weight equals mass times gravitational field strength.

A bit of rearrangement and it gives a gravitational field strength on the alien world of 16 newtons per kilogramme so quite a lot stronger than the gravitational field on Earth.

And finally, your description of the journey could be something like this.

There's quite a lot of information here.

We've got "as the probe moves away from the alien world, its weight will decrease" because the gravitational field will get weaker, "but its mass will stay constant" 'cause there's the same number of atoms in that probe during the whole journey.

In deep space, the probe wouldn't be inside any gravitational fields, so there's no gravitational force and so no weight, but it's still got the same mass.

When it approaches Earth, it goes through Earth's gravitational field, which gets stronger and stronger as you get closer to the surface, so the gravitational force and the weight increases, but the mass stays constant.

Well done if you've got explanations like that.

So, we've reached the end of the lesson now, and here's a summary of all the information you should've seen.

Planets like Earth have gravitational fields that surround them, so different planets have different gravitational fields.

Objects inside those gravitational fields will feel gravitational forces acting on them.

The mass of an object is the amount of material it contains.

It's constant because there's the same number of atoms staying in that object no matter where it is.

The weight of an object is the force you need to lift it, and that depends upon the strength of gravity, and we can calculate the weight of an object using this equation: weight equals mass times gravitational field strength.

So that's the end of the lesson.

Hopefully, I'll see you in future ones.

Bye.