Hello, my name is Mr. Forbes and I'll be leading you through the lesson today.

It's a lesson from the Movement by force unit, and it's all about measuring speed practically.

That means you're gonna carry out an experiment, measure some distances and some times, and use that data to calculate speed.

By the end of this lesson, you're going to be able to measure the speed of a trolley rolling down a ramp.

You're gonna be able to do that as accurately as possible by making sure you measure time accurately and distance accurately.

So when you're ready, let's get on with the lesson.

The keywords you need for a lesson here.

The dynamics trolley is just a little trolley you roll down a ramp.

Gradient is another word for slope, so high gradient is a steep slope.

A timer is anything you can use to measure time, such as your phone or an electronic timer we use in physics.

A metre rule is just a metre rule.

And average speed is the speed of an object over a certain distance, is the distance divided by time.

And here's a set of definitions of those keywords.

You can pause the video here to read through them and you can return to this slide at any point.

So the first thing you need to know is what a dynamics trolley is.

And it's one of these, it's a simple wheeled vehicle we use in physics for quite a lot of our experiments.

It's a standard size and it rolls smoothly over surfaces.

It's got very low friction and that allows it to keep on going over quite a distance when it's very small forces.

And the second piece of equipment is a simple ramp.

A ramp is just a sloped surface.

Something like that, a wooden board.

Sometimes you can buy specialist ones, but any simple sloped surface will do.

When you put a dynamic trolley on it, there's a forward force that acts on it that'll make the trolley roll.

There is a small force of friction as well, but that's so small that we're going to basically ignore it during this experiment.

As we put the trolley on, it will start to roll down and speed up as it goes.

It speeds up because that forwards force is always there, always causing an acceleration.

The average speed of the trolley is going to be able to be worked out from the distance and time measurements that we take.

Now, during this experiment, we are going to use different gradients.

The gradients is the steepness of the slope and we can adjust that by just tilting the ramp.

We can get a small ramp like that with a low gradient and a small forwards force, a medium-sized gradient, giving us a bigger forwards force and a larger gradient that's gonna give us quite a large forwards force.

Now, even though I've described that as a large gradient, we really don't want to make those gradients too steep, otherwise it'd be very difficult to time the trolley.

It'll move too fast.

Okay, first check here, it's gonna be in two parts.

First of all, the true and false part.

Using a ramp with a greater gradient will make the trolley roll faster.

Is that true or false? Pause the video and make your selection and then restart.

Okay, welcome back.

That should have been true.

So well done if you got that.

Now I'd like you to justify that.

I'd like you to give me your reason.

Is it because the higher the gradient of the ramp, the lighter the trolley is, or the higher the gradient of the ramp, the greater the forwards force? Pause the video, make your selection, and then restart.

Okay, well, your answer should have been the higher the gradient of the ramp, the greater the forwards force.

So well done if you've got that.

In the experiment, you're gonna need to measure a distance.

And the simplest way to do that is to use a metre rule.

Because it's a standard fixed length and it's fairly accurate.

So we're gonna use just one of those wooden or plastic metre rules.

What we do is we get the ramp and we put two marks on it, a mark for the start line and a mark for the finish line, just using the ruler to measure those distances out.

There's the start line and there's the finish line.

You're going to need a timer to time how long it takes for the trolley to roll through that distance of one metre.

So you can position the trolley at the start line here and allow it to roll to the finish line.

To measure that time accurately, you'll want to start the timer exactly at the point where you release the trolley.

And to do that, you should use something like a countdown, three, two, one, go, or something like that.

You've also got to be able to stop the timer precisely when the trolley reaches the finish line.

So to do that, the best thing to do is to look directly across the finish line so you can see when the front of the trolley reaches it, and then stop the timer straight away.

Okay, which two of these will help you measure the roll time accurately? Pause the video, make your selection, and then restart when you're happy.

Okay, the two correct answers were looking directly across the finish line, so you can stop the time at just the right time, and counting down to the release of the trolley so that you can start the timer at just the right time.

Well done if you've got both of those.

So here's a set of instructions for the experiment.

I'd like you to read through those carefully before you carry it out.

And to help you, I'll show you a video of someone carrying out the experiment and doing one of the tests.

<v Test Narrator>In this investigation,</v> we're going to be measuring the average speed of a trolley that's rolling down a ramp.

If you look at the ramp, you'll notice that there's two straight lines being drawn onto it, one where the trolley is and one at the bottom of the ramp.

These have been measured with a metre ruler and they're exactly one metre apart.

So what we're going to do is we're going to time how long it takes the trolley to move from the top line, one metre to the bottom line.

So here goes.

<v Test Conductor>Three, two, one.

</v> (trolley wheels rattling) <v Test Narrator>So it took our trolley 2.

74 seconds</v> to travel down that one metre down the slope.

Now you'll have noticed that the trolley sped up as it went down the slope, at least at the start.

And so what we've measured is the average speed.

It's not gone at a steady speed all the way down.

We've just got an average speed.

Now it's always useful to repeat a measurement to double check if you've got it correct.

So let's try again and see if we get a similar time.

<v Test Conductor>Three, two, one.

</v> (trolley wheels rattling) <v Test Narrator>So there we go,</v> exactly 2.

74 seconds again.

I'm actually very surprised at that.

I would've expected it to be a little bit different because of timing errors of the person who was letting go of the trolley, not quite starting at the right moment and not quite getting it right at the other end either.

Let's just have a look at a few more measurements and see what times we get for the next few runs of the trolley down the ramp.

<v Test Conductor>Three, two, one.

</v> (trolley wheels rattling) Three, two, one.

(trolley wheels rattling) Three, two, one.

(trolley wheels rattling) Three, two, one, go.

(trolley wheels rattling) <v Test Narrator>And so, apart from the first two results,</v> they're all slightly different.

And that can be put down to the timing errors when we start the timer and when we stop the timer at the end.

And we can now use those measurements to calculate the average speed of the trolley going down that ramp.

And to do that, we use the equation Speed equals distance divided by time.

Remembering of course that the distance we're measuring is exactly one metre.

<v ->Once you're happy that you've understood everything,</v> you can then get on with the experiment and collect your results.

Okay, you should have carried out the experiment by now.

And here's the results table.

Your table should look something like this, but it won't be exactly the same.

You should have three steepnesses, low, medium, and high, and at least three times for each of the trolleys.

Now your data won't look exactly the same as mine.

And my table also has two extra columns for some calculations we're gonna be doing in the second part of the lesson.

So well done if your table on your results look anything like this.

Okay, we're gonna move on to the second part of the lesson and we're gonna use the data we collected in the experiment to find some average speeds.

Okay, we're gonna find the average time it took for the trolley to roll down the ramp.

And the type of average we're finding is actually called the mean.

And the mean is when you add the values together and divide by how many values there are.

So I'm gonna add the three different time measurements here and then divide by three because there's three of them.

That's on, and that gives me a mean time of 3.

26 seconds, which are right into my table, just there.

Right, to test if you understand how to calculate mean, I'd like you to calculate the mean value of these four time values, please.

Pause the video, work out your answer, and then restart when you're happy.

Welcome back.

Let's have a look at the answer.

It was 2.

44 seconds.

You get 2.

44 like this.

There are four readings so we add all four values together, then we divide by four, and that gives us an answer of 2.

44 seconds.

Well done if you've got that.

So now we're at the point where we can calculate the average speed of the trolley.

We've got the mean times and we know the distance because we measured out the distance during the experiment.

It travelled one metre each time.

So for the lowest slope, we can calculate the average speed by using distance divided by mean time.

The distance was one metre, the mean time was 3.

29 seconds.

So doing that calculation gives us an average speed of 0.

3 metres per second.

Now it's time for you to calculate the average speed of your trolley.

If you don't have any data, you can use the data here.

I'd like you to pause the video, calculate those average speeds, and then restart.

Welcome back.

I'm gonna use my data to calculate the average speeds.

So I can calculate the mean times, fill them in here, that's 3.

33 seconds for the low gradient, and then use that when the speed equation to calculate the average speed.

Speed equal distance divided by time.

Average speed is 1 divided by 3.

33 and that gives me an average speed of 0.

30 metres per second.

And I can repeat those same calculations with the other two gradients of ramp, medium and high.

Okay, we've reached the end of the lesson now, and here's the summary of the information.

You can find the average speed of a dynamics trolley as it rolls down a ramp by measuring a distance and a time.

A higher gradient of the ramp will increase the size of the force on the trolley and that will affect its speed.

You can measure the distance with a metre rule and you can measure the time with a timer, and you can improve timely measurements by carefully observing the start and the finish line and doing things like counting down before you release the trolley.

So well done at reaching the end of lesson and I hope to see you in the next one.

Bye.