A single resultant force has the same effect as several forces acting on an object
If there is no resultant force on an object, it remains still or continues to move at a steady speed in a straight line
If there is a resultant force on an object, it will accelerate in the direction of the force
A resultant force acting in the opposite direction to movement will decelerate (slow down) an object

Common misconception

A common belief is that a resultant force is needed to keep an object in motion, and if no resultant force acts on an object then it must be stationary.

Discuss examples in which friction is very low or absent, such as a puck sliding across an ice rink or an object being dropped during a space walk and extrapolate to what would happen if friction disappeared entirely.

Keywords

Resultant force - the effect of a set of forces on an object
Newton’s First Law of motion - an object will remain at rest, or in motion in a straight line at constant speed, unless there is a resultant force acting on it
Velocity - the rate of change of displacement of an object, measured in metres per second (m/s)
Accelerate - a change in the speed and/or direction of an object
Decelerate - a decrease in the speed of an object

A linear air track, or air hockey table, is an excellent way of showing that objects will move at a constant speed if friction can be eliminated. Try to show this motion to convince pupils that Newton's First Law is valid for moving objects as they can struggle to accept this.

Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

How many newtons (N) are there in 2.5 MN?

2500

25,000

250,000

Correct answer: 2,500,000

25,000,000

Q2.

When two or more forces act on an object, we can imagine a single force that has the same effect as this combination of forces. This single force is called the force.

Correct Answer: resultant, net

Q3.

A coin is dropped into a swimming pool. While the coin is falling downwards through the water, which of the following forces act on it?

air resistance

Correct answer: gravitational force

normal reaction force

Correct answer: drag

Q4.

There is a law of physics which states that forces come in equal and opposite pairs, which act between pairs of objects. What is the name of this law?

Newton’s First Law of motion

Newton’s Second Law of motion

Correct answer: Newton’s Third Law of motion

Newton’s Fourth Law of motion

Q5.

Which of the following are correct definitions of acceleration?

change in displacement divided by time taken

Correct answer: rate of change of velocity

time taken to change velocity by one metre per second

Correct answer: change in velocity per second

Q6.

Which of the following situations involve a change in velocity?

Correct answer: A car increases speed as it travels in a straight line.

A car travels in a straight line at a steady speed.

Correct answer: A car turns a corner without changing speed.

Correct answer: A car slows down while turning a corner.

Exit quiz

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6 Questions

Q1.

The diagram shows four forces acting on an object. What is the magnitude of the resultant force?

Correct answer: 1 N

5 N

6 N

7 N

13 N

Q2.

Which of the following is not caused by a resultant force acting on an object?

Correct answer: moving at constant velocity

changing direction

start moving

stop moving

decelerating and changing direction

Q3.

Which of the following statements are included in Newton’s First Law of motion?

Correct answer: A stationary object remains stationary if there is no resultant force.

Every action has an equal and opposite reaction.

Correct answer: A moving object’s velocity stays the same if no resultant force acts.

When object A exerts a force on object B, B also exerts a force on A.

Q4.

A ship travels at a constant speed without changing direction. Which of the following statements are correct?

There is a steady resultant force acting on the ship.

Correct answer: There is no resultant force acting on the ship.

Correct answer: The force pushing it forwards is equal to the force pushing it backwards.

The force pushing it forwards is greater than the force pushing it backwards.

Q5.

A person pushes a stone across a frozen lake. After they let go, the stone slides across the ice, slowing and stopping before it reaches the other side of the lake. Why does the stone slow and stop?

The person’s pushing force runs out.

The person’s pushing force transfers to the thermal store.

Correct answer: There is a resultant force acting in the opposite direction to the motion.

Other forces cancel out the person’s pushing force.

Q6.

Which of the following vans is decelerating?

Correct Answer: An image in a quiz