The forces on a moving car are balanced. What happens to the car?

It keeps moving at the same speed in the same direction. With no resultant force there is no change in velocity.

A parachutist falls at terminal velocity. Which statement is correct?

Air resistance equals weight. At a steady speed the forces are balanced.

Which force always opposes the motion of an object sliding along a surface?

friction. Friction acts along the surface, against the direction of sliding.

The same resultant force acts on two objects. Object X has twice the mass of object Y. How do their accelerations compare?

X has half the acceleration of Y. a = F ÷ m, so doubling the mass halves the acceleration.

Forces and Newton's laws · Cambridge IGCSE Co-ordinated Sciences (9

In plain words

A force is a push or a pull. If all the forces on something cancel out, it carries on exactly as it was: still, or moving at a steady speed in a straight line. Only an unbalanced (resultant) force changes its motion. A bigger resultant force gives a bigger acceleration; a bigger mass gives a smaller one.

Three things to know

The resultant force is the single force that has the same effect as all the forces together. If it is zero, the object stays still or keeps moving at a steady speed in a straight line.
Resultant force = mass × acceleration (F = ma).
Friction and air resistance oppose motion. A falling object reaches terminal velocity when air resistance equals its weight.

Worked example

A 1200 kg car has a driving force of 3000 N and resistive forces of 600 N. Find its acceleration.

Resultant force = 3000 − 600 = 2400 N.
a = F ÷ m = 2400 ÷ 1200.
= 2 m/s².

Tips and tricks

Find the resultant force first. Forces in opposite directions subtract.
Steady speed means balanced forces, not "no forces". This one catches a lot of people.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Forces and Newton's laws: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

The forces on a moving car are balanced. What happens to the car?
- It slows down and stops.
- It speeds up.
- It keeps moving at the same speed in the same direction. (the answer)
- It changes direction.
With no resultant force there is no change in velocity.
A 2 kg trolley is pushed with a resultant force of 10 N. What is its acceleration?
- 0.2 m/s²
- 5 m/s² (the answer)
- 12 m/s²
- 20 m/s²
a = F ÷ m = 10 ÷ 2.
A parachutist falls at terminal velocity. Which statement is correct?
- There is no air resistance.
- Weight is greater than air resistance.
- Air resistance equals weight. (the answer)
- Air resistance is greater than weight.
At a steady speed the forces are balanced.
Which force always opposes the motion of an object sliding along a surface?
- weight
- friction (the answer)
- upthrust
- tension
Friction acts along the surface, against the direction of sliding.
A 1500 kg car accelerates at 2 m/s². What is the resultant force on it?
- 750 N
- 1502 N
- 3000 N (the answer)
- 30 000 N
F = ma = 1500 × 2.
The same resultant force acts on two objects. Object X has twice the mass of object Y. How do their accelerations compare?
- X has half the acceleration of Y. (the answer)
- X has twice the acceleration of Y.
- They are equal.
- X has four times the acceleration of Y.
a = F ÷ m, so doubling the mass halves the acceleration.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

4 questions, 11 marks. Write your answers on paper, then check them.

Forces and Newton's laws

Cambridge IGCSE Co-ordinated Sciences (9–1) 0973 · 11 marks · papermunch.org

Name ______________________________ Date ______________

Forces of 20 N to the right and 12 N to the left act on a trolley. State the size and direction of the resultant force.[2]
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8 N to the right. 20 − 12.
I got this one
A resultant force of 600 N acts on a car of mass 1200 kg. Calculate its acceleration.[3]
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0.50 m/s². a = F ÷ m = 600 ÷ 1200.
I got this one
Calculate the resultant force needed to accelerate a 70 kg sprinter at 4.0 m/s².[3]
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280 N. F = ma = 70 × 4.0.
I got this one
Explain why a skydiver reaches a steady speed (terminal velocity).[3]
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As she speeds up, air resistance increases. When it equals her weight the resultant force is zero, so she stops accelerating.
I got this one

Answers: Forces and Newton's laws

1. 8 N to the right. 20 − 12.
2. 0.50 m/s². a = F ÷ m = 600 ÷ 1200.
3. 280 N. F = ma = 70 × 4.0.
4. As she speeds up, air resistance increases. When it equals her weight the resultant force is zero, so she stops accelerating.