Motion and Forces

Motion and forces

Physics describes how things move and what makes their motion change. Speed tells you how far something travels in a given time, and it is worked out as distance divided by time. Velocity is speed in a particular direction, and acceleration is how quickly the velocity changes. Motion changes only when a force acts. When the forces on an object are balanced, its motion stays the same; when they are unbalanced there is a resultant force that speeds the object up, slows it down or changes its direction. The bigger the resultant force and the smaller the mass, the greater the acceleration, a link written as force equals mass multiplied by acceleration. This module covers speed, velocity and acceleration, the laws of motion, resultant force and friction, terminal velocity, and turning forces called moments. Take g as 10 newtons per kilogram in any calculation.

Physics words

Learn what each of these means before you use it.

Balanced against unbalanced forces

Whether the forces on an object are balanced decides whether its motion changes.

How to solve a force problem

Most force and motion questions are solved the same way. First, write down what you are given and what you need to find, with the units. Then pick the equation that links them: distance divided by time for speed, mass multiplied by acceleration for a resultant force, mass multiplied by gravitational field strength for weight, or force multiplied by distance for a moment. Put the numbers in, keep the units consistent, and work out the answer. Always give a unit with the answer, because a number on its own earns no marks. Practising this routine turns a wordy problem into a short, safe calculation.

Work out the speed

A runner travels 100 metres in 20 seconds. Speed is distance divided by time. Work out the speed, in metres per second.

Speed or velocity?

A car travels at a steady 30 metres per second in a straight line, then goes round a bend at the same 30 metres per second. Its speed is unchanged, but what has changed?

• its velocity, because velocity includes direction and the direction has changed
• its mass, because going round a bend adds mass
• nothing at all, because the speed is the same
• its weight, because bends increase weight

Match the quantity to its unit

Work out the force

A resultant force acts on a mass of 10 kilograms and gives it an acceleration of 2 metres per second per second. Force equals mass multiplied by acceleration. Work out the force, in newtons.

True about forces

Select the TWO statements that are true about forces.

• A force is measured in newtons
• When forces are balanced there is no resultant force
• A force is measured in kilograms
• Balanced forces always make an object speed up

Complete the physics facts

Speed measures how far an object travels in each unit of _____. Velocity is speed in a stated _____. When the forces on an object are balanced, there is no _____ force and its motion does not change. Force is measured in units called _____.

Order the fall to terminal velocity

Put these stages of a skydiver reaching terminal velocity in order.

• The skydiver starts to fall and speeds up
• As the speed rises, air resistance grows
• Air resistance grows until it balances the weight
• The forces balance and the skydiver falls at a steady terminal velocity

Spot the true physics facts

Tap the TWO statements that are true.

• Acceleration is how quickly the velocity of an object changes
• Weight depends on both the mass and the strength of gravity
• A moment is measured in kilograms
• Balanced forces make an object accelerate

Apply the physics

Read each case and choose the best response, with a reason.

• A car has a driving force forward that is bigger than the friction and air resistance against it. What happens to the car?
• You must find the weight of a 5 kilogram bag. Taking g as 10 newtons per kilogram, which calculation is right?
• A spanner turns a nut more easily when you push at the end of a long handle rather than close to the nut. Why?

Explain motion and forces

A friend is starting GCSE Physics and asks how motion and forces are linked. Explain motion and forces.

• Explain the difference between speed and velocity
• Explain what acceleration is
• Explain the difference between balanced and unbalanced forces
• Explain what the equation force equals mass multiplied by acceleration tells you
• Finish by explaining what terminal velocity is