Forces and Motion

Describing how things move

Before you can explain WHY something moves, you need to describe HOW it moves, and physics is fussy about the words. - Distance is how far something travels. - Speed is how much distance it covers each second. - Velocity is speed in a stated direction, so thirty metres each second NORTH is a velocity, not just a speed. - Acceleration is the rate at which velocity changes, how quickly something speeds up or slows down. ⚠️ Speed and velocity are not the same thing, and that difference matters: a car going round a bend at a steady speed is still changing velocity, because its direction is changing. The rest of this module links these to the forces that cause them, and to the graphs that picture them.

Words for forces

Five terms about forces you need before you use them. Learn what each one means.

Match the motion word to its meaning

Two motion graphs

Motion is often shown on a graph, and the two kinds tell you different things. The trick is to read the slope.

Read the forces

A book rests on a table and does not move. What does this tell you about the forces acting on it?

• The forces are balanced, so the resultant force is zero
• There are no forces acting on it at all
• There is a large resultant force pushing it up
• Only gravity acts on it

Spot the true motion facts

Tap the TWO statements that are true.

• Velocity is speed in a stated direction
• A resultant force is needed to change the motion of an object
• Acceleration means the same thing as speed
• An object with balanced forces must be standing still

The three laws of motion

Isaac Newton set out three laws that link force and motion, and almost every question in this unit comes back to them. - The first law: an object stays at rest, or keeps moving at a constant velocity, unless a resultant force acts on it. - The second law: the resultant force on an object equals its mass multiplied by its acceleration. A bigger force gives a bigger acceleration; a bigger mass gives a smaller one. - The third law: when one object pushes on another, the second pushes back with an equal force in the opposite direction. ⚠️ The first law is the surprising one: keeping a steady velocity needs NO resultant force at all.

Work out the force

A trolley has a mass of 4 kilograms and accelerates at 3 metres per second squared. The resultant force equals the mass multiplied by the acceleration. Multiply 4 by 3 to find the force in newtons.

Working out a speed

Here is a calculation worked through so the method is clear. A runner covers 100 metres in 20 seconds at a steady pace. What is the speed. The rule: speed equals the distance divided by the time. The working: 100 divided by 20 gives 5. The answer: the speed is 5 metres each second. Always check the units make sense: metres divided by seconds gives metres each second, which is exactly what a speed should be. If a question gives you the speed and the time instead, rearrange the same rule by multiplying to find the distance.

Plot the journey

A car travels at a steady speed of 2 metres each second. On this distance-time graph, plot how far it has gone at each time: at time 0 plot 0, at time 1 plot 2, at time 2 plot 4, at time 3 plot 6, and at time 4 plot 8.

Order the journey

A car sets off, cruises, then stops at lights. Put the stages of its velocity in the order they happen.

• It speeds up from rest
• It travels at a constant velocity
• It slows down as it approaches the lights
• It comes to a stop

The forces run

Five questions on forces and motion. Three lives.

Complete the motion facts

The rate at which an object changes its velocity is its _____. The single overall force left when all the forces on an object are combined is the _____ force. The law stating that resultant force equals mass multiplied by acceleration is the _____ law of motion. Speed measured in a stated direction is called _____.

Forces in action

Read each case and choose the best answer, then think about why.

• A car engine drives it forward with 3000 newtons, while friction and air resistance push back with 3000 newtons. What happens to the speed of the car?
• An object of mass 2 kilograms feels a resultant force of 10 newtons. What is its acceleration?
• You push against a wall and it does not move. What is happening, according to the third law of motion?

Explain forces and motion

A student in the year below is confused about forces and motion. Explain the key ideas using what this module has covered.

• Explain the difference between speed and velocity
• Explain what acceleration means
• Explain what a resultant force does to the motion of an object
• Give the rule linking resultant force, mass and acceleration, with a worked example
• Finish by explaining what a distance-time graph and a velocity-time graph each show