1.5 Balanced and unbalanced
Summary: When you wake up in the morning, you need a force to get out of bed and moving. Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe the difference between Balanced and Unbalanced Forces
- explain why an object is in Equilibrium
- explain the changing Motion of objects.
Think back
- What can a force do to a moving object?
- What is the effect of air resistance on a falling object?
- What is the natural object in orbit around Earth?
Key idea More than one force can act on an object. Forces are balanced when they are the same size and act in opposite directions. Forces are unbalanced when they are not the same size.
Key words balanced, equilibrium, unbalanced, driving force, resistive force
When you wake up in the morning, you need a force to get out of bed and moving.
What are balanced forces?
When the forces acting on an object are the same size but act in opposite directions, they are balanced. You can think of balanced forces as two teams at each end of a rope, pulling. If each team pulls with the same force, the rope does not move because the forces cancel out. The object is in equilibrium.
A Name an object on these pages that is in equilibrium.
All stationary (still) objects are in equilibrium. There has to be a support force acting on them to balance out their Weight. The person in Figure 1 is in equilibrium when they lie in bed. The support force and their weight are balanced.
Figure 1You are in equilibrium when lying down.
B Draw a diagram showing the forces acting on a stationary mass hanging on a spring.
What are unbalanced forces?
Figure 2 shows a rocket-powered car. The forces acting on it are unbalanced – they are not the same size. This means that they do not cancel out. The driving force from the engine is shown in blue. The resistive forces from air resistance and Friction are shown in red.
Figure 2The car will change Speed depending on what direction the unbalanced force is in. Full text alternative to the content
C Describe a situation where the forces on your pen are unbalanced.
How do unbalanced forces cause changes in speed?
When the car’s rocket-powered engine starts moving, the driving force will become very big very quickly and the car will speed up. When the driver wants to stop, they will fire a parachute to slow the car down. In both cases, the forces acting on the car are unbalanced.
The driver uses a parachute to slow the car down because it creates a much bigger resistive force than using the brakes alone. This means the car will slow down much faster and stop in a much shorter time.
How do unbalanced forces cause changes in direction?
Isaac Newton worked out that Earth exerts a force on the Moon. The force of Gravity acting on the Moon keeps the Moon in orbit (on a path) around Earth, as you can see in Figure 3. The force that keeps the Moon orbiting Earth is the same force that acts on an apple and pulls it to the ground. It changes the direction of motion, but not the speed.
Figure 3The force of gravity keeps the Moon in orbit around Earth.
Just like the Moon orbiting Earth, every time you go around a corner in a car, the friction between the tyres and the road changes the direction of the car.
Working scientifically Isaac Newton said, ‘For every action there is an equal and opposite reaction’. The forces in an interaction pair are equal and opposite. Is lying in bed an example of this law? No, it is not. Each of the forces acting on you comes from a different interaction pair.
Stretch zone
- Copy and complete the sentences. If the forces on an object are balanced, they are the same direction/size but act in opposite directions/sizes. This is called equilibrium/tension. The forces acting on any stationary object are balanced/unbalanced. If the forces on an object are unbalanced, the object’s mass/speed will change. The object speeds up if the driving/resistive force is bigger than the driving/resistive force. The object slows down if the driving/resistive force is bigger than the driving/resistive force.
- A cyclist is slowing down. Draw a diagram to show the forces acting on the cyclist.Label the forces using the words ‘resistive’ and ‘driving’.Explain why their speed is decreasing.
Stretch zone
- Explain how the forces on an object can be unbalanced even though it is moving at a steady speed.