### KS3 Physics

Energy
Motion and Forces
Waves
Electricity and Electromagnetism
Matter
Space Physics

# Balanced Forces

Balanced forces occur when two forces applied to an object are equal in magnitude but opposite in direction.

Isaac Newton’s First Law of Motion is closely related to the concept of balanced forces. This law states that an object at rest will remain at rest, and an object in motion will remain in motion with the same speed and direction, unless acted upon by an unbalanced force.

In other words, if the forces acting on an object are balanced, the object will maintain its current state of motion, either staying at rest or moving at a constant velocity.

Let’s look at Newton’s first law using a lorry as an example:

Let’s assume that the forces acting on the lorry are balanced.

• If the lorry was originally moving, then it will continue to move in the same direction and at the same speed.
• If the lorry was not moving, then it will remain stationary.

In this example, the two opposing forces are thrust and friction. Friction is a force that resists the motion of surfaces sliding across each other. Due to the roughness of the road surface and the wheels of the lorry, friction acts in the opposite direction to the lorry’s motion. Therefore, the wheels and the road cannot slide across each other easily.

If the forces are balanced, then the thrust and friction are equal but opposite in direction. In this case, two scenarios can occur. If the lorry was initially at rest, it will remain stationary. However, if it was already in motion, it will continue to move at a constant speed.

Another example involves boats in water

A boat floats because the upthrust, also known as buoyancy, from the water balances out the weight of the boat.

This concept is illustrated in the diagram below. Keep in mind that buoyancy is the upward force exerted by a fluid, opposing the weight of an object that is partially or fully immersed.

• On the left – As the buoyant force is greater than the force of gravity, the object will keep going up
• In the middle – As the buoyant force is equal to the force of gravity, the object will float
• On the right – As the buoyant force is less than the force of gravity, the object will sink

## Force Diagrams

The best way to represent this is by using force diagrams.

(hint: arrows are very important)

In a force diagram, the arrows indicate both the direction and the magnitude of the force.

In the diagram above, you’ll notice that the two arrows are of the same length. This tells us that the forces acting on the book are equal (but opposite).

The bigger the arrow size, the greater the force. When forces are balanced, we can say that the overall force is zero. This overall force is known as the resultant force.

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