Newton’s First Law

Balanced forces

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Newton’s First Law states that a body will remain at rest or continue to move at constant speed in a straight line, unless a resultant force acts on it.

This simply means that balanced forces, acting on the same object, will have no effect on the motion of an object.

In practice this means:

  • it could be stationary, or;
  • it could be moving at constant speed in a straight line (constant velocity).

Example

The forces acting on this car are balanced.

A car with 2 forces acting either side of it. On the left is Drag, which measures 5,000 newtons. On the right is Thrust, which also measures 5,000 newtons.Car moving forward at 20 m/s

The thrust from the engine is equal and opposite to the drag caused by air resistance and friction between the road and car tyres.

There is no resultant (or net) force as the forces add up to zero.

The car will continue to travel forward with a speed of 20 m/s in a straight line.

Example

A car with 2 forces acting on it. The downward force is labelled 'Weight'. The upward force is labelled 'Normal reaction force of ground on car'.These forces balance. The car is still.

The upward force equals the downward force and they both act on the car.

The car remains at rest. It does not move upwards or downwards.

Balanced forces have no effect on an object.

If it is at rest, it remains at rest.

If it is moving at constant speed in a straight line, it continues to move at the same speed in the same straight line.