# Effects of forces

A force acting on an object may cause the object to change shape, to start moving, to stop moving, to accelerate or decelerate.

When two objects interact with each other they exert a force on each other, the forces are equal in size but opposite in direction.

Resultant force

The forces acting on an object can be replaced with a single force that causes the object to behave in the same way as all the separate forces acting together did, this one overall force is called the resultant force. All forces (F) are measured in newtons (N).

If the resultant force acting on an object is ZERO then;

• the object will remain stationary if it was stationary when the resultant force became zero
• move at a constant (steady) speed in a straight line  if it was moving when the resultant force became zero

If the resultant force acting on an object is NOT ZERO then;

• the object will accelerate or decelerate (speed up or slow down)

When all the forces that act upon an object are balanced, then the object is said to be in a state of equilibrium. The forces are considered to be balanced if the rightward forces are balanced by the leftward forces and the upward forces are balanced by the downward forces. This, however, does not necessarily mean that all the forces are equal to each other. Consider the two objects pictured in the force diagram shown below. Note that the two objects are at equilibrium because the forces that act upon them are balanced; however, the individual forces are not equal to each other. The 50 N force is not equal to the 30 N force.

If an object is at equilibrium, then the forces are balanced. Balanced is the keyword that is used to describe equilibrium situations. Thus, the net force is zero and the acceleration is 0 m/s/s. Objects at equilibrium must have an acceleration of 0 m/s/s. This extends from Newton’s first law of motion. But having an acceleration of 0 m/s/s does not mean the object is at rest. An object at equilibrium is either …

• at rest and staying at rest, or
• in motion and continuing in motion with the same speed and direction.