Newton's first law

According to Newton's first law of motion, an object remains in the same state of motion unless a resultant force acts on it. If the resultant force on an object is zero, this means:

  • a stationary object stays stationary
  • a moving object continues to move at the same velocity

Resultant forces

Two forces can be added together to find a resultant force. The resultant force is a single force that has the same effect as two or more forces acting together.

Two forces in the same direction

The resultant force of two forces that act in the same direction can easily be calculated - to do this, just add the magnitudes of the two forces together.

Example

Two forces, 3 newtons (N) and 2 newtons (N), act to the right. Calculate the resultant force.

3 N + 2 N = 5 N to the right

Two arrows, one above the other, both pointing to the right, one labelled 2 N and one labelled 3 N. Then an equals sign and then another arrow to the right labelled 5 N.

Two forces in opposite directions

Two forces that act in opposite directions produce a resultant force that is smaller than either individual force. It is often easiest to subtract the magnitude of the smaller force from the magnitude of the larger force.

Example

A force of 5 N acts to the right, and a force of 3 N acts to the left. Calculate the resultant force.

5 N - 3 N = 2 N to the right

Two arrows, one above the other, one pointing to the left, labelled 2 N, the other pointing to the right labelled 3 N. Then an equals sign, with an arrow to the right labelled 1 N.

Inertia - Higher

The tendency of an object to continue in its current state (at rest or in uniform motion) is called inertia. Inertial mass is defined as the ratio of force over acceleration. Inertial mass is a measure of how difficult it is to change the velocity of an object.