In 1687, Isaac Newton created three laws of motion to describe the relationship between a body and the forces acting upon it, and how the body moves in response to those forces.

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A second experiment can be carried out using the apparatus above, to investigate how the acceleration of an object depends on its mass, if the resultant force remains constant.

Use an accelerating force of 5 N and keep this constant.

Record acceleration as additional 0.5 kg masses are added to the trolley.

Plot a graph of mass m in kg on the y-axis against acceleration a in m/s^{2} on the x-axis.

Draw a smooth curve through the points.

The graph is not a straight line through the origin – mass and acceleration are not directly proportional.

Plot a second graph of 1/mass in 1/kg on the y-axis against acceleration a in m/s^{2} on the x-axis. Draw the line of best fit

This graph is a straight line through the origin. Acceleration is directionally proportional to

We say that mass and acceleration are inversely proportional.

If you double the mass, you half the acceleration.

When the forces acting on an object do not balance, the resultant force causes the object to accelerate in the direction of the resultant force.

Acceleration is directly proportional to resultant force if the mass remains constant.

In other words, a resultant force on a body will cause it to change its velocity.

This simply means that unbalanced forces will cause:

- acceleration
- deceleration
- change in direction