forces and motion

Forces can do one of four things to an object:

1. Make it speed up - accelerate.
2. Make it slow down - decelerate.
3. Change its direction.
4. Change its shape.

in order for something to do one of these four things, a resultant force must be acting on it.

a resultant force is the overall force acting on an object.

force is measured in Newtons (N)

Newton's First Law states that:

'Every body continues in a state of rest or uniform motion unless acted upon by an external force.'

so, if an object is moving, then it will continue to keep moving at a constant speed, and in the same direction unless there is a resultant force acting on it.

Also, if an object is stationary, then it will remain stationary unless a resultant force acts on it!

F = ma

this is:- Force = mass X acceleration

• F is the force in Newtons, N.
• m is the mass in kilograms, kg.
• a is the acceleration in m/s².

Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

This shows that if you keep the mass constant and double the applied force the acceleration will double.

You can see here that force is proportional to acceleration. As you double the force the acceleration doubles, as you triple the force the acceleration triples.

You can see here that if you keep the force constant and increase the mass the acceleration will fall. Acceleration is inversely proportional to mass. If you double the mass the acceleration will halve.

Moments make things turn or rotate. They are caused by forces but are not forces themselves. Like forces, moments have a direction. We say they are either clockwise or anti-clockwise, to show which way they will make something turn.

Moment = Force X perpendicular distance

so the smaller the perpendicular distance from the pivot, the greater the force needed to make something turn, e.g a door handle

The bigger the force causing the turning effect the bigger the moment will be.

The further the force is from the pivot the bigger the moment will be.

Moments are usually measured in Nm