4.3 Free-body diagrams

What is weight?

The gravitational force acting on an object through its centre of mass

What is friction?

The force that arises when two surfaces rub against each other.

What is drag?

The resistive force on an object travelling through a fluid (eg. air and water) the same as friction.

What is tension?

The force within a stretched cable or rope.

What is upthrust?

An upwards buoyancy force acting on an object when it is in a fluid.

What is normal contact force?

A force arising when one object rests against another object.

How is each force vector represented in free body diagram?

By an arrow labelled with the force it represents. Each arrow is drawn to the same scale, the longer the arrow the greater the force.

How can you find the forces acting on an object on a smooth slope?

Assume there is no friction, the only force acting on it is its weight. This can be resolved into 2 components.

Force parallel to slope = Wsin theta or Fx = mg sin theta

Force perpendicular to slope = Wcos theta or FYI = mgcos theta

The component of weight down the slope is responsible for the acceleration of the object down the slope. There is no accelration of the object perpendicular to the slope. Therefore, this component of the weight must be equal to the normal contact force N acting on the object, that is

Fy = N = mgcos theta