Forces

Recap of BGE

In the BGE course, Forces were covered in great detail. The following facts were covered:-

A Force can be seen as a push, a pull, or a twist.

A Force can change the shape, speed or direction of an object.

A Force has the units of Newtons.


Measuring Forces

A Force is measured with a Newton balance. A Newton balance consists of a spring with an attached scale. As Force is applied, the spring is extended.

Balanced Forces

When two identical Forces act in opposite directions, they cancel each other out.

Newton's First Law

The above understanding of Forces allowed Isaac Newton to develop the 1st of his three laws of Forces :-

"An object will remain at rest or travel in a straight line with a constant speed, unless an unbalanced Force acts upon it"


Terminal Velocity

When a skydiver jumps out of a plane, they accelerate (see unbalanced Forces below). After a short time, however, they reach their maximum Speed. This maximum Speed is called their Terminal Velocity.

This happens because the two forces acting on the skydiver are balanced.

The Air Resistance that they experience is equal in size and in opposite direction to their Weight:-

Note - By changing their body shape (making themselves more streamlined), the skydiver can alter their air resistance, changing their Terminal Velocity.


Unbalanced Force

When an object experiences an unbalanced Force, the object will Accelerate. This can either be caused by only one Force acting (as in the diagram below), or by two unequal Forces acting in opposite directions.

Newton's Second Law

The above understanding of Forces allowed Isaac Newton to develop the 2nd of his three laws of Forces:-

"An unbalanced Force acting on an object will cause the object to Accelerate. The size of that acceleration will depend on the size of the Force and the Mass of the object."

The above Law is more commonly described as the formula below:-

F = m x a

Where :-

F = Unbalanced Force (N)

m = Mass of Object (kg)

a = Acceleration of Object (ms-2)

Example 1 -

A truck of mass 2000 kg is acted upon by two forces as shown in the diagram below:-

Calculate:-

The unbalanced Force acting on the object.

The Acceleration experienced by the object.

Unbalanced Force = 50000 - 45000

Unbalanced Force = 5000 N (to the right)


F = m x a

a = F / m

a = 5000 / 2000

a = 2.5 ms-2 in the forward direction.

Note - Force is a Vector, so a direction must be given.


Motion Case Study : Skydiving

The video below shows how the motion of a skydiver is affected by the changing forces they experience:-

The following Velocity-Time graph shows the motion of a skydiver during a jump. By observation of the graph, it is possible to identify key times in the jump process:-