As Physics Forces and Motion

AS PHYSICS – FORCES AND MOTION FORCE, WORK AND POWER

DOING WORK

In physics, we often use everyday words that have special meanings.

WORK is such an example

In physics, work is defined as force acting upon an object to cause a displacement.

There are three key words in this definition - force, displacement, and cause.

In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement.

Is work being done is the following examples?

●A teacher applies a force to a wall and becomes exhausted?

●A book on quantum physics falls off a table and free falls to the ground?

●A waiter carrying a tray full of meals above her head with one arm across the room? (Careful! This is a very difficult question)

●A rocket accelerating through space?

Work Done = Force x Distance moved in the direction of the force

W = F x x

The Force (F) is a constant force along the direction of motion.

If the Force (F) was not constant along the direction of motion then the equation W = Fx would no longer hold true.

●A teacher applies a force to a wall and becomes exhausted?

No this is not an example of work. The wall is not displaced

●A book on quantum physics falls off a table and free falls to the ground?

Yes. This is an example of work. There is a force due to (gravity), which acts on the book, which causes it to be displaced in a downward direction.

●A waiter carrying a tray full of meals above her head with one arm across the room?

No. This is not an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray moves horizontally across the room). Yet the force does not cause the displacement

●A rocket accelerating through space?

Yes. This is an example of work. There is a force (the expelled gases push on the rocket), which cause the rocket to be displaced through space.

●Doing work is a way of transferring energy.

Work done = Energy transferred

●The Joule is the unit of work.

●One Joule is equivalent to one Newton of force causing a displacement of one meter.

●One Joule is the amount of energy transferred when a force of 1 Newton moves a distance of 1 meter in the direction of the force.

1 Joule = 1 Newtonx 1 meter

1J = 1 N x m

Q1) A man exerts a force of 450N on a ice cream van, moving the van 3.5m. Calculate the work done by the man. Show full working.

Q2)A donkey exerts a force of 800N on a heavily loaded wagon of mass 300Kg moving it 3m. Calculate the work done by the donkey. Show full working

Q3)A bird of mass 0.3Kg flies 30m vertically upwards. How much work does the bird do against the force of gravity (g=9.8ms-2)? Show full working.

Q4)A elephant drags a load of mass 1500Kg through a distance of 30m in a cobbled street in India. It expends 5000J doing this. Calculate the force exerted by the elephant to do this.