WORKWork = When a force acts upon an object to cause displacement of the object.

3 key ingredients : force, displacement, cause

A force is exerted upon an object to cause the object to be displaced.

WORK?Read the following statements and determine whether or not it is an example of work.• 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 force must cause a displacement in order for work to be done.

WORK?Read the following statements and determine whether or not it is an example of work.• A book free falls to the ground.

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

WORK?Read the following statements and determine whether or not it is an example of work.• A rocket accelerates through space.

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

WORK?A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. (Careful! This is a very difficult question that will be discussed in more detail later.)

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 is moved horizontally across the room). Yet the force does not cause the displacement. To cause a displacement, there must be a component of force in the direction of the displacement.

Play the WORK AnimationAnimation

Work = force x displacementWork can be expressed by the equation, W = f x d.

To do work, forces must CAUSE displacements.The direction of the force CAUSES the direction of the displacement. In the previous example, there is a vertical force (hand holding tray up) and a horizontal displacement (waiter moving across room). Therefore, this is not an example of work.

The Joule (J) is the unit for work.= 1 Newton of force causing a displacement of 1 meter.

1 Joule = 1 Newton * 1 meter

Mechanical Energy= the energy that is possessed by an object due to its motion or due to its stored energy of position.

An object that possesses mechanical energy is able to do work.

Mechanical Energy = Kinetic or Potential

Mechanical energy can be either kinetic (energy of motion) or potential (energy of position).

Potential Energy (PE)= the stored energy of an object as a result of its position or composition.• For example, the heavy ball of a demolition machine is

storing potential energy when it is at an elevated position.• Objects that have potential energy do not use their energy

until they move. That is why it is called “potential.”

2 Types of Potential Energy (PE) 1) Gravitational potential energy = the stored energy of an object as a result of its height and the force of gravity.

2) Elastic potential energy = the stored energy in elastic material as the result of stretching or compressing.

Play the Potential Energy Animation (10)

Animation

Kinetic Energy (KE)= energy of motion (vertical or horizontal motion).• An object that has motion, whether it is vertical or

horizontal motion, has kinetic energy.• The amount of kinetic energy is related to the object’s

mass and to its speed (v).

Play the Kinetic Energy Animation (9)

Animation

Is it Potential or Kinetic Energy?Potential energy (here we see the stored elastic potential energy).

Is it Potential or Kinetic Energy?Potential energy (here we see stored gravitational potential energy).

Is it Potential or Kinetic Energy?Kinetic energy (here we see the energy in motion).

Which of these is an example of WORK?