Work

Mechanical work

• Applying a force on an object that displaces the object in the direction of the force

W = FΔd• Only when the magnitude of the force is

constant.• Scalar (no direction associated with work)• SI unit: N•m• When force and displacement are in the same

direction!

N•m

• Also called the Joule for James Prescott Joule, who studied work, energy, and heat

• 1 N•m = 1 J

• Work is only done when force and displacement are in the same direction!

• If a force is applied on the object but the object doesn’t move, no work is done

activity

• Go around the room and explain in your notes why each of the situations presented is either an example where work is done or no work is done.

example

Example 2

• A curler applies 15.0 N on a stone. The stone accelerates from rest to 8.00 m/s in 3.50 s. How much mechanical work is done?

Example 2

• A curler applies 15.0 N on a stone. The stone accelerates from rest to 8.00 m/s in 3.50 s. How much mechanical work is done?

What if force and displacement are in different directions?

• When would this happen?

This is the only force that causes the suitcase to move horizontally (same direction as the displacement

Fv does no work on the suitcase, because the suitcase isn’t displaced in the vertical direction!

Prove it!

Example with vacuuming

Positive and negative work

• Most objects have several forces acting on them at one time.

• Total work = sum of work done by all the forces acting on an object

• We assume forces act in the same direction as the displacement, or in the opposite direction of the displacement:

sample

• A shopper pushes a shopping cart on a horizontal surface with a horizontal applied force for 41.0 N for 11.0 m. The cart experiences a force of friction of 35.0 N. Calculate the total mechanical work done on the shopping cart.

sample

• A shopper pushes a shopping cart on a horizontal surface with a horizontal applied force for 41.0 N for 11.0 m. The cart experiences a force of friction of 35.0 N. Calculate the total mechanical work done on the shopping cart.

• Both Fa and Ff do work on the cart

• Fa acts in the same direction as the cart’s displacement, and Ff acts in the opposite direction

We will solve this problem in 3 partsa. Calculate work done by the applied force

b. Calculate the work done by the force of friction

c. Calculate the total, or net, work done on the cart by calculating the sum of Wa and Wf