Work Chapter 8.1

Go to www.physicsclassroom.com/Class/. Go to “Lesson 1” and go over the two

“Work” topics.

Determine whether there is “work” done for the following situations. Explain your

answers.

• A truck driver is trying to push his stalled vehicle. Despite his efforts, the truck was not able to move. Was “work” done by the driver?–No. The truck was not displaced by the

force applied by the driver.

Determine whether there is “work” done for the following situations. Explain your

answers.

• An apple is falling from a tree. Is work being done on the apple?

• Yes. Gravity is doing work on the apple. A force, the apple’s weight, is causing the apple to be displaced downward.

Determine whether there is “work” done for the following situations. Explain your

answers.

• A student is carrying a textbook across the classroom.

• No. The force on the textbook (upward force) is not causing the book’s displacement (which is horizontal).

Determine whether there is “work” done for the following situations. Explain your

answers.

• A hot air balloon rises from the ground.• Yes. The force of the air is causing the balloon

to be displaced upward.

Mathematical definition of work

• W = Fd W = work F = force d = displacement

• W = Fd(cosΘ) Θ = angle between force and displacement

Give the different scenarios for the following:

• What does it mean when the angle “theta (Θ) in the work equation is 0o?– The force on the object and the object’s

displacement are in the same direction. Positive work is done.

Give the different scenarios for the following:

• What does it mean when the angle “theta (Θ) in the work equation is 180o?– The force on the object and the object’s

displacement are in opposite directions. Negative work is done.

Give the different scenarios for the following:

• What does it mean when the angle “theta (Θ) in the work equation is 90o?• The force on the object and the object’s

displacement are in right angles from each other. The force does not cause the displacement. No work is done.

You are pulling a sled across the snow (it is a level surface). You are pulling the sled by a rope at an angle of 35 degrees with a force of 300 N. Does this mean that 300 N is the force causing

the sled to move? Explain.

• No. Only the horizontal component of the force applied is causing the sled to move in the horizontal direction.

You are pushing a crate up a ramp that is 20o from the horizontal. When using the

work equation, what value will you use for “theta (Θ)”?

• W = Fd(cosΘ), Θ = 20o

What does “negative work” mean?

• This is when a force acts upon a moving object to hinder a displacement.

• The force on the object acts in the opposite direction on the object’s displacement.

What is the unit of measure for work?

• J = Joules • Newton meter = N m∙ ∙

Calculating Work Done by Forces6. Applied force is doing the work. W = Fd = (30

N)(10.0 m) = 300 J7. Friction force is doing the work. It acts in the

opposite direction of the displacement. W = Fd = (-30 N)(10.0 m) = -300 J

8. No force is causing the 5 m displacement. No work is done.

9. The force of pull is causing the work. W = Fd = (20 N)(5 m) = 100 J