Chapter 5

Work and Energy

What is work?

• List five examples of things you have done in the last year that you would consider work.

• Based on these examples, how do you define work?

5-1 Work

FdW

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1 N ⋅m = 1 joule J( )

What is work?

In physics, work is the magnitude of the force (F) times the magnitude of the displacement (d) in the same direction as the force.W = Fd

~ A force that causes a displacement of an object does work on that object-A student holds a heavy chair at arm’s length for several

minutes, was there work done?~ Work is done only when components of a force are parallel to a displacement

- A man pushing a crate across a concrete floor~ Sign of work can be positive or negative

Kg*m2/s2

How much work is 1 joule?

Lift an apple weighing about 1 N from the floor to the desk, a distance of about 1 m.

5-1 Work

FdW Pushing this car is work because F and d are in the same direction.Why aren’t the following tasks considered work?

A student holds a heavy chair at arm’s length for several minutes.A student carries a bucket of water along a horizontal path while walking at a constant velocity.

5.1 Work Done by a Constant Force

dFW netnet cos1180cos

090cos

10cos

5.1 Work Done by a Constant Force

Example 1 Pulling a Suitcase-on-Wheels

Find the work done if the force is 45.0-N, the angle is 50.0 degrees, and the displacement is 75.0 m.

J 2170

m 0.750.50cosN 0.45cos

sFW netnet

5.1 Work Done by a Constant Force

W = ( F cos Ө) d = Fd

W = (F cosӨ) d = - Fd

5.1 Work Done by a Constant Force

Example 3 Accelerating a Crate

The truck is accelerating ata rate of +1.50 m/s2. The massof the crate is 120-kg and itdoes not slip. The magnitude ofthe displacement is 65 m.

What is the total work done on the crate by all of the forces acting on it?

5.1 Work Done by a Constant Force

The angle between the displacementand the friction force is 0 degrees.

J102.1m 650cosN180 4W

N180sm5.1kg 120 2 mafs

How would you calculate the work in this case?

What is the component of F in the direction of d?

F cos

If the angle is 90°, what is the component of F in the direction of d?

F cos 90° = 0If the angle is 0°, what is the component of F in the direction of d?

F cos 0° = F

Scalar or Vector

Work can be positive or negative but does not have a direction.What is the angle between F and d in each case?

Work is a Scalar!!!!!

Question #1

If a neighbor pushes a lawnmower four times as far as you do but exerts only half the force, which one of you does more work? By how much?

W=Fd

5.1 Work Done by a Constant Force

Question #1

W=Fd

Neighbor W= (1/2F)4d → 2W

You W = Fd → W

The neighbor, does twice as much work

5.1 Work Done by a Constant Force

Question #2

For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value.

A.The road exerts a friction force on a speeding car skidding to a stop.

NEGATIVE

5.1 Work Done by a Constant Force

Question #2

For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value.

B. A rope exerts a force on a bucket as the bucket is raised up a well

POSITIVE

5.1 Work Done by a Constant Force

Question #3

For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value.

C. Air exerts a force on a parachute as the parachutist slowly falls to Earth.

NEGATIVE

5.1 Work Done by a Constant Force

Question #4

For each of the following cases, indicate whether the work done on the second object in each example will have a positive or a negative value.

D. The Earth exerts a force on a bobsled as it moves down a track.

POSITIVE

5.1 Work Done by a Constant Force

Question #5

A worker pushes a 1.50 X103 N crate with a horizontal force of 345 N a distance of 24.0 m. Assume the coefficient of kinetic friction between the crate and the floor is 0.220.

How much work is done by the worker on the crate?

8.28 X 103 J

5.1 Work Done by a Constant Force

Question #6

A worker pushes a 1.50 X103 N crate with a horizontal force of 345 N a distance of 24.0 m. Assume the coefficient of kinetic friction between the crate and the floor is 0.220.

How much work is done by the floor on the crate?

-7.92 X 103 J

5.1 Work Done by a Constant Force

Question #7

A worker pushes a 1.50 X103 N crate with a horizontal force of 345 N a distance of 24.0 m. Assume the coefficient of kinetic friction between the crate and the floor is 0.220.

What is the net work done on the crate?

3.6X 102 J

5.1 Work Done by a Constant Force