Work is the product of the force and the distance through which the object moves

W = F x dW = F x d

WORK

In order for work to be done, three things are necessary:•There must be an applied force.•The force must act through a certain distance•The force must have a component along the distance.

Read the following statements and determine whether or not they represent examples of work.

A teacher applies a force to a wall and becomes exhausted.

A book falls off a table and free falls to the ground.

NO, displacement doesn’t occur.

Yes, displacement in the direction of force.

A truck carries a box in it’s bed 100 m.

NO, This is not an example of work. The force is upward on the box but the displacement is along the ground.

Is work being done or not?

• Mowing the lawn• Weight-lifting• Carrying groceries• Moving furniture up a

flight of stairs• Pushing against a

locked door• Swinging a golf club

• YES• YES• NO• YES

• NO

• YES

W = F d

W = Nm

W = (Newtons )(meters)

The units of work are;

W = Joule (J)

(In English units; W = foot poundsW = ft lb)

Calculating Work

All or part of the force must act in the direction of the movement.

Steve lifts a 50 Nbox from the floor of the closet to a shelf 1.6 m high.

How much work does Steve do?

Given:F = 50Nm = 1.6m

Formula: W = F x d

Work: W = 50N x 1.6m

Answer:80 Joules

Matt lifts a 80 kg barbell upward for 1 meter at a constant speed, how much work does he do?

Given:m = 80kgd = 1mg= 9.8m/s2

First: What force must Matt provide ?

Formulas: W = F x d

F = m x g

Answer:784 J

Work: F = 80kg x 9.8m/s2 = 784N

W = 784N x 1m = 784 J

Work is the product of the force and the distance through which the object moves.

W = F x dW = F x d

Remember . . .

The SI unit of work is the Joule.

POWER is the rate at which work in done.

P = P = W W oror

t t

P = P = Fd Fd t t

The SI unit for power is the Watt (W).

1 Watt = 1 Joule/second

Steve lifts a 50 N box from the floor of the closet to a shelf 1.6 m high in 2 seconds.

How much work does Steve do? How much power does he use?

Given:F = 50Nm = 1.6mt = 2sec

Formulas: W = F x d P = W / t

Work: W = 50N x 1.6m = 80 JoulesPower: P = 80J / 2sec = 40 Watts

Matt uses 50 W of power to lift a barbell in 1.5 sec.

Given:P = 50Wt = 1.5sec

How much work does Matt do on the barbell?

Formula: P = W / t

Answer: W = 75 Joules

W = P x t = 50W x 1.5sec

When you run up a flight of stairs, you do the same amount of work as when you walk up the stairs (same Force, same distance).

But when you run up the stairs, you do the work faster, so you use more power.

The work is the same, but the rate of work (power) changes.