where have we been? unbalanced forces cause acceleration balanced forces = constant velocity we have...

Where have we been?Unbalanced forces cause

accelerationBalanced forces = constant

velocityWe have learned that forces are

vectors too.We can work with forces on

angles.

Work and Power

What your forces do

– and how fast.

What is Work?

only done when a force causes motion in the same direction as the force acting on it.

Force

Motion

It’s Energy transferred by a force

What is Work?

If not moving,

no work is done.

Force

No Motion

It’s Energy transferred by a force

What is Work?

If moving in a direction different from force,

no work is done.

Fo

rce

Motion

It’s Energy transferred by a force

Formula:

W = F x d (cosθ)

Force distanceWork

Angle

So, when does the angle matter?

When 0º, W = Fd (cos 0º)

cos 0 = 1,

so, W = Fd

So, when does the angle matter?

When 90º, W = Fd (cos 90º)

cos 90 = 0,

so, W = 0

Formula:

W = F x d (cosθ)

Force (N)

distance (m)

Work (joules)

Angle (if not 0º, or 90º)

Remember . . .

Work done on an object increases the total energy of the object.

Work done by an object decreases the total energy of the object.

Work is Energy transferred by a force

Work is performed by . . .

Gravity (objects falling)

Friction (opposing motion)

Humans or machines (applying a push or pull)

Net Work isThe sum of work done by

all forces acting on an object

Use appropriate signs

Remember that work by 90º forces is

0J, so don’t waste time solving for it.

Quick quiz!Which of these are examples of work?

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

A rocket accelerates through space. A boy pushes on a large boulder, but the boulder

is too heavy for him to move.

How about this one?

This waiter carries a tray of food across the room. Is the waiter doing work on the tray?

Calculate work

A crane lifts a steel beam with a mass of 1,500 kg.

Calculate how much work is done against gravity if the beam is lifted 50 meters in the air.

735750 JoulesHow many kilo-Joules is this?

Check your understanding!

500 J 433 J 735 J

One more…

Ben carries a 200-N suitcase up three flights of stairs (a height of 10.0 m) and then pushes it with a horizontal force of 50.0 N at a constant speed of 0.5 m/s for a horizontal distance of 35.0 meters. How much work does Ben do on his suitcase during this entire motion?

3750 J

Power

Power is the rate at which work is done.

Power = Work

time

W

P t

Power

Power is the rate at which work is done.

Power = Work

time

W

P t

Power

Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt

Question 1

A woman performed 278 J of work in 40 seconds. Calculate her power.

Question 2

A pulley is used to lift a 620 N box of construction equipment 24 m straight up into the air. The pulley system accomplishes this in 18 seconds. Calculate the power of the pulley system

Question 3

Consider a box that is slid 12 m across the room. Two people apply a force of 19 N each. It takes them a total of 11 seconds to accomplish the feat. What is the combined power of the people?

Question 4

  A certain generator performs 4506.3 kJ of work in 2.3

hours. How much power does the generator have? (Pay close attention to units!)

Question 5

A man pulled a sled at an angle of 32o. The force he exerted was 106 N until he had moved the sled 95 m. It took him 312 s to accomplish this feat. Calculate his power.