CH 8: Energy

Last chapter: How long a force acts makes a

difference in an object’s motion. F x t = Impulse

This chapter: How long (how far) a force acts, which

is known as work.

1. Which takes more work lifting 1 book from the floor to your waist or lifting 10 books from the floor to your waist?

Why?

2. Which takes more work, lifting 1 book from the floor to your waist or lifting 1 book from the floor to over your head?

Why?

3. Which requires more work holding 100 pounds stationary over your head or holding 300 pounds stationary over your head?

Why?

Work A. product of a force acting in the

direction of motion and the distance moved.

B. Work = force x distance W = F x d

C. Units for work Joules (J) = N m

Practice problem: How much work is done on a 50 N

skater that is lifted by her partner 1.5 m into the air?

Another Problem:

Who does more work, a person who lifts 100 pounds 2 m over their head very quickly or a person who struggles and takes a long time to lift the 100 pounds 2 m over their head?

Why?

Power

A. rate at which work is done B. Power = work done

time interval

P = W/ t C. Units for power

watts (w) = joules / t horsepower (hp) 1 hp = 747.7 w

Practice problem: How much power is required to lift

Bertha, (who weighs 70 N) 20 m in an elevator in 6 s?

Energy A. ability to do work

B. Units = joules

C. Mechanical energy

1. Potential Energy a. stored energy b. energy due to position c. examples

rubber band spring fuel - gas, batteries, food a book held high

1. Potential Energy d. potential energy is obtained by work

being done. e. PE = mgh

2. Kinetic energy a. energy of motion b. energy due to movement c. depends on mass and speed d. KE = 1/2 mv2

Conservation of Energy Energy can not be created or

destroyed, only transformed from one form to another.

Total amount of energy never changes.

Conservation of Energy 3. Energy transforms from one form to

another

Sunlight Heat Chemical Mechanical Electrical

Plants Warm air rises Solar cells

IV. Machines A. device used to multiply forces or

change the direction of forces B. basic idea is same as conservation

of energy C. work in = work out

work in = work out

(F . d)in = (F . d)out

F . d = f . d 10 N x 1 m = 1 N x 10 m

D. lever

F . d = f . d

E. Mechanical Advantage MA = output force

input force pg. 112 80 N = 8

10 N MA = input distance

output distance

F. Pulley (bonus question on the test)

G. Efficiency 1. ideal machines have 100%

efficiency Does not happen in the real world

(some energy is ALWAYS lost as heat)

G. EfficiencyWork (out)

Eff = Work (in) X 100%

OR useful work output X 100%

total work input

AMA Eff = TMA X 100%