WORK, POWER, AND MACHINES

WHAT IS WORK? transfer of energy to a body by application

of a force that causes body to move in direction of force.

W = F dSI units: joules (J). 1 J = 1 N•m = 1 kg•m2/s2 F

Wd

WORKImagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?

GIVEN:

W = ?

F = 190 N

d = 2.0 m

WORK:

W = Fd

W = (190 N) (2.0 m)

W = 380 J

FWd

WORKA crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder?

GIVEN:

W = ?

F = 5200 N

d = 25 m

WORK:

W = Fd

W = (5200 N) (25 m)

W = 130,000 J or

1. 3 x 105 JFWd

WORKAn apple weighing 1 N falls through a distance of 1 m. How much work is done on the apple by the force of gravity?

GIVEN:

W = ?

F = 1 N

d = 1m

WORK:

W = Fd

W = (1 N) (1 m)

W = 1 J

FWd

POWER

rate at which work is done or energy is transformed.

power = work/time

SI Unit: watts (W) watt = 1 J/s

pWt

POWERIt takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process?

GIVEN:

p = ?

W = 1 x 105 J

t = 20 s

WORK:

p = W/t

p = 1 x 105 J / 20 s

p = 5 x 103 W or 5 kW

pWt

POWERWhile rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts?

GIVEN:

p = ?

W = 3960 J

t = 60 s

WORK:

p = W/t

p = 3960 J / 60 s

p = 66.0 W

pWt

POWERUsing a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output?

GIVEN:

p = ?

W = 5350 J

t = 50 s

WORK:

p = W/t

p = 5350 J / 50 s

p = 107 W

pWt

MACHINES

Machines multiply and redirect forces. help people by redistributing work put into

them. change either size or direction of input

force.

ALLOWS SAME AMOUNT OF WORK TO BE DONE BY either decreasing distance while increasing

force or by decreasing force while increasing

distance.

FORCE AND WORK

MECHANICAL ADVANTAGE

tells how much a machine multiplies force or increases distance.

mechanical advantage = output force = input distance input force output

distance

ma

id

od ma

of

if

MECHANICAL ADVANTAGECalculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high.

GIVEN:

ma = ?

id = 5.0 m

od = 1.5 m

WORK:

ma = id/od

ma = 5.0 m / 1.5 m

ma = 3.3

ma

id

od

MECHANICAL ADVANTAGECalculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.

GIVEN:

ma = ?

id = 6.0 m

od = 1.5 m

WORK:

ma = id/od

ma = 6.0 m / 1.5 m

ma = 4

ma

id

od

MECHANICAL ADVANTAGEDetermine the mechanical advantage of an automobile jack that lifts a 9900 N car with an input force of 150 N.

GIVEN:

ma = ?

of = 9900 N

if = 150 N

WORK:

ma = of/if

ma = 9900 N / 150 N

ma = 66

ma

of

if