Work & Power
Work is:
Power is:
They say: Work is the product of force and distance.
They say: power refers to the rate at which work is done.
A machine's efficiency is calculated by:
A machine's efficiency is the ratio of the work that is output compared to the input work and is usually given as a percentage.
MA of a pulley
MA of a pulley is equal to the number of supporting ropes in the pulley
Equation of work: Equation of power
SI units:
w = fxd, p = w/t
w: J. p: W
f: N. t: s
d: m
They said: A machine is 70.0% efficient. If the machine requires 15.5 J of work input to run, what is the work output?
Workoutput = e*workinput/100 = 70*15.5 /100 = 10.85 J
How do you find the MA of an incline plane?
What is the MA of an inclined plane with a 7.0-meter length that has a height of 2.0 meters?
MA = input distance/output distance
input distance: the length of the inclined plane
output distance: the change in height from the bottom of the inclined plane to the top
MA = input distance / output distance = 7/2 = 3.5
They said: Kwan is trying to move a couch to vacuum behind it, but it is too heavy for him to move. If he is applying 4.5 N to the couch, how much work is being done?
You lift a book off of your desk, looking for your cell phone. If you lift the book 0.5 m at a force of 1.5 N, how much work is being done?
W = F*d = 4.5 * 0 = 0 j
W = f*d = 1.5 * 0.5 = 0.75 J
List the 6 simple machines
Lever, W&A, Pulley, Screw, Wedge, Incline Plane
MA of a wedge
If the mechanical advantage of a wedge is 5, and its thickness is 0.2 m, what is its length?
MA = length/thickness
They said: Grammy is working out at her local senior center. Calculate the power she is using to lift that pink barbell. She has been working out for a while now, so she is getting a little tired. She is now using 25 N of force to lift the barbell 0.5 m straight up in the air. How much power is using if she can lift the barbell in 2.0 seconds?
w=f*d = 25*0.5 = 12.5 j
p=w/t = 12.5/2 = 6.25 W
Talk about the order of levers and the MA of a lever
First order Lever: Input, Fulcrum, Output
Second order: Fulcrum, Output, Input
Third order: Fulcrum, Input, Output
MA = input arm/output arm.
Input arm = distance from input to fulcrum
Output arm = distance from fulcrum to output
They said: Work is being done at a rate of 36.8 J every 22.0 seconds. How much power is being used?
Wayne and Garth are moving a piano. Wayne is pushing the piano 25 N to the right, and Garth is pulling the piano 20 N, also to the right. They continue to apply this magnitude of force as they move the piano 20 meters in 45 seconds. Calculate the power.
p=w/t = 36.8/22 = 1.67 W
Fnet = 25+20 = 45 N —-> w = f*d = 45 * 20 = 900 j
p=w/t = 900/45 =20 W
They said A wheelbarrow has to lift dirt off the ground in order to transport it, the length of the lever is is 9.825m, the length from the fulcrum to the load is is 9.001m. Look for the value of mechanical advantage.
MA = input arm/ output arm = 9.825/9.001 = 1.09