5 Work and Power Flashcards
Calculate the work done on the block.
Work done = force x distance (in direction of force)
Work done = 15N x 8 m = 120 J
Calculate the GPE gained by a basketball of 1 kg at the height of a basketball net of 3m.
GPE = mgh
GPE = 1 kg x 10 N/kg x 3 m
GPE = 30J
A weight lifter lifted 50 kg dumb bell 2 metres
How would you calculate the work done by the weight lifter?
Weight of dumb bell W=mg
W = 50 kg x 10 N/kg = 500N
Work done = Force x distance
Work done = weight of dumb bell x height lifted
Work done = 500N x 2m = 1000J
How would you calculate the speed of the rollercoaster at the top of the second hill?
roller coaster = 100kg
First hill 65 metres
Second hill 45 metres
GPE at first hill = mgh = 100 x 10 x 65 = 65 000J
GPE at second hill = mgh = 100 x 10 x 45 = 45 000J
GPE lost is 100% transferred to KE = 65 000J-45 000J = 20 000J
KE = 20 000J - use this to find speed
KE = 1/2 mv2
v2 = 2 KE /m
v2 = 2 x 20 000 / 100
v2 =400
v=20 m/s
Define power
Power is the rate of doing work or rate of energy transferred
OR
work done per unit time or energy transferred per unit time
If a light bulbs power is 100W, how much electrical energy is transferred per second?
How much energy would be transferred in 10 seconds?
100 W = 100 joules transferred per second
1000 joules would be transferred in 10 seconds!
If the speed of a car is doubled, what happens to its kinetic store?
It quadruples.
K.E. is proportional to the velocity2
If velocity is doubled, the K.E. is quadrupled
If the height of a bird is doubled, what happens to its G.P.E store?
It also doubles.
G.P.E is proportional to height
What do you need to measure to calculate the power output of a person running up stairs?
Mass of person, height of stairs and time it takes to run up stairs
The person is lifting their own weight
Weight = mass of person x gravity (10 N/kg)
Work = Force x distance (in direction of that force)
Work = weight of person x height of stairs
Power = work / time to run up the stairs
Divide the work done by the time it takes the person to run up the stairs
How would you calculate the total work done by the person pushing the block up the ramp?
(Force applied = 45 N and frictional force = 5N)
Work done = force x distance (in direction of the force)
Force = Push force by person up ramp - frictional force
Force = 45N - 5N = 40N
Work done = Force x distance moved up ramp
Work done = 40 N x 12m = 480J
What equation links gravitational potential energy, mass, gravity and height?
G.P.E = mass x gravity x height
G.P.E. = mgh
What is the equation that link kinetic energy, mass and velocity
K.E. = 1/2 mass x (velocity)2
K.E. = 1/2 mv2
What is the relationship between GPE at the top of a slide and KE at the bottom of the slide?
If 100% energy is transferred then…
GPE=KE
If a child has 1000 J of gravitational potential energy at the top of a slide, how much kinetic energy do they have at the bottom of the slide?
GPE is converted to KE mechanically via gravitational pull
KE at bottom of slide = 1000J
(In reality some energy is lost to the surroundings)
What is the equation that links work, force and distance?
Work = force x distance (in direction of force)
W = Fxd
If an object is dropped from the leaning tower of Pisa, how much initial GPE does an object have 3/4 of the way down?
What percentage has converted to KE?
25% of the GPE is still stored in the object
75% of the energy is transferred to KE
If work is done in half the time, what happens to the power?
Power is doubled
power is inversely proportional to time
Calculate the rate of energy transferred to the block
Rate of energy transferred = power
Power = work done/ time
Work done = 15N x 8 m = 120 J
Power = 120J / 6 s = 720W
or 720 joules per second
How would you calculate the efficiency of the slope?
Total work done by person pushing block up ramp = 480J
Useful work done on block (GPE of block) = 240J
Efficiency = Useful energy out/ total energy in
Efficiency = useful work done on the block / total work done by the person
Efficiency = GPE gained by block/ total energy put in by person
Eff = (240J / 480 J) x100
Eff = 50%
Calculate the KE of a ball of 5kg travelling at 2m/s.
KE= 1/2 mv2
KE = 1/2 x 5kg x (2m/s)2
KE = 2.5 x 4
KE = 10J
What is the equation that links power, work done and time taken?
Power = work done / time taken
P = W/t
How would you calculate the useful work done on the block at the top of the ramp?
Block gained height (GPE)
Work done = force x distance (in direction of the force)
Force = Weight of block (acting downward)
Weight = 5kg x 10 N/kg = 50N
Work done on block = weight x height gained
Work done = 50N x 6 m = 300J
