12 Work, Power, GPE, KE - 4th Form Flashcards
Calculate the work done on the block.
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Work done = force x distance (in direction of force)
Work done = 15N x 8 m = 120 J
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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?
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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
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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
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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)
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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
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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
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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)
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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
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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
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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
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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
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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%
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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