# 5 Work and Power Flashcards

1
Q

Calculate the work done on the block.

A

Work done = force x distance (in direction of force)

Work done = 15N x 8 m = 120 J

2
Q

Calculate the GPE gained by a basketball of 1 kg at the height of a basketball net of 3m.

A

GPE = mgh

GPE = 1 kg x 10 N/kg x 3 m

GPE = 30J

3
Q

A weight lifter lifted 50 kg dumb bell 2 metres

How would you calculate the work done by the weight lifter?

A

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

4
Q

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

A

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

5
Q

Define power

A

Power is the rate of doing work or rate of energy transferred

OR

work done per unit time or energy transferred per unit time

6
Q

If a light bulbs power is 100W, how much electrical energy is transferred per second?

How much energy would be transferred in 10 seconds?

A

100 W = 100 joules transferred per second

1000 joules would be transferred in 10 seconds!

7
Q

If the speed of a car is doubled, what happens to its kinetic store?

A

K.E. is proportional to the velocity2

If velocity is doubled, the K.E. is quadrupled

8
Q

If the height of a bird is doubled, what happens to its G.P.E store?

A

It also doubles.

G.P.E is proportional to height

9
Q

What do you need to measure to calculate the power output of a person running up stairs?

A

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

10
Q

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)

A

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

11
Q

What equation links gravitational potential energy, mass, gravity and height?

A

G.P.E = mass x gravity x height

G.P.E. = mgh

12
Q

What is the equation that link kinetic energy, mass and velocity

A

K.E. = 1/2 mass x (velocity)2

K.E. = 1/2 mv2

13
Q

What is the relationship between GPE at the top of a slide and KE at the bottom of the slide?

A

If 100% energy is transferred then…

GPE=KE

14
Q

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?

A

GPE is converted to KE mechanically via gravitational pull

KE at bottom of slide = 1000J

(In reality some energy is lost to the surroundings)

15
Q

What is the equation that links work, force and distance?

A

Work = force x distance (in direction of force)

W = Fxd

16
Q

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?

A

25% of the GPE is still stored in the object

75% of the energy is transferred to KE

17
Q

If work is done in half the time, what happens to the power?

A

Power is doubled

power is inversely proportional to time

18
Q

Calculate the rate of energy transferred to the block

A

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

19
Q

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

A

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%

20
Q

Calculate the KE of a ball of 5kg travelling at 2m/s.

A

KE= 1/2 mv2

KE = 1/2 x 5kg x (2m/s)2

KE = 2.5 x 4

KE = 10J

21
Q

What is the equation that links power, work done and time taken?

A

Power = work done / time taken

P = W/t

22
Q

How would you calculate the useful work done on the block at the top of the ramp?

A

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