# 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 mv^{2}

v^{2} = 2 KE /m

v^{2} = 2 x 20 000 / 100

v^{2} =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 velocity^{2}

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 mv^{2}

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