Physics - Energy

Question 1

The mass of the ball is 40 g and is held 1.3 m above the ground.

Calculate the gravitational potential energy of the ball.

The gravitational field strength on Earth is 9.8 N/kg.

Answer 1

40 g = 0.04 kg
GPE = 0.04 x 9.8 x 1.3 = 0.51 J

Question 2

What is the formula for gpe? and what are the units?

Answer 2

gpe(J) = mass(kg) x height(m) x gravitational field strength(N/kg)

Question 3

what is the formula for kinetic energy?

Answer 3

kinetic energy = 0.5 x mass x speed squared

Question 4

what is work done?

Answer 4

energy transferred

Question 5

what is a system?

Answer 5

a group of objects

Question 6

what is a closed system

Answer 6

a closed system is where there are no energy transfers between the system and its surroundings. So the total energy of the system is constant, and not lost or gained.

Question 7

Describe how the student could measure the final speed of a ball, just before it hits the ground.

Answer 7

measure the diameter of the ball using a ruler

drop the ball through a light gate and record the time the ball interrupted the gate
repeat and calculate the average time

speed = distance ÷ time

Question 8

Using ideas about forces, explain why the measured value of speed of that falling ball is different to the calculated value.

Answer 8

as the ball accelerates, the air resistance acting on the ball increases

this reduces the resultant force acting on the ball

a lower downward resultant force means a lower acceleration

the measured value of speed will be lower than the calculated value

the calculated value does not account for air resistance

Question 9

What are the five energy stores?

Answer 9

elastic, thermal, kinetic, gravitational potential, chemical

Question 10

what are the four ways of energy transfer?

Answer 10

heat, waves, mechanical, electrical

Question 11

When a parachutist is closer to the ground, they open their parachute.

Explain how the speed of the parachutist changes between opening the parachute and moments before hitting the ground.

Use ideas about forces in your answer.

Answer 11

(when the parachute opens) the air resistance acting on the parachutist increases significantly
there is now an upwards resultant force on the parachutist causing them to decelerate
as the parachutist slows down, air resistance decreases
when the air resistance and weight are equal, the parachutist will travel at constant velocity

Question 12

State the law of conservation of energy.

Answer 12

energy cannot be created or destroyed
only transferred from one form to another

Question 13

what is the formula for weight?

Answer 13

weight = mass x gravititational field strength

Question 14

What are the units for weight?

Answer 14

Newtons

Question 15

A car on earth experiences a downwards weight of 8820 N, what is the mass of the car?

(The Earth’s gravitational field strength is 9.8 N/kg)

Answer 15

8820N / 9.8 = 900kg

Question 16

Every object in the universe exerts a force of gravitational attraction on every other object in the universe.

Which two factors determine the strength of the gravitational force of attraction?

Answer 16

The distance between the two objects and the mass of the objects.

Question 17

what is the formula for elastic potential energy? similar to kinetic energy

Answer 17

0.5 x spring constant(N/m) x extension squared(m)

Question 18

where is wasted energy transferrred to?

Answer 18

surroundings mainly

Question 19

A ball of mass 0.2 kg is thrown upwards with a speed of 2 m/s.

Calculate the maximum height reached by the ball.

Answer 19

Calculate initial kinetic energy of ball:

0.5mv2 = 0.4J
Calculate maximum height reached:
maximum gravitational potential energy = initial kinetic energy

GPE = mass x gravitational field strength x height OR GPE = m x g x h

0.4 = 0.2 x 10 x h
answer: 0.2m

Question 20

What is the formula for work done?

Answer 20

work done = force x distance travelled

Question 21

what is the formula for the change in gpe?

Answer 21

change in gpe = weight(N) x change of height(m)

Question 22

When Brian releases the bow, an arrow of mass 50 g is projected
horizontally towards the target.
Show that the arrow leaves the bow at a speed of 20 m/s.
10J

Answer 22

remember kg for kinetic energy

Question 23

EVERYTHING IS KG

Answer 23

YESSIR

Question 24

Explain why the speed at which the arrow struck the target was lower
than the speed at which it left the bow.

Answer 24

Because some of the kinetic energy of the arrow was transferred [1]
to the thermal energy store of the surrounding air [1]

Question 25

Late for an appointment, Gillian is running up a set of eight steps. As shown in the below diagram, each of the steps is 15 cm high. Gillian has a mass of 60 kg. Calculate the increase in her gravitational potential energy store between the bottom and the top of the set of steps. Take g = 9.8 N/kg Whilst running up the steps, she developed an average power of 250 W. How long did it take her to get from the bottom to the top of the steps?

Answer 25

Total change in height, h = 8 × 0.15 = 1.2 m Increase in GPE = m g h = 60 × 9.8 × 1.2 = 705.6 J E = P t t = E ÷ P = 705.6 ÷ 250 = 2.8 s (1 d.p.)

Question 26

why would a bungee jumper not return to their original height?

Answer 26

some energy would have been lost to the thermal energy store of the surroundings

Question 27

work ?

Answer 27

work = force x distance

Question 28

what are the energy transfers taking place when a car brakes?

Answer 28

the force of friction does work on the brake pad and discs, which causes energy to transfer from the kinetic energy store to the thermal energy store, and then some energy is transferred by heating to the air's thermal energy store

Question 29

What store do the surroundings have?

Answer 29

thermal energy store

Question 30

what in machines always causes energy to be wasted?

Answer 30

friction

Question 31

what would happen in terms of energy transfer and energy stores to a gear box that was insulated so it could not transfer by heating to the surroundings

Answer 31

n a gearbox that is insulated and cannot transfer energy by heating to the surroundings, the primary energy transfer and changes occur within the mechanical system of the gearbox. As the gears rotate and interact, there is a transfer of mechanical energy between the gears. This mechanical energy may involve changes in the rotational speed and torque of the gears. Since the gearbox is insulated and cannot transfer energy by heating, there is no conversion of mechanical energy into thermal energy.

Question 32

how can machines be made to be more efficient?

Answer 32

since the cause of machines losing energy is due to friction, we can add lubrication to reduce friction between moving parts

Question 33

Why would the work done be higher than the calculated value ?

Answer 33

air resistance, friction, etc.

Question 34

EVERYTHINGS IN METRES

Answer 34

YESSIR