changes in energy (topic 1)

Question 1

• a car driver sees a fallen tree lying across the road ahead and makes an emergency stop
• the braking distance of the car depends on the speed of the car
• for the same braking force, explain what happens to the braking distance if the speed doubles
• you should refer to kinetic energy in your answer (4)

Answer 1

• doubling speed increases kinetic energy
• kinetic energy increases by a factor of 4
• work done by brakes to stop the car increases
• work done increases by a factor of 4
• work done is force x distance and braking force is constant
• so if work done increases by 4 then braking distance must increase by 4

Question 2

Answer 2

• energy required increases with load
• increases slowly at first up to 4 newtons the increases rapidly

Question 3

if a ball has a gpe of 0.51j, what is its kinetic energy just before it hits the ground

Answer 3

0.51j

Question 4

describe how a student could measure the final speed of a ball, just before it hits the ground (4)

Answer 4

• 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 5

using ideas about forces, explain why the measured value of speed is different to the calculated value when a ball is dropped (4)

Answer 5

• 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
• as the calculated value does not account for air resistance

Question 6

gfs is the same thing as

Answer 6

acceleration

Question 7

• when the 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 (4)

Answer 7

• (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 8

state the law of conservation of energy

Answer 8

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

Question 9

• as the parachutist falls, both the gravitational potential energy store and the kinetic store of the parachutist decrease, energy is still conserved
• explain how energy is still conserved

Answer 9

• as the parachutist falls, there is friction between the air and the parachutist and the parachute
• energy is transferred to the thermal energy store of the parachute and parachutist

Question 10

how would you calculate the average frictional force acting on something

Answer 10

work done = force x distance

Question 11

• the gravitational potential energy and the kinetic energy of the roller coaster are both now zero
• explain with reference to the conservation of energy how this is possible

Answer 11

• when the brakes are applied work is done on the system
• the kinetic energy of the rollercoaster is transferred to the thermal store of the brakes

Question 12

• a cyclist is cycling along a horizontal road at constant speed.
• state the name of the energy store of the cyclist that is decreasing as they cycle

Answer 12

chemical energy store

Question 13

• the cyclist sees an obstacle in the road and needs to stop quickly
• describe the energy transfer that takes place between the brakes being applied and the bicycle coming to rest

Answer 13

• kinetic energy of the bike decreases
• energy is transferred to the thermal store of the brakes

Question 14

• describe the energy transfer that takes place between A and B
• you may assume no energy is lost to the surroundings

Answer 14

• gravitational potential energy decreases
• kinetic store increases
• gravitational potential energy is transferred mechanically to the kinetic store of the cart

Question 15

how would you calculate the kinetic energy of the roller coaster at B

Answer 15

kinetic energy at B = gravitational potential energy at A - gravitational potential energy at B

Question 16

explain why the roller coaster is travelling at a lower speed than that calculated

Answer 16

• not all of the gravitational potential energy at A is transferred to the kinetic store of the cart
• energy is transferred to the thermal store of the surroundings OR energy is lost as heat due to friction between the cart and the track

Question 17

Answer 17

0.82m

Question 18

explain why the work done on the box is greater than the increase in GPE store of the block

Answer 18

• (the block needs to overcome) friction
• energy is transferred to the surroundings (by mechanical work)

Question 19

Answer 19

work done = initial kinetic energy - final kinetic energy
11,250 - 1,800 = 9,450 J

Question 20

describe the energy transfers that take place as the brakes slow the bicycle from 15 m/s to 6m/s

Answer 20

• kinetic energy store of bicycle decreases
• thermal store of brakes/surroundings increases

Question 21

Answer 21

315N

Question 22

Answer 22

122.5W (convert 49% to 0.49)