Topic 4 - Momentum, Energy, Work and Power

Question 1

What is stopping distance?

Answer 1

The distance it takes to stop a car, made up from thinking distance + braking distance

Question 2

What is thinking distance?

Answer 2

The distance the car travels in the time between the driver noticing the hazard + applying the brakes

Question 3

What factors affect thinking distance?

Answer 3

• Reaction time: affected by tiredness, drugs, alcohol, age

- Fast you’re going: faster you go, further thinking distance

Question 4

What is braking distance?

Answer 4

The distance the car travels during its deceleration whilst the brakes are being applied, time between the brakes applied + car stopping

Question 5

What factors affect braking distance?

Answer 5

• Speed: faster = further braking distance
• Mass of vehicle: w/ same brakes, heavy = longer to stop
• Quality of brakes
• Grip: friction between tyres + road surface to stop (depends on road surface, weather, tyres)

Question 6

What experiment could you do to investigate how much frictional force different surfaces provide?

Answer 6

• Add masses 1 by 1 to mass holder to provide a force on block until eventually slides
• Amount of force (amount of mass) needed to slide block depends on amount of friction between block + surface (like tyres on road surfaces)
• Can use different surfaces
• Smaller frictional force between block + surface = smaller force to make block slide

Question 7

How does a greater mass/greater velocity affect momentum?

Answer 7

More momentum of an object

Question 8

What is a vector?

Answer 8

• Has size

- Has direction

Question 9

Is momentum a vector?

Answer 9

Yes

Question 10

What is the equation for momentum?

Answer 10

Momentum = mass x velocity

Question 11

What is momentum measured in?

Answer 11

kg m/s

Question 12

What is the relationship between momentum before and momentum when no external forces act?

Answer 12

• Equal

- Momentum is conserved

Question 13

What is the equation relating force, chance in momentum and time?

Answer 13

Force = change in momentum / time

  mv-mu F=       t

Question 14

What is force measured in?

Answer 14

Newtons

Question 15

Why are protective features put into cars?

Answer 15

• Slow people over a longer time when they crash

- Take a longer time to change momentum, decreasing force

Question 16

What protective features are out into cars to increase stopping distance and how does this make the car safer?

Answer 16

• Crumple zones: crumple on impact, increase time taken for car to stop
• Seat belt: stretch so increase time taken for wearer to stop reducing force on chest
• Air bags: slow you down more gradually

Question 17

How can bubble wrap and an egg be used to demonstrate how crumple zones work?

Answer 17

• Dropping egg on hard floor means they shatter
• Drop onto cushions in foam or build crumple zone of cardboard + bubble wrap
• Collisions happen over longer time so force on egg is smaller
• Reduces the force on the egg

Question 18

What causes work to be done?

Answer 18

When a force moves an object, energy is transferred and work is done.

Question 19

What is work done measured in?

Answer 19

Joules, J

Question 20

What is the formula for work done?

Answer 20

Work done = force x distance moved in the direction of the force

Question 21

What is the formula linking power, work done and time taken?

Answer 21

Power = work done / time taken

Question 22

What units can be used to measure power?

Answer 22

Watts, W or J/s (joules of energy per second, 1 W = 1 joule of energy per second)

Question 23

What is the equation for kinetic energy?

Answer 23

Kinetic energy = 0.5 x mass x velocity^2

Question 24

Initial kinetic energy = work done by brakes to stop vehicle. What is this in symbols and what happens to work done if you double your speed?

Answer 24

• 0.5 x m x v^2 (velocity) = F (maximum braking force) x d (braking distance)
• Double speed = 2xv, and as v^2 KE increases by factor 4
• F is always maximum possible braking force so d must be increased by factor 4 to balance
• So double speed = 4 x braking distance

Question 25

What is all energy measured in?

Answer 25

Joules

Question 26

What is the equation for gravitational potential energy?

Answer 26

GPE = mass x g (gravitational field strength, 10 N/kg) x height

Question 27

What is the gravitational field strength on earth?

Answer 27

10 N/kg

Question 28

What is the principle of conservation of energy?

Answer 28

Energy can never be created nor destroyed, only transferred from one form to another

Question 29

What is a principle of energy being useful?

Answer 29

Energy is only useful when it’s transferred from one form to another.

Question 30

How does energy transfer in a falling object?

Answer 30

• GPE converted to KE (principle of conservation of energy)
• Some GPE likely dissipated as heat from air resistance
• Kinetic energy gained = Gravitation potential energy lost

Question 31

How is energy lost when it is transferred?

Answer 31

• Energy lost to heat/sound
• Heat lost to cooler surroundings, dissipates as less concentrated
• Due to Principle of Conservation of Energy, total energy stays the same so energy is still there but can’t be used or collected