Topic 5d - Momentum and Car Safety

Question 1

What is momentum?

Answer 1

Momentum = mass x velocity

Question 2

What is the equation for momentum?

Answer 2

p = mv

Question 3

What is the unit for momentum?

Answer 3

kg m/s
OR
Ns

Question 4

What kind of quantity is momentum?

Answer 4

A vector quantity => direction is important. (usually left and right direction).

Question 5

What is impulse?

Answer 5

The change in momentum of an object.

Question 6

What is the unit for impulse / change in momentum?

Answer 6

kg m/s
OR
Ns

Question 7

What is the equation for impulse?

Answer 7

change in p = m (v-u)

Question 8

What must you ensure to do when calculating an impulse problem?

Answer 8

State the directions you are choosing to use (which way you put the + and -)

Question 9

What is a force in relation to momentum?

Answer 9

A force is the change in momentum per unit time/ the rate of change of momentum.

Question 10

What is the equation for Force in relation to momentum?

Answer 10

F = change in p / t
OR
change in P = F t

Question 11

What kind of force is the force used in F = change in p / t?

Answer 11

An average force.

Question 12

What is the principle of conservation of momentum?

Answer 12

Total final momentum of the system (after) = Total initial momentum of the system (before)

Provided there are no external forces acting.

Question 13

In what 2 situations is the principle of conservation of momentum applicable?

Answer 13

• Collisions (when objects join)
• Explosions (when objects move apart)

Question 14

How can propulsion be explained using the example of a balloon? (4)

Answer 14

• Air filled balloon = before. The total momentum of the air + balloon = 0.
• When clamp released, air is ejected at high speed from the neck of the balloon.
• Air at high speed has momentum to the left, so the balloon has the same momentum to the right.
• Releasing the air makes the balloon move at speed in opposite direction to the air due to the principle of conservation of momentum.

Question 15

What is true about all collisions?

Answer 15

They conserve momentum.

Question 16

What do the types of collisions depend on?

Answer 16

The total kinetic energy.

Question 17

What does it mean if a collision is elastic?

Answer 17

Total kinetic energy of the system before = Total kinetic energy of the system after.

Question 18

What does it mean if a collision is inelastic?

Answer 18

Total KE before =/ (NOT) Total KE after

Question 19

What happens to objects in inelastic collisions?

Answer 19

One or more of the objects get deformed.

Question 20

What is the definition of stopping distance?

Answer 20

The total distance travelled during the time it takes for a car to stop in response to some emergency.

Question 21

What is the equation for stopping distance?

Answer 21

Stopping distance = Thinking distance + Braking distance

Question 22

What is an emergency stop?

Answer 22

Where maximum force is applied by the brakes to stop car in shortest possible distance.

Question 23

What is thinking distance?

Answer 23

The distance travelled by a car from when the driver realises they need to break, to when they apply the brakes.

Question 24

What is braking distance?

Answer 24

The distance travelled under the braking force.

Question 25

How would you describe the relation / proportionality between braking force, speed and stopping distance?

Answer 25

For a given braking force, the greater the speed of the vehicle, the greater the stopping distance.

Question 26

What is the definition of reaction time?

Answer 26

A measure of how much time passes between seeing something and reacting to it.

Question 27

What is the human reaction time for an alert person?

Answer 27

Usually in range of 0.2 - 0.9 seconds

Question 28

What are the 5 steps to the reaction time test with a ruler?

Answer 28

1. Person A holds a 30cm ruler vertically, such that the bottom end of the ruler hovers over the top of the hand of Person B. 2. Person A should release the ruler unexpectedly. 3. As soon as Person B sees the ruler move, they should close their hand, catching it. 4. The ruler is marked at the point which it was caught by Person B => in line with top of hand. 5. This gives a measurement of the distance the ruler fell => greater distance, longer reaction time.

Question 29

What is the equation for reaction distance?

Answer 29

Reaction distance = Speed of the car x Driver's reaction time

Question 30

What 2 things affect the thinking distance?

Answer 30

* Speed * Reaction time

Question 31

What 5 factors is braking distance affected by?

Answer 31

* Speed * Condition of brakes * Condition of tyres => minimum tread of depth 1.6 mm * How good the grip is * Load of the vehicle (mass/ weight)

Question 32

How do tread patterns help tyres in wet conditions?

Answer 32

In wet conditions tread patterns help to stop water getting trapped between the tyres and the road => provides a channel through which the water can 'escape'.

Question 33

What will happen if a vehicle has too little tread?

Answer 33

Tyres may lose contact with the ground, causing vehicle to slide.

Question 34

What are 4 factors that affect reaction time?

Answer 34

* Distractions * State of mind => intoxication * Tiredness * Age (not best one)

Question 35

How does a car slow down when brakes are applied? (4)

Answer 35

* When brakes applied, there is a frictional force between brakes and wheels of car. * This frictional force does work on brakes => transfers energy from car to brakes. * So KE of car decreases, and thermal energy of brakes increases. * Means car decelerates.

Question 36

What could a large deceleration lead to?

Answer 36

To the brakes overheating and / or loss of control of the vehicle.

Question 37

What is the equation for work done by the brakes?

Answer 37

Braking force x Breaking distance = 1/2 x Mass x Velocity squared