P3 - Forces for Transport

Question 1

What is the gradient of a distance-time graph represent?

Answer 1

Speed

Question 2

What three different changes may cause acceleration?

Answer 2

• Change in speed - Change in direction - Change in speed and direction

Question 3

What is the equation for acceleration?

Answer 3

acceleration = change in speed / time

Question 4

Define thinking distance.

Answer 4

The distance the car travels in the time between the driver noticing the hazard and applying the breaks.

Question 5

Define braking distance.

Answer 5

The distance taken to stop once the brakes have been applied.

Question 6

List factors that will increase thinking distance.

Answer 6

• Driver tiredness - Alcohol/drugs - Greater speed - Distractions/lack of concentration

Question 7

List factors that will increase braking distance.

Answer 7

• Speed of car - Friction between vehicle and road: Road surface, weather conditions, tyres - How heavy the vehicle is. - How effective the brakes are.

Question 8

Why does thinking distance increase linearly as speed increases?

Answer 8

The thinking ‘time’ remains roughly the same regardless of changes in speed. Therefore, increasing the speed of the vehicle simply means the vehicle travels further in that same thinking time.

Question 9

Why does braking distance increase as a squared relationship with speed?

Answer 9

KE = 1/2 * mass * velocity squared Therefore, if you increase the speed by a factor of two (double it), the KE will quadruple. As a result, four times as much as energy is needed to stop the car (must be applied by breaks).

Question 10

What is power?

Answer 10

Power = work done / time - measured in watts - a measure of the rate of doing work

Question 11

Explain factors which affect fuel consumption.

Answer 11

• Chemical energy from fuel needs to be converted into kinetic energy. - KE = 1/2 * mass * velocity squared - Therefore, heavier vehicles consume more fuel. - And the higher the speed, the more fuel consumed. - Also, the energy required to do work against friction will affect fuel consumption. If more energy needed, more fuel consumed. - Acceleration uses up more energy, therefore frequent breaking/acceleration will increase fuel consumption.

Question 12

Explain why biofuelled cars can be considered more sustainable:

Answer 12

• Burning biofuels producing pollution. - But we have overall net reduction in pollution because plants grown to make biofuel do not produce CO2, instead they take in CO2. - However, some pollution created in production of vehicles. - BUT, overall reduction in CO2 is the most likely outcome.

Question 13

Describe arguments for and against the use of battery powered cars.

Answer 13

Against: - Limited performance compared to combustion cars. - Electric cars expensive to make and buy. - Frequent charging means shorter distances can be traveled in one go. For: - They do not release any pollution at the point of use. - Electricity can be generated from renewable sources (e.g. solar). - Technology is improving.

Question 14

Why do seat belts need to be replaced after a crash?

Answer 14

Seat belts need to be replaced after a crash because they will stretch as a result of the large force absorbed. This makes them less effective at absorbing large forces in the future.

Question 15

What is momentum?

Answer 15

Momentum (kg m/s) = Mass (kg) * Velocity (m/s) - Measured in kg m/s - Has both size and direction (vector)

Question 16

Use Newton’s second law of motion to explain the equation: Force = Change in Momentum / Time

Answer 16

• Force = mass * acceleration - Therefore, any force applied to object increases acceleration. - And acceleration is just change in velocity over time. - A change in momentum can be caused by a change in velocity, so any force applied to an object causes a change in momentum.

Question 17

Describe how different safety features help reduce forces.

Answer 17

• Crumple zones crumple and change shape on impact, increasing the time taken for the car to stop. - Seat belts stretch slightly, increasing the time taken for the wearer to stop. This reduces the forces acted on the chest. - Air bags also slow you down more gradually, reducing the forces.

Question 18

Why does increasing the stopping or collision time reduce the risk of injury to the driver/passengers?

Answer 18

• If the vehicle slows down over a long period of time, its change in momentum will be spread over a large time distance. As a result, the overall force will be lower. Furthermore, many injuries can be caused by rapid deceleration of the body - if you increase the collision time, acceleration reduces (a = v/t).

Question 19

Describe how test data may be gathered and used to identify and develop safety features for cars.

Answer 19

• Test/crash dummies used. - Different cars/speeds/dummies used to test effectiveness of safety features. - Slow motion film footage to analyse tests.

Question 20

Describe the functions of ABS brakes:

Answer 20

• Make it possible to keep control of the steering of a vehicle in hazardous situations (e.g. when braking hard or going into a skid) - Work by the brakes automatically pumping on and off to avoid skidding - They can sometimes reduce the braking distance, preventing a collision with the car in front.

Question 21

Explain, in terms of balance of forces, why objects reach a terminal speed:

Answer 21

• As speed increases, the force of drag (air resistance) increases. - The larger the area of the object, the greater the drag. - Eventually, the forces balance out, so that weight of falling (accelerating force) is equal to the resistance force (drag).

Question 22

What can and can’t affect gravitational field strength?

Answer 22

• Unaffected by atmospheric changes. - Varies slightly at different points on Earth. - Force slightly different depending on height/altitude.

Question 23

What is the gravitational field strength on Earth?

Answer 23

10 N/kg

Question 24

What is the equation for weight?

Answer 24

Weight = 10N/kg * mass (kg)

Question 25

Describe what happens to kinetic energy and gravitational potential energy when a body is falling through the atmosphere at terminal velocity.

Answer 25

• KE does not increase (any increases in KE are made null by equal increases in air resistance). - Instead, GPE is transferred into increased thermal energy of surrounding air particles via friction.

Question 26

What is the equation for GPE?

Answer 26

GPE (J) = height (mass) * 10N/kg * mass (kg)

Question 27

Describe the effect of changing mass and speed on kinetic energy (KE):

Answer 27

• If you double the mass, the kinetic energy also doubles. - BUT, if you double the velocity, the kinetic energy quadruples.

Question 28

What equation would be used to find out how high an object must be dropped from to reach a specific speed?

Answer 28

h = v²/ 2g

This equation derives from the fact that:

mgh = 1/2 * m * v²

Therefore, gh = 1/2 * v2

So, h = 1/2 * v²/ g

or

h = v²/ 2g