P10 Force and motion

Question 1

How to calculate the resultant force on an object from its acceleration and its mass

Answer 1

resultant force = mass x acceleration
resultant force (N), mass (kg), acceleration (m/s²)

Question 2

Newton’s 2nd Law

Answer 2

States that the acceleration of an object is:
- proportional to the resultant force on the object
- inversely proportional to the mass of the object

Question 3

Inertia

Answer 3

The tendency of an object to remain in its same state of uniform motion or rest

Question 4

Inertial mass

Answer 4

• A measure of how hard it is to change an object’s velocity.
• It is defined as the ratio of force over acceleration.
• Force / acceleration

Question 5

When would the velocity of an object change?

Answer 5

If the velocity of an object changes, it must be acted on by a resultant force. Its acceleration is always in the same direction as the resultant force.

Question 6

When would an object accelerate?

Answer 6

• The velocity of the object increases (it accelerates) if the resultant force is in the same direction as the velocity
• Its acceleration is positive because it is in the same direction as its velocity

Question 7

When would an object decelerate?

Answer 7

• The velocity of the object decreases (it decelerates) if the resultant force is in the opposite direction to its velocity
• Its acceleration is negative because it is in the opposite direction to its velocity

Question 8

How to calculate the force due to gravity

Answer 8

weight = mass x gravitational field strength
weight (N), mass (kg), gfs (N/kg)

Question 9

What is the weight of an object?

Answer 9

The force acting on the object due to gravity

Question 10

What is the mass of an object?

Answer 10

The quantity of matter in the object

Question 11

What does the stopping distance of a vehicle depend on?

Answer 11

The thinking distance and the braking distance

Question 12

How to calculate the stopping distance of a vehicle

Answer 12

stopping distance = thinking distance + braking distance

Question 13

Thinking distance

Answer 13

• The distance travelled by the vehicle in the time it takes the driver to react
• Because the car moves at constant speed during the reaction time, the thinking distance is equal to the speed x reaction time
• Thinking distance is proportional to the speed

Question 14

Braking distance

Answer 14

The distance travelled by the vehicle during the time the braking force acts

Question 15

Factors affecting stopping distances

Answer 15

• Tiredness, alcohol, and drugs affect the brain and increase reaction times. Distractions like using a mobile phone also increase reaction time and cause serious accidents. All these factors increase the thinking distance, so the stopping distance is greater
• The faster the vehicle is travelling, the further it travels before it stops. This is because the thinking distance and the braking distance both increase with increased speed
• In adverse road conditions, for example on wet or icy roads, drivers have to brake with less force to avoid skidding. So stopping distances are greater in poor weather and road conditions
• Poorly maintained vehicles, for example with worn brakes or tyres, take longer to stop because the brakes and tyres are less effective

Question 16

What are the forces that oppose the driving force of a vehicle?

Answer 16

Friction and air resistance

Question 17

What does the braking force of a vehicle depend on?

Answer 17

• The speed of the vehicle when the brakes are first applied
• The mass of vehicle

Question 18

How to calculate momentum

Answer 18

momentum = mass x velocity
momentum (kg m/s), mass (kg), velocity (m/s)

Question 19

Law of conversation of momentum

Answer 19

States that in a closed system, the total momentum before an event is equal to the total momentum after the event

Question 20

Which equation links extension, force, and spring constant

Answer 20

force = extension x spring constant
force (N), extension (m), spring constant (N/m)

Question 21

What is meant when an object is called elastic?

Answer 21

An object is elastic if it returns to its original shape when the forces deforming it are removed

Question 22

How to measure the extension of an object when it is stretched

Answer 22

extension = length of the object - original length

Question 23

Hooke’s Law

Answer 23

The extension of a spring is directly proportional to the force applied, as long as its limit of proportionality is not exceeded

Question 24

Explain what is meant by ‘elastically deformed’

Answer 24

Will return to its original shape when the force is removed

Question 25

How do seatbelts reduce the force on people in car accidents?

Answer 25

• Seatbelts are elastic and are designed to stretch a bit in a collision. This increases the time taken for the body’s momentum to reach zero, and so reduces the forces on it
• The seatbelt acts across the chest, so it spreads the force out. Without the seatbelt, the force would act on the person’s head when it hit the windscreen

Question 26

How do airbags reduce the force on people in car accidents?

Answer 26

• Air bags, in a car are designed to protect the driver and the front passenger. In a car crash, an inflated air bag spreads the force of an impact across the upper part of a person’s body.
• It also increases impact time, increases the time taken for the head’s momentum to reach zero. So the effect of the force is lessened compared with just using a seatbelt
• They also act as a soft cushion and percent cuts