P10 - Forces and motion

Question 1

State Newton’s second law of motion:

Answer 1

the acceleration of an object is proportional to the resultant force acting on it, and inversely proportional to its mass

F = ma

Question 2

What is inertia?

Answer 2

tendency for an object to stay in a state of rest/uniform motion

uniform motion means a constant velocity

Question 3

What is inertial mass?

Answer 3

-a measure of how difficult it is to accelerate an object

-defined as m = F/a (Newton’s 2nd law of motion)

Question 4

What is terminal velocity? Describe the forces acting on a body at this velocity:

Answer 4

-the maximum velocity an object can reach while in freefall in a fluid
-the weight of the object would be equal to the frictional/drag force on the object (resultant force is 0, no further acceleration)

Question 5

Define stopping distance:

Answer 5

sum of the distance travelled while the driver reacts (reaction distance) and the distance travelled under the braking force (braking distance)

Question 6

What factors can affect a driver’s reaction time?

Answer 6

-tiredness
-drugs/alcohol
-distractions (eg mobile phone, conversations)

Question 7

Give some factors that can affect braking distance:

Answer 7

-wet/icy road/weather conditions or poor vehicle condition (brakes/tires)

-downhill slope
-mass of vehicle
-speed of the vehicle

Question 8

Why might having poor brakes or wet roads increase braking distance?

Answer 8

decreased friction on the road

Question 9

Explain how having a car with more mass increases braking distance:

Answer 9

-car has more kinetic energy
-this means more work needs to be done to stop the car

Question 10

Explain why a driver’s reaction time affects the thinking distance:

Answer 10

distance = speed x time

-increasing the time taken to react will increase the distance travelled whilst the driver reacts to the situation

Question 11

Why does having a steeper downhill slope increase braking distance?

Answer 11

-a larger component of the car’s weight force acts against the braking force
-resultant braking force on the vehicle is reduced
-takes longer to decelerate the car (Newton’s 2nd Law)

Question 12

How do brakes work?

Answer 12

-work is done between the brakes and the tires, due to the added frictional force applied over a certain amount of time
-kinetic energy transferred to thermal energy
-speed decreases, but temperature of brakes increases

Question 13

What is momentum? Give its equation and unit:

Answer 13

the quanitity of motion of a moving body, taking into account both its mass and velocity

p = mv

kg m s⁻¹

ie mass in motion

Question 14

How does inertia differ from momentum?

Answer 14

when an object is not moving, it has no momentum but it still has inertia

Question 15

What is the principle of conservation of momentum?

Answer 15

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

Question 16

What happens to an object’s momentum when an external force is applied to it?

This is not a question about the conservation of momentum

Answer 16

-it changes by a certain amount depending on how long the force is applied for

F = ∆p/t (rearranging gives ∆p = Ft, so if the force is applied over a longer time then the momentum changes more)

Question 17

In an impact, what effect does increasing the time of the impact have?

Answer 17

F = ∆p/t
-impact force is inversely proportional to time

-increasing impact time (time to reduce velocity to 0) means the impact force is reduced

This is why falling on the floor hurts more than falling on a bed - the bed takes longer to reduce your velocity to 0, so impact time is increased, and thus decreases impact force

Question 18

Why do airbags work?

Answer 18

-airbags are compressible, increases impact time
-rate of change of momentum is decreased, so force is also decreased
-force is distributed across a large area of the body
-reduces pressure on person
-reduces the chance of an injury

Seat belts work in the same way as they are slightly stretchy

Question 19

What is an elastic object?

Answer 19

an object that returns to its original shape when the forces deforming it are removed

Question 20

Describe Hooke’s law:

Answer 20

the extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded

F = ke

Question 21

What is the limit of proportionality?

Answer 21

the extension of a spring where going past it means Hooke’s law is no longer followed, meaning the relationship between extension and force is non-linear

Question 22

What is the difference between elastic and inelastic deformation? Give an example for each:

Answer 22

-elastic deformation is when an object returns to its original shape after being stretched/bent/compressed by 2 external forces (eg rubber band)

-inelastic is where the object does not return to its original shape (eg crushing a can)

Question 23

When can an elastic object undergo inelastic deformation?

Answer 23

when it goes past its elastic limit (the furthest point it can be stretched while still being able to return to its original shape)

This could happen when a metal spring is stretched too far and isn’t springy anymore

Question 24

RP6 - How can you determine the spring constant of an elastic object?

Answer 24

-set up the equipment as shown in the diagram (without the masses), using another clamp stand to hold the ruler
-measure the spring’s initial length
-add a mass to the base of the spring, calculate its extension, and repeat for at least 5 added masses
-calculate the mass’s weight force
-plot force vs extension, calculate gradient of line of best fit, which will be the spring constant

Question 25

RP6 - How could you improve the accuracy of measuring the spring’s extension?

Answer 25

-use set square to line up bottom of the spring with ruler

or

-attach horizontal pointer to bottom of spring so the pointer goes across the ruler scale

Question 26

Why might a spring be useful in a mass balance?

Answer 26

-deforms elastically so it can return to its original shape
-compression of spring is directly proportional to the weight force acting on it, so it gives a linear relationship making it easy to use it as a scale

Question 27

RP7 - How would you set up the equipment required to investigate Newton’s 2nd Law?

Answer 27

-put a trolley on rails
-attach a string from the trolley to a pulley and add a mass on the other end to supply a force
-put 2 light gates near the end of the rails (measures Δv, then a)

Question 28

RP7 - How would you change the trolley on rails experiment to investigate how the mass of an object affects its acceleration?

Answer 28

-put a constant mass on the pulley’s end of the string
-pull trolley up to a fixed point on the rails and release it

-take reading of acceleration
-add masses to the trolley and repeat
-plot graph of acceleration vs mass added

Question 29

RP7 - How would you change the trolley on rails experiment to investigate how the force on an object affects its acceleration?

Answer 29

-don’t change the mass of the trolley
-pull trolley up to a fixed point on the rails and release it

-take reading of acceleration
-add masses to the string to increase the force exerted on the trolley, and repeat
-plot graph of acceleration vs force applied

Remember to convert the mass to weight

Question 30

Why do cars have a top speed?

Answer 30

-maximum driving force provided by car motor
-as speed increases, frictional force increases too
-eventually, magnitude of air resistance force = driving force
-no more acceleration as resultant force = 0 (by Newton’s first law of motion)

Question 31

Why does paper fall slower than an object like a coin?

Answer 31

air resistance has the greatest effect on paper due to its increased surface area

They would fall at the same rate in a vacuum though

Question 32

What happens to the braking distance of a car if the speed is doubled and why?

Answer 32

-doubling speed will quadruple Ek (Ek∝v²)
-work done by brakes to stop car must also quadruple
-Work done = force x distance, assuming brake force is constant, then braking distance must quadruple to make work done quadrupled