forces - newtons laws, suvat, stopping distance - acceleration experiemetns

Question 1

state the equation that links final velocity, initial velocity, acceleration and distance

if an object is accelerating at a constant rate

Answer 1

(final velocity) ^2 − initial (velocity) ^2
= 2 × acceleration × distance
v^2 − u^2 = 2 a s

final velocity, v, in metres per second, m/s
initial velocity, u, in metres per second, m/s
acceleration, a, in metres per second squared, m/s2
distance, s, in metres, m

Question 2

What is the acceleration of objects that fall towards the surface of the Earth (falling freely under gravity)

Answer 2

When any object falls towards the surface of the Earth, it initially accelerates at around 9.8m/s^2

Question 3

what does 9.8m/s^2 represent

Answer 3

the acceleration of objects that fall towards the surface of the Earth (falling freely under gravity)

Question 4

Describe the acceleration of an object falling through a fluid

Answer 4

An object falling through a fluid initially accelerates due to the weight which acts downwards (due to the force of gravity acting on the object) .

As the object falls, it experiences an upward force of friction with air particles - this is called air resistance

After some time, the force of air resistance balances the force due to gravity

At this point the object stops accelerating (the resultant force is 0) and moves at a constant velocity. This is called the terminal velocity

Question 5

Describe how different objects may reach different terminal velocities

Answer 5

Some objects experience a greater force of friction than others due to their shape so will have a lower terminal veloctiy

Question 6

State Newton’s First Law of Motion

Answer 6

Newton’s First Law states:

If the resultant force acting on a stationary object is zero then the object will remain stationary

If the resultant force acting on a moving object is zero, then the object will continue moving in the same direction at the same speed (the object will continue to move at the same velocity)

Question 7

What can be said about the forces if the resultant force = 0

Answer 7

If the resultant force = 0, all the forces are said to be balanced

Question 8

When the velocity of an object change

Answer 8

The velocity of an object will only change if a resultant force is acting on the object

Question 9

What is the resultant force if the forces are balanced

Answer 9

Because the forces are balanced, the resultant force = 0

Question 10

A car is moving at a constant speed

What must there be if the car is moving at a constant speed

Answer 10

Because the car is moving at a constant speed, there must be an equal force acting to the right (an equal and opposite force to the driving force)

Question 11

What does a resultant force cause to a stationary object

Answer 11

A resultant force causes an object’s speed to change/causes the object to accelerate

Question 12

What does a resultant force cause to a moving object

Answer 12

A resultant force causes an object’s speed to change/causes the object to accelerate or decelerate

Question 13

When an vehicle travels at a steady speed what balances the driving force

Answer 13

So, when a vehicle travels at a steady speed the resistive forces
balance the driving force.

Question 14

What can a resultant force cause

Answer 14

A resultant force causes an object’s speed to change or an object’s direction to change

Question 15

State Newton’s Second Law

Answer 15

Newton’s Second Law states:

The acceleration of an object is proportional to the resultant force acting on the object and inversely proportional to the mass of the object

Question 16

What will happen:
resultant force of 20N acting on object A
resultant force of 10N acting on object B

Object A and object B are identical

Answer 16

Object A will experience twice the acceleration of Object B

if there is a greater resultant force acting on the object, the object will experience a greater acceleration

Question 17

When an vehicle travels at a steady speed what balances the driving force

Answer 17

So, when a vehicle travels at a steady speed the resistive forces
balance the driving force.

Question 18

What will happen:
resultant force of 20N acting on object A with a mass of 1kg
resultant force of 20N acting on object B with a mass of 2kg

what will happen

Answer 18

The top object will experience twice the acceleration of the bottom object

if the mass is larger, the acceleration will be smaller

Question 19

state the equation that links resultant force, mass and acceleration

Answer 19

F = ma

resultant force = mass × acceleration
F = m a
force, F, in newtons, N
mass, m, in kilograms, kg
acceleration, a, in metres per second squared, m/s2

Question 20

State the estimate speeds/acceleration/forces for everyday road transport

Cars on a main road - UK
cars on a motorway

to accelerate from a main road to a motorway

for a typical family car that would require a force of ___

Answer 20

Cars travel at approx. 13m/s on a main road in the UK and approx. 30m/s on a motorway

to accelerate from a main road to a motorway involves a typical acceleration of approx. 2m/s^2

For a typical family car, that would require a force of approx. 2000N

Question 21

Describe this property of objects: inertia

Answer 21

Objects will stay stationary or continue moving at the same speed and direction unless a resultant force is applied

Question 22

Define inertial mass

Answer 22

Inertial mass is a measure of how difficult it is to change the velocity of an object

The ratio of the force needed to accelerate an object over the acceleration produced

Question 23

State the different between an object with a large inertial mass and an object with a smaller inertial mass

Answer 23

An object with a large inertial mass will require a larger force to produce a given acceleration than an object with a smaller inertial mass

Question 24

State Newton’s third law

Answer 24

Newton’s third law states:

Whenever two objects interact, the forces they exert on each other are equal and opposite

Question 25

Describe a man rowing a boat in terms of newtons third law

Answer 25

The man is using the paddle to push on the water
At the same time, the water pushed back on the paddle
This force is equal in magnitude but opposite in direction to the force the man is using for the paddle

Question 26

Describe a skateboarder jumping off a skateboard in terms of newtons third law

Answer 26

When the skateboarder jumps off a skateboard, they apply a push force onto the skateboard
This causes the skateboard to move to the right

At the same time, the skateboard pushes back on the skateboarder.
This force is equal in magnitude but opposite in direction
This causes the skateboarder to move to the left

Question 27

Describe a car driving using newtons third law

Answer 27

When a car is driving, the wheel exerts a force in the reverse direction on the road
At the same time, the road exerts a force in the forward direction on the wheel
These two forces are equal in magnitude but opposite in direction

Question 28

Describe how the forces acting on a skydiver change with velocity

Answer 28

As soon as the skydiver jumps out of the play, the only force acting on them is weight (due to gravity - this force will not change during the journey)

Because of weight, the skydiver experiences a resultant force acting downwards, so they accelerate towards the ground

As they fall, the skydiver experiences friction with air particles. This force is called air resistance and it acts upwards.

The weight is still greater than the air resistance so the skydiver continues to accelerate toward the ground

As the skydiver’s velocity increases, the air resistance also increases

At a certain point, the air resistance is equal to the weight acting downwards

Now the resultant force = 0, so the velocity stays constant. The sky diver has reached terminal velocity

This velocity is extremely great, and the skydiver will die if they hit the ground. At this point the skydiver opens their parachute

The surface area now increases, causing air resistance to increase massively.

At this point the air resistance is now greater than the weight.
So there is a resultant force acting upwards
This causes the skydiver to decelerate (their velocity decreases)

Because the velocity decreases, the air resistance also decreases
At some point, the air resistance will balance the weight and the resultant force will be zero

At this point the velocity will will stay constant
Now the skydiver is falling at a lower terminal velocity - this is now safe for them to hit the ground
The skydiver lands

Question 29

Represent the motion of a skydiver on a velocity-time graph

Question 30

What are the variables for investigating how varying the force affects the acceleration of an object of constant mass

Answer 30

Dependent variable - acceleration
Control variable - displacement, mass of the vehicle, surface of the board
Independent variable - mass

force - the weight of the mass on the end of the string

Question 31

The hailstone stops accelerating and reaches terminal velocity.
Explain why the hailstone reaches terminal velocity.

Answer 31

as the velocity of the hailstone
increases air resistance
increases

until air resistance becomes
equal to the weight of the
hailstone

so the resultant force is (equal
to) zero

Question 32

Why does terminal velocity increase with mass?

Answer 32

As mass increases the weight of a hailstone increases

Question 33

Explain the difference in the maximum kinetic energy of a hailstone with a mass of box
10 g and a hailstone with a mass of 20 g

Answer 33

kinetic energy depends on both
mass and velocity

as mass increases so does
terminal / maximum velocity

kinetic energy ∝ m and kinetic
energy ∝ v2 so as mass doubles
kinetic energy more than
doubles

Question 34

1 Joule = ___

Answer 34

1J = 1Nm

Question 35

Describe the method for investigating how varying the force affects acceleration of an object of constant mass

Answer 35

Set up the pulley system by securely clamping the pulley to the wooden board

Position the trolley at the opposite end of the board and using a ruler, measure the distance from the starting point of the trolley to the pulley

Start with the 20 gram mass that is attached to the string on the pulley

Using a stopwatch measure the time taken for the trolley to cover the distance

Repeat the previous steps, altering the masses, then calculate appropriate mean values and calculate the acceleration using the equation - s = ut + 1/2at^2

Question 36

Investigating how varying the force affects acceleration of an object of constant mass

WHAT DOES OBJECT REFER TO

Answer 36

The toy car, the string and the mass on the end of the string - since they are all attached to each other

Question 37

What is the conclusion to investigating how varying the force affects acceleration of an object of constant mass

Answer 37

Newtons second law tells us that the acceleration of an object is proportional to the force applied

The force in this case is the weight of the mass on the end of the string

The acceleration of the toy car is proportional to the mass on the other end of the string due to newtons 2nd law

Question 38

Describe the variables for investigating how varying the mass of an object affects the acceleration produced by a constant force

Answer 38

independent: the mass on the trolley
dependent: acceleration
control : mass of the pulley/hanger, displacement, surface of the board

Question 39

Risk assessment for investigating how varying the mass of an object affects the acceleration produced by a constant force

Answer 39

Make sure the masses do not fall off the trolley onto the floor or ourselves

Make sure the board is securely on the table -

Answer 40

Set up the pulley system by securely clamping the pulley to the wooden board

Position the trolley at the opposite end of the board and using a ruler, measure the distance from the starting point of the trolley to the pulley

keep the mass that is attached to the string on the pulley constant (keep the force constant)e.g. using a 100g mass at the end of the string

Now attach a mass to the toy car e.g. 200 grams.

Using a stopwatch measure the time taken for the trolley to cover the distance

Repeat the previous steps, altering the masses attached to the toy car, then calculate appropriate mean values and calculate the acceleration using the equation - s = ut + 1/2at^2

Question 41

conclusion for investigating how varying the mass of an object affects the acceleration produced by a constant force

Answer 41

Newtons second law tells us that the acceleration of an object is inversely proportional to the mass of the object

With this experiment, we should find that as we increase the mass of the toy car, the acceleration decreases

Question 42

Draw the graph for
investigating how varying the mass of an object affects the acceleration produced by a constant force

Answer 42

curve
like lowercase l

Question 43

Draw the graph for investigating how varying the force affects the acceleration of an object of constant mass

Answer 43

straight line

Question 44

Define stopping distance

Answer 44

The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking
distance)

Question 45

define thinking distance

Answer 45

the distance the vehicle travels during the driver’s reaction time (thinking distance)

Question 46

define braking distance

Answer 46

the distance the vehicle travels under the braking force (braking
distance)

Question 47

assuming the same braking force is applied, how does the speed of the vehicle, affect the stopping distance

Answer 47

The greater the speed of the vehicle, the greater the stopping distance (assuming that the same braking force is applied)

Question 48

At 30mph how many metres does it take a typical family car to stop

Answer 48

At 30mph a typical family car takes around 23m to stop - the equivalent of 6 car lengths

Question 49

state factors that affect reaction time/thinking distance

Answer 49

A driver’s reaction time can be affected by:

tiredness
drugs and alcohol. Distractions in the car (e.g. a mobile phone) may also affect a driver’s ability to react.

Question 50

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

Answer 50

distance = speed × time

(so) longer reaction time =
longer distance

Question 51

typical values for a person’s reaction time

Answer 51

Typical values range
from 0.2 s to 0.9 s.

Question 52

state factors that affect braking distances

Answer 52

wet or icy road conditions
poor condition of tyres
poor condition of breaks brakes
increased mass of car
negative gradient of road

Question 53

Explain how conditions on the road affect braking distances

Answer 53

Wet or icy conditions reduce the friction between the tyres and the road and increase the braking distance

Question 54

Explain the effect of two other factors on the braking distance of a car

Answer 54

• poor condition of tyres
• poor road surface
• wet or icy road
• poor/worn brakes

because of decreased friction

increased mass of car/passengers

• increases kinetic energy of car
• more work needs to be done to stop car
• road slopes downhill
  (a component of) gravity opposes the braking force
• resultant (braking) force is reduced

Question 55

Explain how the condition of tyres/brakes will affect braking distance

Answer 55

The braking distance will also increase if a car has worn tyres
This is because this reduces the friction between the tyres and the road

Question 56

What does kinetic energy depend on

Answer 56

Kinetic energy depends on the velocity (squared)

if you double the velocity of the car, the kinetic energy quadruples

Question 57

what happens when a force is applied to the brakes of a vehicle

Answer 57

During braking, the brakes press against the wheel
The force of friction now acts between the brake and the wheel
The kinetic energy of the car is now converted to thermal energy in the brakes
This causes the temperature of the break to increase
At the same time, the car slows down as it loses kinetic energy

Question 58

explain the dangers of large deceleration

Answer 58

A large braking force causes the car to decelerate rapidly
At the same time, a large amount of kinetic energy is transferred to thermal energy in the brakes
This can cause the brakes to overheat

It can also cause the driver to lose control of the vehicle

Question 59

how does speed affect braking distances

Answer 59

The greater the speed, the greater the braking force needed to stop the car in a certain distance

Question 60

what can a large braking force cause

Answer 60

A large braking force causes the car to decelerate rapidly

Question 62

estimate total stopping distance

Answer 62

thinking siatance is proportional to speed
braking distance is proportional to the square of the speed

Question 63

A driver performs an emrergency stop
His thinking distance and braking distance are both 6m
Estimate his total stopping distance if he had been travelling three times quickly

page119 cgp
page 157 textbook

Answer 63

Stopping distance = thinking distance + braking distance

thinking distance = 6 x 3 = 18m

stopping distance = 6 x 3^2 = 54 m

stopping distance = 18 + 54 = 72 m

Question 64

A car with a mass of 1000jg is travelling at 30m/s on the motorway

They decelerate to leave the motorway
Their velocity decreases to zero in ten seconds

calculate the forces involved in the deceleration of
road vehicles in typical situations on a public road.

Answer 64

in this case they have decelerated but that will not affect the calculation

F = ma
F = 1000 X 3
F = 3000N for this deceleration