forces - newtons laws, suvat, stopping distance - acceleration experiemetns Flashcards
state the equation that links final velocity, initial velocity, acceleration and distance
if an object is accelerating at a constant rate
(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
What is the acceleration of objects that fall towards the surface of the Earth (falling freely under gravity)
When any object falls towards the surface of the Earth, it initially accelerates at around 9.8m/s^2
what does 9.8m/s^2 represent
the acceleration of objects that fall towards the surface of the Earth (falling freely under gravity)
Describe the acceleration of an object falling through a fluid
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
Describe how different objects may reach different terminal velocities
Some objects experience a greater force of friction than others due to their shape so will have a lower terminal veloctiy
State Newton’s First Law of Motion
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)
What can be said about the forces if the resultant force = 0
If the resultant force = 0, all the forces are said to be balanced
When the velocity of an object change
The velocity of an object will only change if a resultant force is acting on the object
What is the resultant force if the forces are balanced
Because the forces are balanced, the resultant force = 0
A car is moving at a constant speed
What must there be if the car is moving at a constant speed
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)
What does a resultant force cause to a stationary object
A resultant force causes an object’s speed to change/causes the object to accelerate
What does a resultant force cause to a moving object
A resultant force causes an object’s speed to change/causes the object to accelerate or decelerate
When an vehicle travels at a steady speed what balances the driving force
So, when a vehicle travels at a steady speed the resistive forces
balance the driving force.
What can a resultant force cause
A resultant force causes an object’s speed to change or an object’s direction to change
State Newton’s Second Law
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
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
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
When an vehicle travels at a steady speed what balances the driving force
So, when a vehicle travels at a steady speed the resistive forces
balance the driving force.
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
The top object will experience twice the acceleration of the bottom object
if the mass is larger, the acceleration will be smaller
state the equation that links resultant force, mass and acceleration
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
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 ___
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
Describe this property of objects: inertia
Objects will stay stationary or continue moving at the same speed and direction unless a resultant force is applied
Define inertial mass
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
State the different between an object with a large inertial mass and an object with a smaller inertial mass
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
State Newton’s third law
Newton’s third law states:
Whenever two objects interact, the forces they exert on each other are equal and opposite
Describe a man rowing a boat in terms of newtons third law
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
Describe a skateboarder jumping off a skateboard in terms of newtons third law
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
Describe a car driving using newtons third law
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
Describe how the forces acting on a skydiver change with velocity
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
Represent the motion of a skydiver on a velocity-time graph
What are the variables for investigating how varying the force affects the acceleration of an object of constant mass
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
The hailstone stops accelerating and reaches terminal velocity.
Explain why the hailstone reaches terminal velocity.
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
Why does terminal velocity increase with mass?
As mass increases the weight of a hailstone increases
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
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
1 Joule = ___
1J = 1Nm
Describe the method for investigating how varying the force affects acceleration of an object of constant mass
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
Investigating how varying the force affects acceleration of an object of constant mass
WHAT DOES OBJECT REFER TO
The toy car, the string and the mass on the end of the string - since they are all attached to each other
What is the conclusion to investigating how varying the force affects acceleration of an object of constant mass
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
Describe the variables for investigating how varying the mass of an object affects the acceleration produced by a constant force
independent: the mass on the trolley
dependent: acceleration
control : mass of the pulley/hanger, displacement, surface of the board
Risk assessment for investigating how varying the mass of an object affects the acceleration produced by a constant force
Make sure the masses do not fall off the trolley onto the floor or ourselves
Make sure the board is securely on the table -
f varying the mass of an object on the acceleration
produced by a constant force.
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
conclusion for investigating how varying the mass of an object affects the acceleration produced by a constant force
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
Draw the graph for
investigating how varying the mass of an object affects the acceleration produced by a constant force
curve
like lowercase l
Draw the graph for investigating how varying the force affects the acceleration of an object of constant mass
straight line
Define stopping distance
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)
define thinking distance
the distance the vehicle travels during the driver’s reaction time (thinking distance)
define braking distance
the distance the vehicle travels under the braking force (braking
distance)
assuming the same braking force is applied, how does the speed of the vehicle, affect the stopping distance
The greater the speed of the vehicle, the greater the stopping distance (assuming that the same braking force is applied)
At 30mph how many metres does it take a typical family car to stop
At 30mph a typical family car takes around 23m to stop - the equivalent of 6 car lengths
state factors that affect reaction time/thinking distance
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.
Explain why the driver’s reaction time affects the thinking distance of a car
distance = speed × time
(so) longer reaction time =
longer distance
typical values for a person’s reaction time
Typical values range
from 0.2 s to 0.9 s.
state factors that affect braking distances
wet or icy road conditions
poor condition of tyres
poor condition of breaks brakes
increased mass of car
negative gradient of road
Explain how conditions on the road affect braking distances
Wet or icy conditions reduce the friction between the tyres and the road and increase the braking distance
Explain the effect of two other factors on the braking distance of a car
- 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
Explain how the condition of tyres/brakes will affect braking distance
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
What does kinetic energy depend on
Kinetic energy depends on the velocity (squared)
if you double the velocity of the car, the kinetic energy quadruples
what happens when a force is applied to the brakes of a vehicle
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
explain the dangers of large deceleration
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
how does speed affect braking distances
The greater the speed, the greater the braking force needed to stop the car in a certain distance
what can a large braking force cause
A large braking force causes the car to decelerate rapidly
estimate total stopping distance
thinking siatance is proportional to speed
braking distance is proportional to the square of the speed
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
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
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.
in this case they have decelerated but that will not affect the calculation
F = ma
F = 1000 X 3
F = 3000N for this deceleration
