P4 - Explaining Motion Flashcards
What 2 things does velocity tell you?
- Speed
- Direction of travel
If a lorry travels along a straight road at 15m/s, what will the velocity be? (HT)
+15m/s in one direction and -15m’s in the other direction
Does it matter which direction (in terms of velocity) is called positive or negative? (HT)
No - as long as opposite directions have opposite directions have opposite signs.
Explain how direction is described in terms of velocity. (HT)
- Changes in distance in one direction are described as positive
- In the opposite direction they’re negative
What two things do you need to know in order to find out the speed of an object?
- The distance it has travelled
- The time it took to travel that distance
How can speed be calculated?
Using the following formula:
speed (m/s) = distance travelled (m) / time taken (s)
What does the “speed = distance / time” formula calculate?
An average speed over the total distance travelled, even if the speed of a n object isn’t constant
True or False?
You cannot calculate the average speed of an object over the total distance travelled if the speed is not constant.
False - You can calculate the average speed of an object over the total distance travelled, even if the speed is not constant.
The speed of an object at a particular point in time is called the what?
The instantaneous speed
A car travels 10 metres in 5 seconds. What is the average speed?
Speed = distance / time = 10m / 5s = 2m/s
What is a ‘slope’ on a graph also known as?
The gradient
What is the gradient?
The ‘slope’ on a graph
What is the gradient (or ‘slope’) on a distance-time graph used to measure?
The speed of an object
The gradient (or ‘slope’) on a distance-time graph can be used to measure the speed of an object.
a) The steeper the slope…
b) The flatter the slope…
a) …the greater the speed
b) …the slower the speed
How can speed be calculated by using a distance-time graph? (HT)
By working out what the gradient (or ‘slope’) is
Explain how you can calculate speed by working out the gradient on a distance-time graph. (HT) (4 stages)
- Take any two points on the gradient
- Read off the displacement travelled between these points
- Note the time taken between these points.
- Divide the displacement by time
What is the instantaneous velocity of an object?
Its instantaneous speed together with an indication of direction.
When the line of a distance-time graph is curved, what does that mean?
The speed of the object is changing
What does the gradient (or ‘slope’) of a speed-time graph represent?
How quickly an object is increasing in speed (i.e. accelerating)
What would you call it when an object is increasing in speed?
Accelerating
Complete the sentence in relation to a speed-time graph.
The steeper the slope/gradient…
…the faster its speed is increasing
Where might a speed-time graph be used?
In lorry tachographs
For what reasons might a speed-time graph be used in lorry tachographs?
To ensure that drivers:
- don’t exceed the speed limit
- rest for suitable amounts of time
A student plots a speed-time graph. The graph gives a horizontal straight line. What does that tel you about the acceleration?
It was zero - his speed was constant
A man walks North at 2m/s for 10 seconds, then rests for another 20s. What is his displacement? (HT)
20m North
The quantity velocity has both speed and _______. A change in either (or both) speed and ________ of travel will cause a change in _______. (HT)
- Direction
- Direction
- Velocity
Can a velocity-time graph have negative values for the velocity axis?
Yes
Give the formula used to calculate acceleration.
acceleration (m/s[2]) = change in speed (m/s) / time taken (s)
A car is travelling at 2m/s when the driver accelerates to 10m/s in 4 seconds/ What is the acceleration?
Change in speed / time taken = 10 - 2 / 4 = 8 / 4 = 2m/s[2]
When do forces occur?
When two object interact with each other.
Whenever one object exerts a force on another…
…it always experiences a force in return.
A force occurs when two object interact with each other. Whenever one object exerts a force on another, it always experiences a force in return. Therefore forces work in an _________ ________.
Interaction pair
How do forces work?
In interaction pairs
What do the 2 forces within an interaction pair have to be?
- Equal in size
- Opposite in direction and they act on different objects
Give some examples of forces.
- Gravity (weight)
- Air resistance (drag)
- Rocket and jet engines
How does gravity (weight) work as a force?
Two masses are attracted to each other, e.g. you are attracted to the Earth and the Earth is attracted to you with an equal and opposite force.
Give an example of how air resistance (drag) works as a force.
The air tries to slow down a skydiver by pushing upwards against him/her. The skydiver pushes the air out of the way with an equal and opposite force.
Explain how forces play a part in rocket and jet engines.
The engine pushes gas backwards (action) and the gas pushes the rocket forwards (reaction).
How does a person move? (HT)
A person moves by applying a force to the ground (they push on the ground). There will be an equal and opposite force generated (the ground pushes back on the person). The person moves because they have a much smaller mass than the Earth.
True or False?
Some forces only occur as a response to another force.
True.
What 2 things happen when an object is resting on a surface?
- The object is pulled down onto the surface by gravity
- The surface pushes up on the object with an equal force
When an object is resting on a surface:
- the object is pulled down onto the surface by gravity
- the surface pushes up on the object with an equal force
What is this called?
The reaction of the surface.
What happens when 2 objects try to slide past one another?
Both objects experience a force that tries to stop them moving.
When 2 objects try to slide past one another, both objects experience a force that tries to stop them moving. What is the name of this force?
Friction.
Do objects have to be moving to experience friction?
no
Give an example of how an object can experience friction, even if the object is not moving.
The friction from a car’s brakes stops it rolling down a hill.
Friction and the ________ ___ _ ______ arise in response to the action of an ______ force, and their size matches the applied force up to a ______.
- Reaction of a surface
- applied
- limit
What is used when drawing diagrams of forces to show size and direction of the force?
Arrows
What does the size of an arrow represent on a diagram about forces?
The size of the force
What does the direction of an arrow represent on a diagram about forces?
The direction of the force
a) If more than one force acts on an object they will…
b) The overall effect of adding or subtracting these forces is called the…
a) - add up if they are acting in the same direction
- subtract is they are acting in opposite directions.
b) …resultant force
What is momentum?
A measure of the motion of an object.
How can the momentum of an object be calculated?
momentum (kg m/s) = mass (kg) x velocity (m/s)
a) If a car and a lorry are travelling at the same speed, which will have more momentum?
b) Why is this?
a) The lorry
b) Because it has a bigger mass
A car has a mass of 1200kg and is travelling at a velocity of 30m/s. What is its momentum?
1200kg x 30m/s = 36,000kg m/s
If the resultant force acting on an object is zero, what will happen to the momentum?
It will not change
If the resultant force acting on an object is zero…
…the momentum will not change
If the resultant force acting on an object is zero, what will not change?
The momentum
The momentum will not change if the _________ ______ acting on an object is zero.
Resultant force
The momentum will not change if the resultant force acting on an object is what?
Zero
If the resultant force acting on an object is zero, its momentum will not change. So if the object is
a) stationary, it will…
b) already moving, it will…
a) …remain stationary
b) …continue moving in a straight line at a steady speed.
If the resultant force acting on an object is zero, its momentum will not change. So if the object is stationary it will remain ________.
Stationary
If the resultant force acting on an object is zero, its momentum will not change. So if the object is already moving, it will continue ________ in a ___________ line at a ______ speed.
- moving
- straight
- steady
If the resultant force acting on an object is not zero, what will happen to the momentum?
It causes a change of momentum in the direction of force.
If the resultant force acting on an object is not zero…
…it causes a change of momentum in the direction of force.
If the resultant force acting on an object is not zero, it causes a change of momentum in the direction of force. What 3 things could this do?
- Make a stationary object move
- Increase or decrease an object’s speed
- Change an object’s direction
What 2 things does the extent of change in momentum depend on?
- The size of the resultant force
- The length of time the force is acting on the object
Give the formula used to calculate the change in momentum.
change in momentum (kg m/s) = resultant force (newtons) x time the force acts for (seconds)
What can collisions cause?
Changes in an object’s momentum
A car with a mass of 1000kg traelling at 10m/s has a momentum of what?
10,000kg m/s
A car with a mass of 1000kg traelling at 10m/s has a momentum of 10,000kg m/s. What happens if the car is involed in a collision and comes to a sudden stop?
The car would experience a change in momentum of 10,000kg m/s.
Sudden changes in momentum as a result of a collision can affect what 2 things?
- The car
- The passengers (leading to injuries)
If the change in momentum is spread out over a longer period of time, what will the resultant force be?
Smaller
If the change in momentum is spread out over a longer period of time…
…the resultant force will be smaller.
How can the force of a collision impact on a human body be reduced?
By increasing the time of the impact.
By increasing the time of a collision impact, what effect will be applied to a human body?
The force will be reduced
How can the force of a collision impact on a human body be increased?
By reducing the time of the impact.
The force of the impact on the human body in a collision can be reduced by…
…increasing the time of the impact.
What is the purpose of road safety devices?
To increase the time of the impact, reducing the force of the impact a collision can have on the human body.
Give some examples of road safety devices that reduce the force of a collision on humans.
- Seat belts
- Crumple zones (where vehicles crumple on impact – e.g. motorcycle and bicycle helmets)
- Air bags
What 2 forces do both cars and bicycles contain?
- Driving force
- Counter force
Explain how the driving force is produced within cars.
It is produced by the engine
Explain how the driving force is produced within bicycles.
It is produced by the energy of the cyclist
What happens if the driving force of a car or bike is bigger than the counter force?
The vehicle speeds up
What happens if the driving force of a car or bike is equal to the counter force?
The vehicle travels at a constant speed in a straight line
What happens if the driving force of a car or bike is smaller than the counter force?
The vehicle slows down
How is the counter force within cars and bikes caused?
By friction and air resistance
What type of energy does a moving object contain?
Kinetic energy.
The amount of kinetic energy an object has depends on its what?
- Mass
- Velocity
The greater the mass and velocity of an object…
…the more kinetic energy it has.
How can kinetic energy be calculated?
kinetic energy (joules) = 1 / 2 x mass (kg) x velocity [2] (metres per second)
A bicycle of mass 50kg is moving at a velocity of 8m/s. How much kinetic energy does it have?
1 / 2 x 50kg x (8m/s) [2] = 1 / 2 x 50 x 64 = 1600J
Explain what happens when a person throws a ball up into the air.
As the ball leaves the hand, an initial force is applied vertically upwards. There will be two forces opposing the motion:
- air resistance
- gravity
As soon as the ball leaves the hand, these forces will cause the ball to lose speed. There is no upward force once the ball has left the hand.
What 2 forces would oppose the motion of a ball being thrown upwards into the air?
- Air resistance
- Gravity
What would happen as a skydiver initially jumps out of a plane?
As the skydiver jumps out of the plane, only one force is acting in the vertical direction - the unbalanced force of gravity. The skydiver will start to accelerate downwards.
What would happen as a skydiver starts to accelerate downwards after jumping out of a plane? (4 stages)
- As the skydiver falls, he/she will start to experience a new force of air resistance. The faster the skydiver falls, the greater this air resistance force becomes.
- The force of gravity remains the same.
- Eventually the two forces of gravity and air resistance become equal and opposite.
- The skydiver now stops accelerating and travels at a constant speed.
Work is done by a force to move an object, resulting in what?
The transfer of energy
What happens when work is done ON an object?
The object GAINS energy
What happens when work is done BY an object?
The object LOSES energy
What is meant by the term ‘conservation of energy’?
The total amount of energy stays the same
Complete this formula:
Amount of energy transferred (joules, J) = ?
Amount of energy transferred (joules, J) = work done (joules, J)
Complete this formula:
? = work done (joules, J)
Amount of energy transferred (joules, J) = work done (joules, J)
Give the formula for the amount of energy transferred.
Amount of energy transferred (joules, J) = work done (joules, J)
What happens when a force makes an object’s velocity increase? (2 things)
- Work is done on the object by the force
- The object gains kinetic energy
What happens if you ignore drag and friction?
The increase in kinetic energy will be equal to the work done by force (but, in reality, some of the energy will be dissipated/lost as heat).
Give the formula that shows the relationship between work done, force and distance.
work done (joules, J) = force (newtons, N) x distance moved in the direction of the force (metres, m)
Air resistance or ________ will cause the gain in an object’s _______ energy to be ____ than the work done on it by an applied force in the direction of motion because some energy is __________ through heating.
a) friction
b) kinetic
c) less
d) dissipated/lost
What happens when an object is lifted above the ground? (2 things)
- Work is done by the lifting force against gravity
- The object has the potential to do work when it falls, e.g. a diver standing on a diving board
When an object is lifted above the ground:
- Work is done by the lifting force against gravity
- The object has the potential to do work when it falls, e.g. a diver standing on a diving board
What is this called?
Gravitational Potential Energy (GPE)
What does GPE stand for?
Gravitational Potential Energy
Give the formula by which GPE (Gravitational Potential Energy) can be calculated.
Change in GPE (joules) = weight (newtons) x vertical height difference (metres)
Do you use weight or mass to find the GPE?
Weight, not mass
If an object is dropped…
…its GPE decreases and converts into kinetic energy
What happens to an object’s GPE when it is dropped?
It decreases and converts into kinetic energy
An object is dropped from a height of 5m. It has a mass of 2kg and weighs 20N. How much kinetic energy does it gain?
Change in GPE = weight by vertical height difference
= 20N x 5m
= 100J
An object is dropped from a height of 5m, with a mass of 2kg and a weight of 20N. It gains 100J of kinetic energy.
How much GPE is lost?
100J
An object is dropped from a height of 5m, with a mass of 2kg and a weight of 20N. What is lost and what is gained?
- Loses 100J of gravitational potential energy
- Gains 100J of kinetic energy
What can the kinetic energy formula also help us to work out? (HT)
The velocity of a falling object.
Give the formula for finding kinetic energy. (HT)
kinetic energy = 1 / 2 x mass x velocity [2]
With a rocket, the engine pushes gas backwards. What is the name of that force?
Action
A force of 12N acts on a truck for two seconds. What is the change in momentum?
24kg m/s
A cyclist of mass 60kg is moving at a speed of 4m/s. How much kinetic energy does the cyclist have?
1 / 2 x 60 x 4[2] = 480J
A person weighing 800N gains 16,000J of gravitational potential energy as they’re carried up a lift. How high did they go? (HT)
20m
A person has 800J of kinetic energy. If they have a mass of 100kg, how fast are they travelling?
4m/s
A car of mass 1500kg is travelling along a road at a velocity of 45m/s.
a) What is the momentum of the car?
b) What is the acceleration if the speed increases from 45m/s to 55m/s in 4 seconds?
a) Momentum = 1500 x 45 = 67,500kg m/s
b) Acceleration = 55 - 45 / 4 = 2.5m/s[2]
True or False?
The change in momentum depends on the size of the force acting and the time it acts for.
True
True or False?
For an object moving in a straight line, if the driving force is larger than friction, the object will slow down.
False
True or False?
If the resultant force on a car is zero, its momentum is constant.
True
True or False?
The energy of a moving object is called work.
False
A ball of weight 40N is dropped from a height of 20m. Calculate the velocity of the ball just before it hits the ground. Take the mass of the ball to be 4kg. (HT)
PE lost = weight x height = 40 x 20 = 800J PE lost = KE gained 800 = 1/2mv[2] 800 = 1/2 x 4 x v [2] v = 20m/s
A 150kg dodgem car travelling at 3m/s collides with a rubber wall in the fairground and rebounds with a speed of 2m/s.
a) What is the change in momentum? (HT)
b) If the collision lasted for 0.5 seconds, what force acted on the dodgem car? (HT)
c) Why did the driver bend his knees during the impact?
a) Change in momentum = 150 x 3 - (150 x -2) {Remember the velocity of rebound will be negative}
Change in momentum = 450 + 300 = 750kg m/s
b) Force x 0.5 = 750, so force = 1500N
c) To increase the impact time, which reduces the force on his body
