P5 Flashcards
What is a vector quanity
A quanity that has both magnitude and direction
What is a scalar quanity
A quantity that has only magnitude and NO direction
How are vectors usually represented
By an arrow - the length of the arrow shows the magnitude,and the direction of the arrow shows the direction of the quantity
State examples of a vector quantity
- Force
- Velocity
- Displacement
- Acceleration
- Momentum
State examples of scalar quanitites
- Speed
- Distance
- Mass
- Temperature
- Time
What is a force?
A force is a push or pull that acts on an object due to the interaction with another object. All forces between forces are either contact or non contact forces
What is a contact force?
When two objects touch each other in order for a force to act,it is a contact force
What is a non-contact force?
If the objects do not need to be touching for a force to act,it is a non-contact force
State examples of contact forces
- Friction
- Air resistance
- Tension
- Normal contact force
State examples of a non-contact force
- Magnetic force
- Gravitational force
- Electrostatic force
Is force a scalar or vector quantity?
Vector quanity
What is an interaction pair?
A pair of forces that are equal and opposite and act on two interacting objects
(This is basically newtons third law)
A tennis ball is dropped from a height.Name one contact and one non-contact force
Contact force - Air resistance
Non-contact force - Gravitational force
(The air resistance is going up whilst the gravitational force is going down)
Note:
AQA says that students should be able to describe the interaction between pairs of objects which produce a force on each object. The forces are to be represented as vectors
What is weight?
Weight is the force acting on an object due to gravity (the pull of the gravitational force on the object).
(Close to Earth, this force is caused by the gravitational field around the Earth)
What does the weight of an object depend on?
The weight of an object depends on the strength of the gravitational field at the location of the object. This means that the weight of an object changes with its location
( For example, an object has the same mass whether it’s on Earth or on the Moon - but its weight will be different. A 1 kg mass will weigh less on the Moon (about 1.6 N) than it does on Earth (about 9.8 N), simply because the gravitational field strength on the surface of the Moon is less.)
What does weight measure in?
Newtons
State the equation to find the weight of an object
Weight(N) = Mass(kg) x Gravitational field strength(N/kg)
W = mg
What is the centre of mass?
A point at which you assume the whole mass is concentrated
( For a uniform object (one that’s the same density,throughout and is a regular shape), this will be at the centre of the object.)
What is a similarity of mass and weight?
They are directly proportional to each other
( This means Increasing the mass of an object increases its weight. If you double the mass, the weight doubles too)
How is weight measured?
Using a calibrated spring balance ( or newtonmeter)
How is mass measured?
It’s measured in kilograms with a mass balance (a pair of balancing scales).
Note : Mass is not a force
What is the resultant force?
The overall Force on a Point or Object
Note:
AQA says that students should be able to calculate the resultant of two forces that act in a straight line
What are the different types of forces acting on an isolated object?
When you jump out of a place you accelerate. Your motion changes because there is a resultant for on you, the air exerts a force on you, but the earth exerts a larger force. As you accelerate the force of the air increases. Eventually, the force of the air on you equals the force of the earth on you, and your motion no longer changes. You have reached terminal velocity. Without air resistance, you would reach the speed of sound in about 30 seconds.
When a rocket takes off there is a resultant force on it that produces a large acceleration, the burning fuel pushes exhaust gases out of the bottom of the rocket, the gases pushing on the rocket pushing on the gases are another example of newtons third law. When the force of the gases on the rocket is bigger than the force of the earth on the rocket then the rocket will accelerate
Describe, using free body diagrams, examples where two or more forces lead to a resultant force on an object
A leaping animal uses its back legs to exert a force on itself. Its motion changes because there is a resultant force. A resultant force can change the speed of an object and the direction of motion of an object. If the speed or direction of an object changes when it is accelerating. For objects on which a resultant force is acting, you can use free body diagrams. The international space station orbits the earth roughly every 90 minutes which means that it is moving at a steady speed. However, its direction of motion is contantly changing
Describe, using free body diagrams, examples of the special case where forces balance to produce a resultant force of zero ( qualitative only)
Suppose that you draw a free body diagram for a car parked in a car park. Then, you draw another free body diagram for a feather falling at a steady speed. In both cases, the resultant force is zero so the motion does not change. Both objects are in equilibrium
What can a single force be resolved into?
Two components acting at right angles to each other. The two component forces together have the same effect as the single one
Note:
AQA says that students should be able to use vector diagrams to illustrate resolution of forces, equilibrium situations and determine the resultant of two forces, to include both magnitude and direction(scale drawings only)
When a force causes an object to move through a distance, what is done?
‘Work’ is done, meaning energy is transferred. So a force does work on an object when the force causes a displacement of the object
State the equation to calculate work done
Work done(J) = Force(N) x Distance(m)
W = Fs
What is 1 Joule equal to?
1 Newton-meter
(One joule of work is done when a force of one newton causes a displacement of 1 metre)
What happens when work is done against frictional forces acting on an object?
This causes a rise in temperature of the object
What are the forces involved in stretching,compressing or bending an object
Tensional force - Stretching
Stretching - (Forces in opposite directions away from the object)
Bending - (Forces that distort the object)
Compressing - (forces in opposite directions towards the object)
Why, to change the shape of an elastic object,(stretching,bending or compressing) more than one force has to be applied? - This is limited to stationary objects only
In order to stretch,compress or bend an elastic object, we need to apply multiple forces; this is because if we only applied one force, we would just move the elastic object in which the force were to be applied
What is elastic deformation?
It can go back to its original shape and length after the force has been removed.
What is inelastic deformation?
If the object doesn’t return to its original shape and length after the force has been removed.
What is the extension directly proportional to?
Force applied / Load, provided that the limit of proportionality is not exceeded
State the equation for force
Force(N) = Spring constant (N/m) x Extension(m)
F = ke
What is the equation for forces relationship also apply to?
It also applies to the compression of an elastic object, where ‘e’ would be the compression of the object
Note:
A force that stretches(or compresses) a spring does work and elastic potential energy is stored in the spring. Provided the spring is not inelastically deformed, the work done on the spring and the elastic potential energy stored are equal
What is the linear relationship between force and extension?
Extension of an elastic object is directly proportional to the force applied to it. If no force is applied, there is no extension. The graph of force against extension is a straight line through the origin, which shows a linear relationship
What is the non-linear relationship between force and extension?
When an elastic object is stretched beyond its elastic limit, the object does not return to its original length when the force is removed. In this instance, the relationship between force and extension changes from being linear to being non-linear
How do you find the spring constant in linear cases?
Rearrange the equation to get k = F/x
NOTE:
AQA requires students to interpret data from an investigation of the relationship between force and extension
State the equation for elastic potential energy
Elastic potential energy = 0.5 x spring constant x extension^2
E(e) = 1/2ke^e
Note:
AQA requires students to be able to calculate relevant values of stored energy and energy transfers
What is distance?
Distance is how far an object has moved. It’s a scalar quantity so it doesn’t involve direction
What is displacement?
Displacement measures the distance and direction in a straight line from an object’s starting point to it finishing point. This means it’s a vector quantity.
(E.g the plane flew 5 metres north. The direction could be relative to a point, e.g. towards the echool, or a bearing (a threa-digit angle from north, 8. g. 035)
Is distance a scalar or vector?
Scalar
Is displacement a scalar or vector?
Vector
What is your distance and displace if you walk 5m north then 5m south?
Displacement is 0m
Distance travelled is 10m
What is speed?
Speed how fast you’re going (e.g. 30 mph or 20 m/o) with no regard to the direction
Is speed constant?
The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing
What affects the speed for a person to walk,run or cycle?
- Age
- Terrain
- Fitness
- Distance travelled
State 6 typical speeds for:
1) A person walking
2) A person running
3) A person cycling
4) A car
5) A train
6) A plane
A person walking - 1.5 m/s
A person running - 3 m/s
A person cycling - 6 m/s
A car - 25 m/s
A train - 30 m/s
A plane - 250 m/s
What speeds of things vary?
The speed of sound,speed and wind
What can wind speed be affected by?
Temperature, atmospheric pressure and if there are its large buildings or structures nearby e.g. forests reduce the speed of the air travelling through them
What is the typical value for the speed of sound?
330 m/s
Note:
AQA says that students should be able to make measurements of distance and time then calculate speeds of objects
State the equation to find the distance travelled
Distance travelled(m) = speed(m/s) x time(s)
To find the average speed for non-uniform motion what equation do you use?
Average speed = Total distance covered/Total time taken
What is velocity?
Velocity is speed in a given direction, (e.g. 30 mph north or 20 m/s, 060°)
Is velocity a scalar or vector?
Vector
Give an example where a circle moves in a constant speed but in a changing velocity
For example, a car travelling on a roundabout will move at a constant speed, but with a changing velocity, as its direction is constantly changed. The centripetal force that acts inwards is due to the friction between the car’s tyres and the road. This force keeps the car moving in a circular path
What can you do if an object moves along a straight line?
The distance can be represented by a distance-time graph
How can the speed of an object be calculated on a distance-time graph?
By calculating the gradient
If an object is accelerating, how can you find its speed at any time on a distance-time graph?
By drawing a tangent and measuring the gradient of the distance-time graph at that time
Note:
AQA says that students should be able to draw distance-time graphs from measurements and extract and interpret lines and slopes of distance-time graphs,translating information between graphical and numerical form
Note:
AQA says that students should be able to determine speed from a distance-time graph
State the equation to find acceleration
Acceleration(m/s^2) = change in velocity(m/s) / time taken(s)
a = ꕔv/t
How can the acceleration of an object be calculated in a velocity-time graph?
By finding the gradient
How do you find the distance travelled by an object or displacement of an object?
From the area under a velocity-time graph
Note:
AQA says that students should be able to draw velocity-time graphs from measurements and interpret lines and slopes to determine acceleration. Also,you must interpret encloses areas in velocity-time graphs to determine distance travelled or displacement. Also, measure, when appropriate, the area under a velocity-time graphs by counting squares
State the equation that applies to uniform acceleration
(Final velocity)^2 (m/s) - (Initial velocity)^2 (m/s) = 2 x acceleration(m/s^2) x distance(m)
Near the earths surface, any object falling freely under gravity has an acceleration of what?
9.8m/s^2
For an object falling through an object what happens?
An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity
What is newtons first law?
If the resultant force acting on an object is zero and:
- the object is stationary, the object remains stationary
- the object is moving, the object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity
Because of newtons first law, what does this result in?
So, when a vehicle travels at a steady speed the resistive forces balance the driving force
So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object
Note:
AQA says that students should be able to apply Newton’s first law to explain the motion of objects moving with a uniform velocity and the objects where the speed and/or direction changes
What is intertia?
The tendency for motion to remain unchanged
What does an objects intertial mass measure?
How difficult it is to change the velocity of an object
What is newtons second law?
Acceleration happens when force happens on an object
(This means that acceleration is proportional to the resultant force though inversely proportional to the mass of the object)
State the equation to find inertial mass / resultant force
Resultant force(N) = Mass(kg) x Acceleration(m/s^2)
F = ma
What is inertial mass defined as?
The ratio of force over acceleration
What is the equation for estimating acceleration
Acceleration(m/s^2) = Change in velocity(m/s) / Time taken(s)
Force(N) = mass(kg) x acceleration(m/s^2)
How do you say estimated speed in short terms?
The squiggly dash
State the maximum legal speed on a single carriageway(m/s),mass(k/g) and acceleration of a family car(m/s^2)
Maximum legal speed on a single carriageway(m/s) - 27
Mass(k/g) - 1600
Acceleration(m/s^2) - 3
State the maximum legal speed on a single carriageway(m/s),mass(k/g) and acceleration of a lorry(m/s^2)
Maximum legal speed on a single carriageway(m/s) - 22
Mass(k/g) - 36000
Acceleration(m/s^2) - 0.4
What is newtons third law?
When two objects interact, the forces they exert on each other are equal and opposite
( If you push something, say a shopping trolley, the trolley will push back against you, just as hard. And as soon as you stop pushing, so does the trolley )
EXAMPLE FOR NEWTONS THIRD LAW
When skater A pushes on skater B (the ‘action’ force), she feels an equal and opposite force from skater B’s hand (the ‘normal contact force). Both skaters feel the same sized force, in opposite directions, and so accelerate away from each other. Skater A will be accelerated more than skater B, though,because she has a smaller mass - remember a = F/m.
EXAMPLE OF NEWTONS THIRS LAW IN EQUILIBRIUM
An example of Newton’s Third Law in an equilibrium situation is a man pushing against a wall. As the man pushes the wall, there is a normal contact fore acting back on him. These two forces are the same size. As the man applies a force and pushes the wall, the wall ‘pushes back’ on him with an equal force.
Why is a book resting on a ground not newtons third law?
As the two forces are different types (normal contact force and weight) and both acting on the book.
What is stopping distance equal to?
Stopping Distance = Thinking Distance(reaction time) + Braking Distance(car)
The greater the speed, the greater the…
Stopping distance
Is everybodys reaction time the same or different?
Reaction times vary from person to person. Typical values range from 0.2s to 0.9s. A drivers reaction time can be affected by tiredness,drugs and alcohol. Distractions may also affect a drivers ability to react
What is thinking distance?
How far the car travels during the driver’s reaction time
What is braking distance?
The distance taken to stop under the breaking force (once the brakes are applied)
( Typical car braking distances are: 14 m at 30 mph, 55 m at 60 mph and 75 m at 70 mph)
What is thinking distance affected by?
1) Your SPEED - the faster you’re going the further you’ll travel during the time you take to past
2) Your REACTION TIME - the longer your reaction time, the longer your thinking distance
3) Drugs
4) Alcohol
5) Distractions
6) Tiredness
What is braking distance affected by?
1) Your SPEED - for a given braking force, the fanter a vehicle travels, the longet it takes to stop.
2) The WEATHER or ROAD SURFACE - if it is wet or icy ,or there are leaves or oil on the road, there is less grip (and so less friction) between a vehicle’s tyres and the road, which can cause tyres to skid.
3) The CONDITION of your IYRES - if the tyres of a vehicle are bad (they don’t have any tread left) then they cannot get rid of water in wet conditions. This leads to them skidding on top of the water
4) How good your BRAKES are - if brakes are worn or faulty, they won’t be able to apply as much force as well-maintained brakes, which could be dangerous when you need to brake hard.
What factors affect stopping distance?
- Icy conditions
- Driving close to other cars ( especially in icy conditions)
- Speed limits
( The longer your stopping distance, the more space you need to leave in front in order to stop safe )
What does braking in a car rely on?
The friction between the brakes and wheels
Why does braking rely on the friction between the brakes and wheels?
When the brake pedal is pushed, this causes brake pads to be pressed onto the wheels.
This contact causes friction, which causes work to be done. The work done between the brakes and the wheels transfers energy from the kinetic energy store of the wheels to the thermal energy stores of the brakes. The brakes increase in temperature
The faster a vehicle is going, the more energy it has in its kinetic stores, so the more work needs to be done to stop it. This means that a greater braking force is needed to make it stop within a certain distance
A larger braking force means a larger deceleration. Very large decelerations can be dangerous were they may cause brakes to overheat (so they don’t work as well or could cause the vehicle to skid.
What are methods used to measure human reaction times
The ruler drop test
Explain the steps for the ruler drop test
- Work with a partner
- Person A holds out their hand with a gap between their thumb and first finger
- Person B holds the ruler with the zero at the top of person A’s hand
- Person B drops the ruler without telling person A and they must catch it
- Repeat 5 times
- Record into a suitable table
READ the example results for the ruler drop test
With noise (cm)
1 - 25
2 - 38
3 - 36
4 - 31
5 - 38
Average - 33.6
Without noise (cm)
1 - 18
2 - 15
3 - 22
3 - 24
4 - 24
5 - 13
Average 18.4
5 - 13
Explain the factors which affect the distance required for road transport vehicles to come to rest in emergencies and the implications for safety
- Poor vehicle conditions
- Poor road and weather conditions
Safety features in vehicles are designed to increase collision times, which reduces the force and so reduces the risk of injury, e.g seat belts stretch, air bag slows you down and crumple zones crumple up easily
Why does the distance required for road vehicles to stop in an emergency varies over a range of typical speeds
The braking distance increases by a factor of four each time the starting speed doubles.
For example, if a car double its speed from 30mph to 60 mph, the thinking distance will double from 9m to 18m and the braking distance will increase by a factor of four from 14m to 56m
What are the forces involved in the deceleration of road vehicles in typical situations on a public road
Large decelerations can cause serious injuries. This is because a large deceleration requires a large force (f=ma)
Safety features in vehicles are designed to increase collision times, which reduces the force and so reduces the risk of injury, e.g seat belts stretch, air bag slows you down and crumple zones crumple up easily
State the equation to find momentum
Momentum(kgm/s) = mass(kg) x velocity (m/s)
p = mv
What is the conversion of momentum?
In a closed system,the total momentum before an event will be the same as after the event
In snooker how is momentum used in it?
Lets take 2 balls as an example.A white ball and a red ball.The red ball is stationary so it has zero momentum.But we hit the white ball so it is moving with a velocity v,so it has a momentum of p = mv.Now the white ball hits the red ball,causing it to move.The red ball has momentum,the white ball continues movies but at a much small velocity (and so at a much smaller momentum.The combined momentum of the red and white ball is equal to the original momentum of the white ball,mv
If the momentum before an event is zero,what will it be after the event?
Zero
(For example in an explosion,the momentum before is zero.After the explosion,the pieces fly off in different direction,so that the total momentum cancels out to zero)
