Physics: Motion, Forces And Conservation Of Energy Flashcards
What do vector quantities have?
Magnitude (size) and direction
What do scalar quantities have?
Magnitude (size)
Two bikes are travelling in different directions. Would their velocity be different or the same?
Different, because they’re moving in different directions
What is distance? Scalar or vector?
How far an object has moved . Scalar.
What is displacement? Scalar or vector?
Distance and direction in a straight line for a starting point to finishing point. Vector
Someone walks 5m north then 5m south. What is their distance? What is their displacement?
Distance = 10m, displacement = 0m
What is speed? Scalar or vector?
How fast you’re going. scalar.
What is velocity? Scalar or vector?
How fast you’re going (speed) in a given direction
Give an example of velocity
30mph north
If something has a constant speed but changing velocity, what is it doing?
Traveling at the same speed but changing direction
For an object travelling at a constant speed, what is the formula?
(Average) speed = distance traveled/ time
Units for distance travelled?
M
Units of (average) speed?
m/s
Units for time?
s
What is the typical average speed for walking? Give answer in m/S and km/h
1.4m/S, 5km/h
What is the typical average speed for running? Give answer in m/S and km/h
3M/s, 11km/h
What is the typical average speed for cycling? Give answer in m/S and km/h
5.5m/S, 20km/h
What is the typical average speed for cars in a built up area? Give answer in m/S and km/h
13m/S, 47km/h
What is the typical average speed for cars on a motorway? Give answer in m/S and km/h
31m/S, 112km/h
What is the typical average speed for trains? Give answer in m/S and km/h
55m/S, 200km/h
What is the typical average speed for aeroplanes? Give answer in m/S and km/h
250m/S, 900km/h
What is the typical average speed for ferries? Give answer in m/S and km/h
15m/S, 54km/h
What is the typical average speed for wind speed? Give answer in m/S
5-20m/S
What is the typical average speed for speed of sound in air? Give answer in m/S
340m/S
What is uniform acceleration?
Speeding up or slowing down at a constant rate
What is acceleration? Two ways of saying this.
How quickly you’re speeding up, change in velocity in a certain amount of time
What is the formula for average acceleration of an object?
Acceleration = (change in velocity)/time
How do you work out initial velocity?
Final velocity - Initial velocity
What are the units for acceleration?
m/s^2
What are the units for change in velocity?
m/S
Something is slowing down. What will we know about the change in velocity?
It’ll be negative
A car is travelling 15m/S, when it collides with a tree and comes to a stop. Estimate the deceleration of the car.
1) estimate how long it would take the car to stop.
Car comes to a stop in ~1s
2) put this number into acceleration equation
a=(v-u)/t
a=(0-15)/1
3) car has slowed down, acceleration is negative
=-15m/s^2
Deceleration = 15m/s^2
What can we say about acceleration due to gravity in objects that are in a free fall?
Acceleration due to gravity is uniform/constant
What is acceleration due to gravity equal to? What else does ‘‘tis value represent?
Roughly 10m/s^2
Gravitational field strength
What is the equation for uniform acceleration?
Final velocity - initial velocity = 2 x acceleration x distance
V^2-u^2=2xaxx
A van travelling at 23m/S starts decelerating uniformly at 2.0m/s^2 as it heads towards a built up area 112m away. What will it’s speed be when it reaches the built up area?
1) rearrange equation so v^2 is on one side
V^2=u^2+ (2 x a x X)
2) put in numbers - a is negative due to declaration
V^2 = 23^2 + (2 x -2.0 x 112)
=81
3) Square root the whole thing
V= square root of 81 = 9m/S
What do distance/time graphs tell you?
How far something has travelled + speed of an object
What do the different parts of a distance/time graph show?
The motion of the object
What does the gradient (slope) at any point on a distance time graph tell us?
The speed of an object
If a line is flat on a distance time graph, what does it show?
It has stopped
If a line is steep on a distance time graph, what does it show?
Going faster
If a line is curved on a distance time graph, what does it show?
Acceleration
If a curve is getting steeper on a distance time graph, what does it show?
Speeding up
If a curve is levelling off (flattening) on a distance time graph, what does it show?
Slowing down
If the distance time graph is a straight line, what is the speed?
Equal to the gradient of the line
What is the equation for gradient of a line?
Speed=gradient= change in vertical/change in horizontal
If a distance/time graph is curved, how do we find the speed at a certain time?
Draw a tangent to the curve at that point, and then find the gradient of the tangent
What is a tangent?
A line that is parallel to the curve at that point
How do we calculate average sheep of an object when it has a non uniform motion (accelerating)?
Divide total distance traveled by the time it takes to travel that distance.
Sketch a distance/time graph for an object that initially accelerates, then travels at a constant speed, then decelerates to a stop
Continuous line that initially curves upward, which curves downward at the end until it becomes horizontal. Straight middle section.
What kind of gradient can velocity/time graphs have?
Positive or negative gradient
What is shown on a velocity/time graph?
How an objects velocity changes over time
What does the gradient on a line on a velocity time graph tell us?
Acceleration, since acceleration=change in velocity/time
On a velocity time graph, what do flat sections represent?
A steady speed
What can we say about steepness the velocity/time graph in relation to acceleration.
Steeper the graph, the greater the acceleration or deceleration
On a velocity time graph, what do uphill ( / ) sections show?
Acceleration
On a velocity time graph, what do downhill ( \ ) sections represent?
Deceleration
On a velocity time graph, what does a curve represent?
Changing acceleration
If a velocity/time graph is curved, how can we work out the acceleration at that point?
Use a tangent to the curve
Where on a velocity time graph is the distance travelled represented?
The area under the graph
What is the area under any section of a velocity/time graph (or all of it) equal to?
Distance travelled in that time interval
A velocity/time graph has bits where the accelerations constant. How can we work out the area under the graph?
Split it into rectangles and triangles
A stationary car starts accelerating increasingly for 10s until it reaches a speed of 20m/S. It travels at this speed for 20 seconds until the driver sees a hazard and brakes. He decelerates uniformly, coming to a stop 4s after braking. Draw the velocity/time graph for this journey. Using the graph, calculate the deceleration of the car when it breaks.
Upward curved acceleration line to 20m/S, straight line representing steady speed, straight line representing uniform deceleration a(cceleration)=gradient of the line =change in vertical/change in horizontal =(0-20)/(34-30) =-20/4=-5m/s^2 Deceleration = 5m/s^2
What is the simple way of putting Newton’s first law?
A resultant force is needed to make something start moving, speed up or slow down.
What happens to a stationary object of the resultant force on it is zero?
Object will remain stationary
What happens to a moving object of the resultant force on it is zero?
It’ll carry on moving at the same velocity (same speed and direction)
What will a non-zero resultant force always do?
Produce acceleration or deceleration in the direction of the force
What are the five different forms acceleration of an object with a non zero resultant force acting on it can take?
Starting, stopping, speeding up, slowing down ad Changing direction
What is the simple way of putting Newton’s second law?
Acceleration is proportional to the resultant force
What can we say about how size of resultant force on an object relates to its acceleration
Larger resultant force = more acceleration
What can we say about how mass of an object relates to its acceleration
Larger mass = accelerate less
What is the formula for resultant force?
Resultant force = mass x acceleration
What is mass measured in?
Kg
What is the general rule about large deceleration? Why is this the case?
THey can be dangerous, because large deceleration requires
Large force
How can the force of an object in a large deceleration be lowered?
Slowing the object down over a longer time (decreasing deceleration
What do safety features In vehicles do to reduce risk of injury?
Increase collision time, decreasing force
What are the three main safety features of a car and how do they work?
Seat belts - stretch slightly
Ari bags - slow you down gradually
Crumple zones - crumple up easily in a collision, increasing time taken to stop
Estimate the resultant force acting on a car stopping quickly from 15m/S
1) estimate deceleration of car
Car comes to a stop in ~1s
a=(v-u) divided by t= (0-15) divided by 1 = -15m/s^2
2) estimate mass of car
~1000kg
3) put these numbers into Newton’s second law
F=mxa
=1000x-15=-15 000N
Force is negative because it acts in opposite direction to motion of car
How do large decelerations affect breaks of car
Ma cause breaks to overheat and cause vehicle to skid
Find the resultant force needs to accelerate an 80kg man on a 10kg bike at 0.25m/s^2
F=ma=(80+10)x0.25
=22.5N
What is mass?
The amount of ‘stuff’ in an object. Will have the same value anywhere in the universe.
What type of quantity is mass? What measured in? What measured with?
Scalar quantity, measured in kilograms with a mass balance
What is weight?
The force acting on an object due to gravity (pull of gravitational force on the object)
What is weight close to Earth caused by?
Caused by the gravitational field around earth
What is weight measured in?
Newton’s
How can we think of weight? (Acting)
Force acting from a single point on the object, called its centre of mass (point we assume whole mass is concentrated at)
What is used to measure weight?
A calibrated spring balance (newton meter)
What is the equation for weight?
Weight = mass x gravitational field strength
SI Units for gravitational field strength?
(N/Kg)
What can we say about gravitational field strength and location + object size?
It varies with location. Stronger the closer you are to the mass causing the field. Bigger object = stronger field
What is the weight in Newton’s of a 2.0kg chicken on earth?
G=10N/Kg
W=m x g = 2.0 x 10 = 20N
A chicken has a weight of 16N on a mystery planet. What is the gravitational field strength of the planet?
g= w / m
= 16 / 2.0
=8.0N/kg
What is velocity?
Both the sped and direction of an object
What can we say about direction, velocity and acceleration for an object travelling in a circle at a constant speed?
Constantly changing direction, constantly changing velocity. It is accelerating
What can we say about resultant force for an object travelling in a circle at a constant speed?
It acts towards the centre of a circle
What is centripetal force?
The force that keeps something moving in a circle
What are the five steps of the ‘Investigating the motion of a trolley on a ramp’ practical?
1) measure the mass of trolley, unit masses and hanging hook. Measure length of piece of card that interrupts light gates
2) set up apparatus: ramp up at one end, trolley on high end, light gate in front of trolley and one at low end, string attached to trolley goes through pulley on low end, hanging mass on hook on end of string
3) adjust hight of ramp so trolley jut starts to move. Mark line on ramp jut before first light gate.
4) attach trolley to hanging mass by string. Hold trolley still at start line, then let go so it rolls down ramp
5) using results from light gate, you can calculate acceleration of trolley
In the ‘Investigating the motion of a trolley on a ramp’ practical, what do. light gates record?
Initial speed of trolley as it begins to move (roughly 0m/S) (first light gate), final speed of trolley (second light gate), time it takes the trolley to travel between two light gates
In the ‘Investigating the motion of a trolley on a ramp’ practical, what is the purpose of adjusting ramp so trolley just begins to move?
Force due to gravity caused by hanging mass will be main cause of trolley accelerating
In the ‘Investigating the motion of a trolley on a ramp’ practical, how do we investigate effect of trolleys mass?
Add masses one at a time to trolley. Keep mass on hook constant so accelerating force is constant
In the ‘Investigating the motion of a trolley on a ramp’ practical, how do we investigate effect of the accelerating force?
Start with all the masses loaded onto the trolley and transfer the masses to the hook one at a time
In the ‘Investigating the motion of a trolley on a ramp’ practical, what can we estimate about what happens when the accelerating force increases? What does this mean?
As accelerating force increases, the acceleration increases for the given trolley mass. So, force and acceleration are proportional
In the ‘Investigating the motion of a trolley on a ramp’ practical, what can we estimate about what happens when the mass of the trolley increases? What does this mean?
It’s acceleration decreases, so mass and acceleration are inversely proportional
What are the four ways to measure distance and time?
Light gates, rolling tape measures and markers, stopwatch, video frames
Wat is inertia?
Until acted on by a resultant force, objects at rest stay at rest and objects moving at a constant velocity stay at that velocity. This is called inertia
What does inertial mass measure?
How difficult it is to change the velocity of an object
How do you find inertial mass?
M = f / a, The ratio of force over acceleration
What is Newton’s third law?
When two objects interact, the forces they exert on each other are equal and opposite
What can you say about a shopping trolley pushing back on you when you push against it?
The trolley is pushing back on you just as hard as you’re pushing the trolley
Two skaters meet, and push their hands against each other. Skater A has a mass of 55kg. Skater B has a mass of 65kg. Describe the forces taking place, and how this will effect acceleration
Skater A pushes on skater B, this is the action force.She feels an equal and opposite force from skater Bs hand, this is the normal contact force. They both feel the same sized force, in opposite directions, so accelerate away from each other.
Skater A will accelerate more than skater B, because she has a smaller mass, and a = f/m
There is a book on the table. Describe the forces in action here, as well as forces due to Newton’s Third law.
Weight of book pulls book down, normal reaction force from table pushes book up. Forces are equal - they’re in equilibrium.
Newton’s third law force pairs:
Book pulled down by weight due to gravity of Earth, book pulls up on Earth
Normal contact force from Table pushes up on book, normal contact force from book pushing down on table
What is he equation for momentum?
Momentum = mass x velocity
What is momentum a product of?
Ass and velocity
Give the si units for momentum
Kg m/S
Give the si units for mass
Kg
Give the si units for velocity
m/s
How can we say mass and velocity relates to amount of momentum
Greater mass = greater velocity = more Monteur,
What type of quantity is momentum?
Vector
A 50kg cheetah is running at 60m/S, calculate its momentum
P = m x v = 50 x 60
= 3000 kg m/S
A boy has a mass of 30kg and a momentum of 75kg m/S. Calculate his velocity
V = p/m = 75 / 30 = 2.5m/S
In a closed system, what can we say about total momentum before and after an event? What is this called?
Total momentum before = total momentum after. This is conservation of momentum.
Explain how conservation of momentum acts in one snooker ball hitting a stationary snooker ball
Red ball is stationary, has 0 momentum. White ball is moving with velocity v, so has a momentum of p = m x v
White ball hits red ball, causing it to move. Red ball has momentum. White ball continues moving with slower velocity, and so slower momentum. Combine momentum or red and white ball is equal to original momentum of white ball, m x v
A 1500kg car, travelling at 25m/S, crashes into the mass of a parked car. Parked car has mass of 1000kg. The Two cars lock together and continue moving in same direction as original moving car. Calculate velocity that the two cars move with.
1) calculate momentum before the collision
P = m x v = 1500 x 25 = 37 500 kg m/S
Total momentum before = total momentum after
2) find the combined mass of the cars
New mass of joined cars = 2500 kg = m
3) rearrange equation to find velocity of cars
4) v = p / m = 37 500 / 2500 = 15m/s
What is the equation to prove that a resultant force on an object causes it to accelerate? What is another way of writing this equation?
Force = mass x acceleration Force = ( mass x change in velocity) / time
Mass x change in velocity is equal to what?
Change in momentum
Give the equation for force using change in momentum
Force = change in momentum / time
What is si units for change in momentum?
kg m/S
What can we say about how fast a given change in minuets happens relating to the force causing the change?
Faster change happens, bigger force causing change must be
What can we say about size of acceleration or deceleration compared to force size?
Larger acceleration requires larger force needed to produce it
What is stopping distance of a vehicle? What’s it include?
The distance covered between the driver first spotting a hazard and the vehicle coming to a complete stop. It includes thinking distance and braking distance
What is the equation for stopping distance
Stopping distance = thinking distance + breaking distance
What is thinking distance? What two main things is it affected by?
The distance the car travels in the drivers reaction time (Time between noticing hazard and applying breaks.)
Drivers reaction time and vehicles speed
What is breaking distance? What four things is it affected by?
Distance taken to stop once breaks have been applied
Speed, mass of car, condition of breaks, how much friction is between tyres and road
Give the seven steps of the ruler reaction time practical
1) you Sit with arm resting on edge of table, someone else holds ruler so it hangs between your thumb and forefinger, lined up with zero
2) without warning, ruler is dropped. You catch as quickly as possible
3) measurement on ruler at point where it was caught is how far ruler dropped in time it took to react
4) longer distance = longer reaction time
5) you can calculate reaction time (how long ruler was falling for) because acceleration is a constant and equal to 10m/s^2
6) repeat lots of times and take average distance ruler fell. Use this average in calculations
7) use same ruler each time and have same person dropping it
Give the eight energy stores, and give brief explanation of each
Kinetic - anything moving
Thermal - any object. Hotter = more energy
Chemical - anything that can release energy via chemical reaction
Gravitational potential - anything in a gravitational field (can fall)
Elastic potential - anything stretched
Electrostatic - eg) two charges that attract or repel each other
Magnetic - e.g.) two magnets that attract or repel each other
Nuclear - atomic nuclei release nuclear energy in nuclear reactions
When does energy travel do kinetic energy store? When does it travel away?
Speeding up = transferred to, slowing down = transferred away
What does amount of kinetic energy in the kinetic energy store depend on?
Objects mass and speed
How does mass and speed relate to kinetic energy amounts?
Great mass + high speed = high kinetic energy
What is the equation for kinetic energy?
Kinetic energy = 0.5 x mass x (speed)^2
What are si units for kinetic energy?
J
If you double the mass of something, how does it effect the amount of kinetic energy?
Doubles it
If you double the speed of something, how does it effect the amount of Kinetic energy?
It quadruples (increases by a factor of 4)
A car of mass 1450kg is travelling at 28m/s. Calculate energy in its kinetic energy store, giving your answer to 2s.f
Kinetic energy 0= 0.5 x mass x (speed)^2
= 0.5 x 1450 x 28^2 = 568 400 = 570 000 J (to 2 s.f)
What can we say about a certain energy store of an object that is at any height above the earths surface?
It will have energy in its gravitational potential store
What is the equation for change in gravitational potential energy?
Change in gravitational potential energy = mass x gravitational flies strength x change in vertical height
What is change in vertical height measured in?
Meters
What is gravitational field strength measured in?
N/kg
Give the rules of conservation of energy
Energy can be stored, transferred between stores and dissipated, but it can never be created or destroyed. Total energy of a closed system has no net change
What are the four ways energy can be transferred between stores? Give a brief explanation
Mechanically - force acting on an object and doing work
Electrically - charge doing work against resistance
Heating - enemy transferred from hotter to colder object
By radiation - energy transferred by waves
Explain what’s going on in the energy transfer of a ball rolling up a slope
Ball does work against the gravitational force, so energy is transferred mechanically from the kinetic energy store of the ball to its gravitational potential energy store
Explain what’s going on in the energy transfer of a bat hitting a ball
Bat has energy in kinetic store. Some is transferred mechanically to balls kinetic energy store. Some is also transferred mechanically to the thermal energy store of bat and ball, and surroundings by heating. The rest is carried away by sound
Explain what’s going on in the energy transfer of a rock dropped from a cliff
Assuming there’s no air resistance, gravity does work on the rock, so rock constantly accelerates towards ground. Energy transferred mechanically from rocks gravitational potential energy store to its kinetic energy store
Explain what’s going on in the energy transfer of a car slowing down (without breaking)
Energy In kinetic energy store of car is transferred mechanically (due to friction between tyres and road), then by heating, to thermal energy stores of car and road
Explain what’s going on in the energy transfer of a kettle boiling water
Energy transferred electrically from mains to heating element of kettle, then by heating to the thermal energy store of water
What represents what in an energy transfer diagram?
Boxes = energy stores Arrows = energy transfers
When is energy useful?
When it’s transferred from one store to a useful store
What happens in unuseful energy transfers?
Some of the Input Energy is dissipated or wasted, often to thermal energy stores of the surroundings
What does dissipated mean?
Spread out/lost
Give the calculation for efficiency. How do you change the decimal answer to a percentage?
Useful energy transferred by device / total energy supplied to device = efficiency
Times by 100
A toaster transfers 216 000 J of energy electrically from the mains. 84 000 J of energy is transferred to the breads thermal energy store. Calculate efficiency of the toaster.
Efficiency = useful energy transferred by device / total energy supplied to device = 84 000/ 216 000 = 0.388… = 0.39 = 39%
Why can efficiency never be equal or higher to 1(100%)?
Because some energy is always wasted
What are the two main ways to reduce unwanted energy transfers?
Lubrication and thermal insulation
Draw an energy efficiency diagram (Sankey diagram) to show an electric motor with 80% efficiency. Total energy supplied to motor = 100J
Total energy supplied to motor = 100J
Thick arrow to the right /8/ boxes high labelled ‘useful Energy transferred to kinetic energy stores = 80J
Arrow splitting off downwards, /2/ boxes thick labelled ‘energy wasted to thermal energy shows = 20J
Explain How does lubrication help to reduce unwanted energy transfers?
Reduces energy transfers by friction. When something moves, frictional force acts against it. This transfers energy mechanically (work done by friction) to thermal energy store of objects involved, which is dissipated by heat to surroundings. Lubricants reduce this friction.
Explain how insulation reduces unwanted energy transfers?
Reduces rate of energy transfers by heating. One side of an object is heated, particles in hotter part vibrate, colliding with each other. This transfers energy from kinetic stores. This is thermal conductivity. Thermal insulation slows the rate of this energy transfer
What is thermal conductivity?
How well a material transfers energy by conduction
What are the two non renewable energy resources?
Fossil fuels (coal. Oil, natural gas) and nuclear fuels (uranium, plutonium)
What are 5 positives of non renewable energy resources? Give 3 negatives
Positives: reliable, won’t run out for a while, can be stocked allowing fast response to change in energy demand, cost to extract /fossil fuels/ is low and fossil fuel power plants are cheap to build and run
Negatives: nuclear power plants are expensive and sometimes dangerous. Fossil fuels release carbon dioxide, contributing to global warming. Burning coal + oil causes sulfur dioxide, leading to acid rain.
Give five sources of renewable energy
1) biofuels
2) wind
3) the sun
4) hydro-electricity
5) tides
What are two positives of renewable energy resources? Give one negative
Will never run out, less harmful to environment than non-renewable sources
Don’t provide as much energy and those that rely on weather are unreliable
What are bio fuels? What do they do?
Renewable energy resources created from plant products and animal dung. They can be solid, liquid or gas and can be Burnt to make electricity or run cars.
What is often debated about bio fuels?
Are they carbon neutral, as this is only true if you grow plants or raise animals s the rate you burn things
What are 3 positives and 4 negatives of bio fuels?
Positive: fairly reliable, can be stored/mass produced
Negative: can’t respond to immediate energy demand, cost to refine them is very high, cause of deforestation, increased methane and carbon dioxide emissions.
