Forces P2 Flashcards
Define distance
How far an object moves, not including direction
Define displacement
Includes both the distance an object moves measured in a straight line and the direction
Define magnitude
The size of a quantity
Define scalar
Physical quantity with no direction but with size/magnitude
Define vector
Physical quantities with direction along with size/magnitude
Give 5 examples of scalars
Distance, speed, time, energy, mass
Give 4 examples of vectors
Displacement, acceleration, velocity, force
What can vectors be represented by?
Arrows :length represents magnitude of quantity and direction of arrow represents direction of quantity
What do you do if 2 vectors are doing in opposite directions?
Subtract
What do you do if two vectors are going in the same direction?
Add them
What do you do if two vectors are perpendicular?
Use Pythagoras theory
How do you work out angle of the direction when the vectors are perpendicular?
Depending on which trigonometry method, divide one side by the other then do the inverse of the trigonometry method you used with the answer you got
What three things so forces do?
Change an objects shape (stretch or squash), change motion (accelerate/decelerate), change state of rest (start moving)
Why is sandpaper difficult to rub together?
Friction
Why does a parachute fall slowly?
Air resistance increases as the parachute opens (slowing down is descent) however gravity is still acting upon it so the parachute still falls
Why do two blue poles of magnets repel?
Due to magnetic force because they are alike
Why do a blue and red pole of a magnet attract?
Due to magnetic force, they are opposites
What can a ruler do when charged and why?
Pick up bits of paper due to electrostatic force
What are two examples of contact forces?
Air resistance, friction. For a force to be a contact force, objects have to be touching for that force to act
What are two examples of non contact forces?
Gravitational, magnetic
What is Newton’s third law?
When two objects interact with eachother they exert equal and opposite forces on eachother for example when you push on a wall there is a normal contact force acting back on you and these two forces are the same size
What is gravity?
A force between two objects with mass trying to pull them toward each other. Only noticeable when one of the masses is really big e.g a planet or star
What is weight measured in?
Newtons
What is earths gravitational field strength?
10
What is the equation for weight?
Mass x gravitational field strength
What is mass?
The amount of matter an object contains
What is mass measured in?
Kg
What is the relationship between weight and mass?
The more mass an object has the greater its weight
Define centre of mass
The mass of an object may be considered to act at a single point referred to as its centre of mass
Where is centre of mass for flat objects that are symmetrical?
Where all lines of symmetry meet
What are the 4 steps to finding the centre of mass for an irregular shape?
Put hole in one corner of shape and suspend from the stand rod, use a plumb like placed by the card shape to draw a vertical line down it, repeat the procedure hanging it from different corners, test to see if the point where the lines intersect is the centre of mass by resting it on a slender flat surface like the flat end of a pencil to see if it’s stable
Define stability
A measure of how likely it is for an object to topple over when pushed or moved
What two things is stability of an object affected by?
Width of base, height of centre of mass
How can an object be made more stable?
Lower it’s centre of mass, widen its base
Why is a double decker bus likely to topple over?
Centre of mass is outside wheelbase because the car is higher up and less wide so will topple so a resultant moment is produced which topples the car
Why is a sports car unlikely to topple over?
Centre of mass is within wheelbase as the car is lower and wider so won’t topple. No resultant force is produced to topple the car
How could a tractor be redesigned to be made more stable?
Place the engine lower as this has a high mass therefore this would lower the objects centre of mass
Define resultant force
A force produced in a certain direction due to a combination of forces
What happens if the resultant force is 0?
The object will remain stationary of move with constant velocity because it is at equilibrium
What happens if the resultant force is not 0?
The object will accelerate
What is Newton’s first law of motion?
When forces are balanced an object will remain at rest or moving with a constant velocity unless acted on by an external force
What is Newton’s second law of motion?
If forces are unbalanced an object will accelerate in the direction of the resultant force. The acceleration will be directly proportional to the resultant force
Equation for force, mass and acceleration
Force = mass x acceleration
When does stretching happen?
When an object is pulled
When does compression happen?
When the material or object is squashed
What happens if you pull or squeeze too hard?
The object may not return to its original size and shape or it may snap -no longer elastic deformation. Until this point hookes law applies
What is hookes law?
The extension of an object is directly proportional to the force applied, providing that its elastic limit has not been reached
What is the relationship between extension and mass/force?
Extension increases as mass/force increases
What is the independent variable in hookes law experiment?
Mass/force
What is the dependant variable in hookes law experiment?
Extensión
What is the control variable in hookes law experiment?
The place you are measuring from
Equation for acceleration
Change in velocity (m/s) divided by time taken
What three things can forces do?
Change the speed, direction or shape of an object
What is acceleration?
Change in velocity measured in m/s^2
What is the acceleration due to gravity?
9.81m/s^2
Why do all objects exert a force when they fall?
They accelerate downward
What is weight?
A force an object exerts due to its mass (directly proportional relationship) and the strength of the gravitational field the object is in. You can think of it as acting from a single point AKA an object’s centre of mass
What is gravity on earth?
9.81N
What is inertia?
The tendency for an object to stay at rest or to continue in uniform motion (constant velocity) Inertial mass refers to the measure of difficulty in changing an objects velocity and can be found by mass = force divided by area
What are the five steps to the Hookes Law experiment?
Use a ruler to measure initial length of spring, attach the spring to the clamp stand and add a 10g mass to the free end, allow the spring to come to a rest and measure the new length, calculate extension by taking the original length from the new length, repeat method from a total mass of up to 100g
What is the relationship between resultant force and acceleration?
The greater the force, the greater the acceleration
What are the seven steps to the acceleration and resultant force practical?
Measure intervals on the bench and draw straight lines at each, attach the bench pulley to the end of the bench, tie a length of string to a trolley, pass the string over the pulley and attach the weight stack to the other end of the string making sure the string is horizontal and in line with the trolley, hold the trolley at start point and attach the full weight stack (1.0N) to the weight stack, release the trolley as you start the stopwatch and press the stop watch (lap mode) at each interval and for the final time at 100cm, repeat steps 5-8 for decreasing weights on the stack and place the weights you remove onto the top of the car
What are 3 errors with our acceleration and resultant force practical?
Different people have stop watches so different reaction times, friction between trolley and bench (this could be reduced by using an air track which is a track hovering a trolley on jets of air), trolley not travelling in straight line
Independent variable of the acceleration and resultant force practical?
Force
Dependant variable of the acceleration and resultant force practical?
Time
Control variable of the acceleration and resultant force practical?
Distance
What is the relationship between mass and acceleration?
The greater the mass, the greater the acceleration
What are the six steps to the acceleration and mass practical?
Measure intervals on the bench and draw straight lines at each, attach the bench pulley to the end of the bench, tie a length of string to a trolley, pass the string over the pulley and attach the weight stack to the other end of the string making sure the string is horizontal and in line with the trolley, hold the trolley at start point and attach the weight stack (1.0N) to the trolley and put a 100g mass on the car, release the trolley as you start the stopwatch and press the stop watch (lap mode) at each interval and for the final time at 100cm, repeat steps 5-8 for increasing masses on the car
What is an error if the acceleration and mass practical?
Different reaction times because 8 people were using the stop watches
Independent variable in the acceleration and mass practical?
Mass
Dependent variable in the acceleration and mass practical?
Time
Control variable in the acceleration and mass practical?
Distance
How does a helmet protect a person?
It is designed to increase impact time. With a longer impact time, the rate of change of momentum per second is reduced. The helmet therefore reduces the force of the impact
How does a seatbelt protect a passenger?
Stops its wearer from continuing forwards when the car suddenly stops. The decelerating force is reduced by wearing a seatbelt. The seatbelt acts across the chest so spreads the force out. Without a seatbelt, force acts on the persons head as it hits the windscreen
How do air bags protect a passenger?
Inflated air bags spread out the force of an impact across the upper part of a persons body and increases impact time. The effect of the force is lessened compared with just using a seatbelt
How do crumple zones, side inspect bars and collapsible steering wheels make cars more safe?
Increase the impact time to lessen the force of an impact, increase stopping distance
What can happen if you break too harshly?
Skidding can occur which can result in a collision
What is stopping distance?
The average smallest distance for a vehicle to make an emergency stop
Equation for stopping distance
Thinking distance + braking distance
What is the relationship between thinking distance and speed?
Proportional
What is the relationship between braking distance and the square of the speed?
Proportional
How are playgrounds made more safe?
Fitted with cushioned surfaces which makes the duration of the impact longer
Identify 8 safety features in cars
Traction control, safety cage, electric windows, air bags, paddle shift controls, anti lock brakes, seatbelts, crumple zones
What do seatbelts, air bags and crumple zones all do?
Change shape to absorb energy and reduce injuries
How do most safety devices work?
Increase distance needed to stop, increase time to stop, decrease acceleration
What does a force lead to?
Change in momentum
Equation for impulse
Force x time = change in momentum
Equation for momentum
Mass x velocity
Unit of impulse
Ns
Unit of momentum
kg m/s
Identify 4 factors which affect thinking distance
Poor visibility, caffeine speeds it up, tiredness slows it down, drugs and alcohol slow down reaction time
Define thinking distance
The distance you travel while reacting to a hazard
Identify 4 factors which affect braking distance
Road material, speed, quality of brakes, weather (icy or wet roads)
Define braking distance
The distance you travel while the braking force is applied
Describe terminal velocity
When falling objects first set off the force of gravity is much more than the frictional force slowing them down so they accelerate, as the speed increases the friction builds up which gradually reduces the acceleration until eventually the frictional force is equal to accelerating force so resultant force is zero, it will have reached its maximum speed/terminal velocity and will fall at a constant speed
What is the acceleration of an object in terminal velocity?
Zero
What does an object have to fall through to be in terminal velocity?
Resistance which equals the gravitational pull on the object
What is meant by conservation of momentum?
The total momentum before and after a collision or explosion is equal, provided no external forces act (in a closed system)
Define closed system
One where no external forces act
Define explosion
Two things which are at rest push away from eachother
How does momentum stay at 0 when two things which are at rest push apart?
The momentum must be equal in opposite directions
How much momentum does an object at rest have?
None because momentum is the product of velocity and mass
Define momentum
A vector quantity which is the product of velocity and mass
Why do two teenagers riding identical skateboards, moving at the same speed with the same mass not have the same momentum?
Moving in opposite directions
Why does a skateboard slow down and it’s kinetic energy decreases when it recoils?
Work is done against friction so some energy is transformed into kinetic
What is the connection between thinking distance and speed?
Higher the speed, longer the thinking distance
Why is data from the Highway Code reliable?
Data will have been checked and repeated
State two ways a spring changes after a mass is suspended from it
Increased length of coils, increased gap between coils
Why should you give mean values to two significant figures?
Makes the data easier to plot
How does gravitational field strength vary?
It is stronger the closer you are to the mass causing the field and is stronger for larger masses
What forces act on a skydiver?
His weight acts on him pulling him towards the ground and air resistance (drag) acts on him in the opposite direction to his motion
What forces act on a moving car?
Normal contact force upwards, weight downwards, driving force forwards, friction backwards
What happens when a force is applied to an object?
To make something move a force must be applied, the thing applying the force needs a source of energy, the force does work to move the object and energy is transferred from one store to another
Why does the temperature of an object increase when pushing it along a rough surface?
Work is being done against frictional forces so energy is transferred to the kinetic energy store of the object because it starts moving but some is also transferred to thermal energy stores due to friction which causes overall temp of the object to increase (like rubbing your hands together)
Equation for work done
Force x distance moved along the line of action of the force
What is a scale diagram like when the resultant force is 0?
On a scale diagram the tip of the last force you drew should end where the tail of the first force you drew begins so for three forces the diagram forms a triangle
How do you deal with a force acting at an awkward angle?
Resolve it (split it into two components acting at right angles to each other) and measure these lengths then convert to N using a scale
How can an object be stressed, compressed or bent?
More than one force needs to be acting on it otherwise the object just moves in the direction of the applied force instead of changing shape. Work is done when an object stretches or compresses and causes energy to be transferred to the elastic potential energy store of the object
What is elastic deformation?
If the object can go back to its original shape and length after the force has been removed. Objects which can be easily deformed are elastic objects and all the energy from work done is transferred to the object’s elastic potential store when it is elastically deformed
What is inelastic deformation?
If an object doesn’t return to its original shape and length after the force has been removed
How can a spring be stretched?
If it is supported at the top and a weight is attached at the bottom
What does spring constant depend on?
The material you are stretching -the stiffer the material, the greater the spring constant
Equation for compression
Force in N = spring constant in N/m x compression in m (difference between natural and compressed lengths)
What is the limit of proportionality?
There is a maximum force which can be applied to an object for it to keep increasing proportionally
What must you be careful with when doing calculations with springs?
Measurements must be in m for extension not cm or mm
What should you do before the Hooke’s law experiment?
Do a pilot experiment to make sure your masses are an appropriate size for your investigation. Use an identical spring to the one you will be using and load it with 5 masses one at a time and measure increase in extension each time, if any of them have a bigger increase than the previous ones you’ve gone past the spring’s limit of proportionality so you will have to use smaller masses for your real experiment otherwise you won’t get enough measurements for the graph
How can you check whether the deformation is elastic or inelastic in the Hooke’s Law experiment?
Remove each mass temporarily to check if the spring goes back to the previous extension
When can you have objects travelling at a constant speed but with a changing velocity?
When an object is changing direction but keeps the same speed, An object moving in a circle at a constant speed has a constantly changing velocity as the direction is always changing
What are the typical speeds for a person walking, running and cycling?
Walking 1.5m/s, running 3m/s, cycling 6m/s
What are the typical speeds for a car, train and plane?
Car 25m/s, train 55m/s, plane 250m/s
Identify 4 things which affect a person’s speed
Fitness, age, distance travelled, terrain
Identify 3 things which affect wind speeds
Temperature, atmospheric pressure, whether there are large buildings nearby
Define uniform acceleration and what is the equation?
Constant acceleration. final velocity squared - initial velocity squared = 2 x acceleration x distance
How does friction cause things to slow down?
If an object has no force propelling it along it will always slow down and stop due to friction, friction always acts in the opposite direction to movement. To travel at a constant speed frictional forces need to balance the driving force. You get friction when two forces are in contact or when an object passes through a fluid (drag)
What is drag?
Resistance you get in a fluid (gas or liquid). Air resistance is a type of drag which is the frictional force produced by air acting on a movement object
What is the most important factor in reducing drag?
Keeping the shape of the object streamlined meaning it is designed to allow fluid to flow easily across it however parachutes work in the opposite way as they want as much drag as they can get
What happens to friction when speed increases?
IT increases so the engine has to work harder to maintain a steady speed
How do shape and area affect terminal velocity?
The less streamlined an object, the lower its terminal velocity so objects with larger surface areas tend to have lower terminal velocities as there is more air resistance acting on them so they spend less time accelerating and don’t speed up as much because the air resistance is large enough to equal the accelerating force
What is the difference between resultant force on a stationary object and resultant force on a moving object?
The stationary object will remain stationary whereas the moving object will move with a constant speed
What five forms can acceleration in the direction of a resultant force take?
Starting, stopping, speeding up, slowing down, changing direction
What is acceleration inversely proportional to?
Mass of an object because Newton’s Second Law shows that force = mass x acceleration
What are the two pairs of contact forces when a book rests on a table?
Weight of book is pulled down by gravity and the book pulls back up on the earth, the normal contact force from the table pushing up on the book and the normal contact force from the book pushing down on the table
How can Newton’s Second Law explain the results of the acceleration and mass/force investigation
Force=weight of the hanging masses, mass=mass of whole system, acceleration=acceleration of the system. By adding masses to the trolley the mass of the whole system increases but the force applied stays the same so this should lead to a decrease in acceleration. By transferring masses to the hook you are increasing the accelerating force without changing the mass of the whole system so increasing force should lead to increase in acceleration
How do icy conditions affect safety?
Ice increases chance of skidding and increase stopping distance so driving close to other cars is dangerous. The longer the stopping distance, the higher the risk of crashing into whatever is in front of you so the more space you need to leave in front in order to stop safely
What is the typical reaction time?
Between 0.2 and 0.8 seconds
How can you calculate reaction time?
Sit with your arm resting on the edge of a table to stop it moving up and down during the test, get someone to hold a ruler so it hangs between you r thumb and finger and lines up with zero, without giving any warning the person holding the ruler drops it and the other person tries to catch it by closing their thumb and forefinger as quick as possible, the longer the distance to where the ruler was caught the longer the reaction time, you can calculate how long the ruler falls for because acceleration due to gravity is constant (9.81m/s^2), do lots of repeats and calculate mean reaction time, the results will be better if the ruler falls straight down so you could add a blob of modelling clay to the end to stop it waving about
What happens when you brake?
When the brake pedal is pushed the brake pads are pressed onto the wheels which causes work to be done which transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes so they increase in temperature. The faster an object is going, the more energy it has in its kinetic energy store so the harder it is to stop it within a certain distance
Why is a larger braking force dangerous?
Means a larger deceleration which can cause brakes to overheat so they don’t work as well or cause a vehicle to skid
What happens with momentum when a car crashes into the back of a parked car?
The cars lock together and move in the direction the original moving car was travelling but at a lower velocity. The mass of the moving object has increased but its momentum is equal to momentum before the collision so there is a decrease in velocity
What happens with momentum after an explosion?
Pieces of the object fly off in all directions so its mass decreases so the total momentum cancels out to zero even though the velocity has increased
