P5 Forces Flashcards
what are the two components of a vector
magnitude and direction
D A M V F
what are the vector quantities
force,velocity,displacement,acceleration, and momentum
S T T D M
what are the scalar quantities
speed,distance,mass,temperature,time
what is the relationship between mass and weight
they are directly proportional
what is work done
when a force moves an object through a distance
how do you know if an obejct is in equilibrium
if the forces are balanced
what is the difference between elastic and inelastic deformation
if an object has been elastically deformed it can go back to its original shape and length after the force has been removed. if an object has been inelastically deformed it doesn’t go back to its original shape after the force has been removed.
what is the relationship between extension and force
they are directly proportional
what is the limit of proportionality
the limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally. it is shown on a graph on a point marked P.
what is the method for the extension of a sting practical
1) measure the natural length of the string (when no load is applied) with a millimetre ruler clamped to the stand. make sure you take the reading at eye level and add a marker to the bottom of the string to make it more accurate.
2) add a mass to the string and allow it to come to rest. record the mass and measure the new length of the sting. the extension is the change in the length
3) repeat this until you have added at least 6 weights to the string
4) plot the results on a force-extension graph
what is a moment
the turning effect of a force. a larger force or a larger distance would mean a larger moment.
how do you get the maximum moment of a force
push at right angles
how do you know if an object is balanced
if the total anticlockwise moment equals the total clockwise moment about a pivot
what do levers do
levers increase the distance from the pivot at which the force is applied. this means that levers make it easier to do work
what happens when a fluid is fully submerged in water
the pressure of the fluid exerts a force on it from every direction. pressure increases with depth,so the force exerted on the bottom of the object is larger than the force acting on the top of the object. this causes a resultant force, also known us upthrust.
what happens when the weight of an object is equal to the upthrust
the object floats because the forces are balanced. if the weight is greater than the upthrust then the object will sink. this means that whether or not an object will sink or float, depends on its density
what happens when an object is less dense than the fluid it is in
it displaces a volume of fluid that is equal to its weight before it is completely submerged, at this point the weight is equal to the upthrust so the object floats
what happens to atmospheric pressure when the height is increased
the atmospheric pressure has decreased, this is because as the altitude increases, the atmosphere gets less dense so there are fewer air molecules that are able to collide with the surface.
is distance a scalar or a vector quantity
scalar
is displacement a scalar or a vector quantity
vector
is speed a scalar or a vector quantity
scalar
is velocity a scalar or a vector quantity
vector
what are the average speeds for:
a person walking
a person running
a person cycling
a car
a train
a plane
a person walking - 1.5m/s
a person running - 3m/s
a person cycling - 6m/s
a car - 25m/s
a train - 55m/s
a plane - 250m/s
what is acceleration
the rate of change of velocity. negative acceleration is known as deceleration, when this happens you are slowing down instead of speeding up.
what is the average acceleration due to gravity
9.8m/s
what do the following things tell you on a distance time graph
- gradient
- flat lines
- straight upwards lines
- downwards curves
- upwards curves
- gradient = the speed ( the steeper the graph, the faster the object is going
- flat lines = the object is stationary ( it has stopped)
- straight upwards lines = the object is travelling at a steady speed
- downwards curves = the object is accelerating
- upwards curves = the object is decelerating
what do the following things tell you on a velocity time graph
- gradient
- flat lines
- straight upwards lines
- straight downwards lines
- curves
- gradient - the acceleration ( because acceleration is the change in velocity divided by time)
- flat lines - the object is moving at a constant speed
- straight upwards lines - the object is accelerating at a constant speed
- straight downwards lines - the object is decelerating at a constant speed
- curves - the objects deceleration or acceleration is changing
how does friction act according to movement
it always acts in the opposite direction
where does friction occur
in two surfaces in contact or when on object passes through a fluid (drag)
what is drag
the resistance you get in a fluid that stop things moving smoothly
what is terminal velocity
when an object reaches its maximum speed and falls at a constant rate
describe how terminal velocity works in falling objects
1) when falling object first set off, the force of gravity is much greater than the frictional force slowing then down, so they accelerate
2) as the speed increases the friction builds up
3) the gradually reduces the acceleration, until eventually the frictional force is equal to the accelerating force ( the resultant force is zero)
4) at this point it will have reached its terminal velocity and will fall at a steady speed
how can the terminal velocity be determined
by the drag ( e.g air resistance, in comparison to its weight )
how do parachutes decrease the terminal velocity of a skydiver
without a parachute, a skydivers terminal velocity is about 120mph, once the parachute is applied the air resistance becomes much greater which decreases the terminal velocity to about 15mph which is a safer speed to land at.
what is newtons first law of motion
if the resultant force on a stationary object is zero, the object will remain stationary. the the resultant force on a it will carry on moving at the same velocity
what is newtons second law of motion
the acceleration of an object is both proportional to the resultant force acting on an object and inversely proportional to the mass of the object
what is inertia
the tendency for motion to remain unchanged. an objects intertial mass measures how difficult it is to change the velocity of an object. intertial mass is the ratio of force over acceleration
what is newtons third law of motion ( the law jefferson mentions in hamilton)
when two objects interact, the forces they exert on one another are equal and opposite ( every action has an equal opposite reaction)
when can you NOT apply newtons third law
when an object is in equilibrium
what is the method for the RPA that investigates the affect of mass/ force on acceleration
1) place a trolley on a level bench and attach the pulley to the end of the bench. then attach a string to the trolley whilst ensuring that the mass does not touch the ground when moving through the light gates
2) attach a 20g mass hanger to the paper clip so that it hangs over the edge of the pulley and hold the trolley at the start.
3) place the 100g of masses onto the hanger, let the trolley go and record the acceleration using the data logger.
4) repeat until you have 5 readings and calculate a mean
5) remove 20g of mass from the trolley and add it to the mass hanger
6) plot a graph of acceleration against force applied. draw a line of best fit and calculate the gradient to find the mass of the system.
how does newtons second law explain the results of the RPA of mass/force on acceleration
by adding masses to the trolley, the mass of the whole system increases but the force applied to the system stays the same. this leads to a decrease in the acceleration of the trolley
by transferring masses to the hook, you are increasing the accelerating force without changing the mass of the whole system. so increasing the force should lead to an increase in the acceleration of the trolley.
what is thinking distance
how far the car travels during the drives reaction time ( the time between seeing a hazard and applying the brakes.
what is braking distance
the distance taken to stop under the braking force
what factors affect thinking distance
the drivers speed - the faster you are going the longer you will take to react
the drivers reaction time - the longer your reaction time, the longer your thinking distance. reaction time can be affected by tiredness or being under the influence of drugs or alcohol.
what factors affect braking distance
speed - the faster a vehicle travels the longer it takes to stop
the road conditions - if it is icy or wet there is less grip between the road and the tires which can cause skidding
the condition of the tires - if the tires are bald then they cannot get rid of water properly which leads to skidding
the conditions of the brakes - if they are worn or faulty then they work be able to apply as much force which means it takes longer for you to stop
how do brakes work
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 stores of the wheels to the thermal energy stores of the brakes. this causes the brakes to increase in temperature. the faster a vehicle is going the more energy it has in its kinetic energy store so there is more work that is needed 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. large decelerations can be dangerous becasue they may cause brakes to overheat or could cause the vehicle to skid
what is momentum
the effect of mass in motion
what is momentum like in a closed system
the total momentum before a collision is the same as the total momentum after the event. this is called conservation of momentum.
what is the relationship between forces and the rate of change of momentum
the force causing the change is equal to the rate of change of momentum
