Topic 5 - Forces Flashcards
What type of quantity is
force?
(scalar or vector)
vector
Define
vector quantity.
a quantity which has a magnitude and a direction
What are some examples of
vector quantities?
(5)
- force
- velocity
- displacement
- acceleration
- momentum
Define
scalar quantity.
a quantity which only has magnitude and no direction
What are some examples of
scalar quantities?
(5)
- speed
- distance
- mass
- temperature
- time
Define
force.
a push or a pull on an object that is caused by it interacting with something
Define
contact force.
when two objects have to be touching for a force to act
What are some examples of
contact forces?
(3)
- friction
- air resistance
- tension in ropes
Define
non-contact force.
when the objects do not need to be touching for the force to act
What are some examples of
non-contact forces?
(3)
- magnetic force
- gravitational force
- electrostatic force
Define
interaction pair.
a pair of forces that are equal and opposite and act on two interacting objects
What are the
two important effects of gravity
around a planet?
- on the surface of a planet, it makes all things
- it gives everything a weight
What is
mass?
the amount of ‘stuff’ in an object
for any given object this will have the same value anywhere in the universe
Define
gravity.
the pull of the gravitational forces on the object
Define
weight.
the force acting on an object due to gravity
close to earth, this force is caused by the gravitational field around the earth
How does
gravitational field strength vary?
it varies with location
it’s stronger the closer you are to the mass causing the field
it’s stronger for larger masses
What is
weight
dependent 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
What is an object’s
centre of mass?
a point at which you assume the whole mass is concentrated
How is
weight
measured?
a calibrated spring balance
(or a newtonmeter)
How is
mass
measured?
a mass balance (kg)
mass is not a force
What equation links
mass, weight and gravitational field strength?
weight = mass x gravitational field strength
W = mg
W: N
m: kg
g: N/kg
What is the relationship between
weight and mass?
they are directly proportional
What equation links
distance, force and work done?
work done = force x distance
W = Fd
W: joules, J
F: newtons, N
d: metres, m
How do you convert between
joules and newton metres?
1J = 1Nm
How would you use a
scale drawing to find the resultant force?
- Choose a sensible scale.
- Draw all of the forces acting on an object, to scale, ‘tip to tail’.
- Draw a straight line from the start of the first force to the end of the last force (this is the resultant force).
- Measure the length of the resultant force on the diagram to find the magnitude.
- Measure the angle of the resultant force on the diagram to find the direction of the force.
Define
equilibrium.
when all of the forces on an object combine to give a resultant force of zero
When has an object been
elastically deformed?
when the object can go back to its original shape and length after the force has been removed
When has an object been
inelastically deformed?
when the object doesn’t return to its original shape and length after the force has been removed
What equation links
force, spring constant and extension?
force = spring constant x extension
F = ke
F: newtons, N
k: newton metres, N/m
e: metres, m
Define
limit of proportionality.
the maximum force above which a force against extension graph curves, showing that extension is no longer proportional to force
Describe an experiment that investigates the
link between force and extension.
(5 steps)
- Measure the natural length of the spring (when no load is applied) with a millimetre ruler clamped to the stand.
- Add a mass to the spring and allow it to come to rest.
- Record the mass and measure the new length of the spring. The extension is the change in length.
- Repeat this process until you have enough measurements (at least 6).
- Plot a force-extension graph of your results.
it will only start to curve if you exceed the limit of proportionality
How would you interpret a
force-extension graph?
- when the line of best fit is a straight line it means that there is a linear relationship between force and extension (they’re directly proportional), so the gradient of the straight line is equal to the spring constant
- when the line begins to bend, the relationship is now non-linear between force and extension
What is the equation for the
work done in stretching a spring?
(as long as the spring is not stretched past its limit of proportionality)
EPE = (ke^2) /2
(this is equal to the area under a force-extension graph)
EPE: joules, J
k: newton metres, N/m
e: metres, m
Define
moment.
the turning effect of a force
What equation gives the
size of the moment of the force?
moment = force x perpendicular distance from pivot to the line of action of the force
M = Fd
M: newton metres, Nm
F: newtons, N
d: metres, m
How do
levers make it easier for us to do work?
they increase the distance from the pivot at which the force is applied
this means that less force is needed to get the same moment
What is a
gear?
a circular disc with teeth around its edge
What do
gears do?
and how?
they transmit the rotational effect of a force from one place to another
this is because their teeth interlock so that turning one causes another to turn, in the opposite direction
Define
fluid.
a substance that can ‘flow’ because its particles are able to move around
(liquids or gases)
Define
pressure.
force per unit area
What equation allows you to calculate the
pressure at the surface of a fluid?
pressure = force normal to surface / area of that surface
p = F/A
p: pascals, Pa
F: newtons, N
A: metres squared, m^2
How does
density affect pressure?
(in a liquid)
the more dense a given liquid is, the more particles it has in a certain space, this means that there are more particles that are able to collide so the pressure is higher
How does
density vary in a liquid?
it doesn’t
for a given liquid, the density is uniform and it doesn’t vary with shape or size
How does
depth affect pressure
in a liquid?
as the depth of the liquids increases, the number of particles above that point increases, the weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth
What equation allows you to calculate the
pressure at a certain depth due to the column of liquid above?
pressure = density of the liquid x gravitational field strength xheight of column of liquid (the depth)
p = ρgh
p: pascals, Pa
ρ: kg/m^3
g: N/kg
h: metres, m
What is
upthrust?
the resultant force of the force exerted on the bottom of a submerged object and the force acting on the top of the object
(the force on the bottom is larger)
this is equal to the weight of fluid that has been displaced by the object
When does an object
float?
when the upthrust on an object is equal to the object’s weight
(the forces balance)
When does an object
sink?
when the object’s weight is more than the upthrust
What does
an object’s ability to float depend on?
its density
An object that is less dense than the fluid it is placed in weighs less than the equivalent volume of the fluid. This means that it displaces a volume of fluid that is equal to its weight before it is completely submerged.
At this point, the object’s weight is equal to the upthrust, so the object floats.
An object that is denser than the fluid it is placed in is unable to displace enough fluid to equal its weight. This means that its weight is always larger than the upthrust, so it sinks.
How do
submarines make use of upthrust to sink and rise?
Submarines hold large tanks.
To sink, these tanks are filled with water to increase the weight of the submarine (but not the volume) so that it is more than the upthrust.
To rise to the surface, the tanks are filled with compressed air to reduce the weight so that it’s less than the upthrust.
What is the relationship between
atmospheric pressure and altitude?
and why?
atmospheric pressure decreases with altitude
As the altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface.
There are also fewer air molecules above a surface as the height increases. This meanse that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude.
What is the
atmosphere?
a layer of air that surrounds the Earth
this is thin compared to the size of the Earth
What is
atmospheric pressure?
the pressure that is created on a surface by air molecules colliding with the surface
Define
distance.
what type of quantity is this?
how far an object has moved
this is a scalar quantity (so it doesn’t involve direction)
What is
displacement?
and what type of quantity is this?
the distance and direction in a straight line from an object’s starting point to its finishing point
this is a vector quantity
the direction could be relative to a point, or a bearing
What is a
bearing?
a three-digit angle from north
e.g. 035°
What is
speed?
and what type of quantity is this?
how fast you’re going with no regard to the direction
this is a scalar quantity
What is
velocity?
and what type of quantity is this?
a speed in a given direction
this is a vector quantity
What equation links
distance, time and speed?
distance = speed x time
What is the typical speed for
a person walking?
(in m/s)
1.5 m/s
What is the typical speed for
a person running?
(in m/s)
3 m/s
What is the typical speed for
a person cycling?
(in m/s)
6 m/s
What is the typical speed for
a car?
(in m/s)
25 m/s
What is the typical speed for
a train?
(in m/s)
30 m/s
What is the typical speed for
a plane?
(in m/s)
250 m/s
What does
the speed at which a person can travel depend on?
(4 things)
- their fitness
- their age
- the distance travelled
- the terrain
What are some examples of things that affect
wind speed?
(3 things)
- temperature
- atmospheric pressure
- if there are any large buildings or structures nearby
What is
uniform acceleration?
speeding up (or slowing down) at a constant rate
Define
acceleration.
the change in velocity in a certain amount of time
What equation allows you to calculate
the average acceleration of an object?
acceleration = change in velocity/time
a = Δv / t
a: m/s^2
v: m/s
t: s
What is
deceleration?
negative acceleration
(if something slows down, the change in velocity is negative)
What is
acceleration due to gravity near the Earth’s surface?
roughly 9.8 m/s^2
(the same value as gravitational field strength)
this is uniform for objects in free fall
What equation links
final velocity, initial velocity, distance and acceleration?
(only for uniform acceleration)
(final velocity)^2 - (inital velocity)^2 = 2 x acceleration x distance
v^2 - u^2 = 2ad
What does the
gradient of the line represent on a distance-time graph?
gradient = speed
(the steeper the graph, the faster it’s going)
What does a
flat section represent on a distance-time graph?
the object is stationary
What does a
straight line represent on a distance-time graph?
the object is travelling at a steady speed
What does a
curve represent on a distance-time graph?
an acceleration or a deceleration
a steepening curve means it’s speeding up
a levelling off curve means it’s slowing down
What does the
gradient of the line represent on a velocity-time graph?
gradient = acceleration
the steeper the graph, the greater the acceleration or deceleration
uphill sections are acceleration
downhill sections are deceleration
What does a
flat section represent on a velocity-time graph?
the object is travelling at a steady speed
What does a
curve represent on a velocity-time graph?
changing acceleration
How could you calculate
distance travelled using a velocity-time graph?
by calculating the area under any section of the graph
In what direction does
friction work?
the opposite direction to the movement
When do you get
friction?
(2 cases)
between two surfaces or when an object passes through a fluid
Define
drag.
the resistance you get in a fluid
(air resistance is a type of drag)
What is the most important factor in
reducing drag?
keeping the shape of the object streamlined
What makes an object
streamlined?
the object is designed to allow fluid to flow easily across it
(this reduces drag)
parachutes work in the opposite way
What is the relationship between
friction and speed?
(from fluids)
frictional forces from fluids always increase with speed
How does a
falling object reach its terminal velocity?
- When the falling object first sets off, the force of gravity is much more than the frictional force slowing it down, so it accelerates.
- As the speed increases the friction builds up.
- This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force (so the resultant force is zero). It will have reached its maximum speed or terminal velocity and will fall at a steady speed.
What is
Newton’s First Law?
If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it’ll just carry on moving at the same velocity.
(a resultant force is needed to make something start moving, speed up, or slow down)
What is the relationship between
resultant force and acceleration?
they are directly proportional
What is the relationship between
acceleration and mass of the object?
(for a fixed resultant force)
acceleration is inversely proportional to the mass of the object
so an object with a larger mass will accelerate less than one with a smaller mass (for a fixed resultant force)
What equation describes
Newton’s Second Law?
(linking acceleration, resultant force and mass)
resultant force = mass x acceleration
F = ma
F: N
m: kg
a: m/s^2
Define
inertia.
the tendency to continue in the same state of motion
What does
inertial mass
measure?
how difficult it is to change the velocity of an object
How can you calculate
inertial mass?
by rearranging Newton’s Second Law
m = F/a
(inertial mass is just 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
Describe how you would carry out the
trolley experiment (which tests Newton’s second law).
(7 steps)
- Set up two light gates and connect them to a computer.
- Set up the trolley so it holds a piece of card in the middle that will interrupt the signal on the light gates.
- Connect the trolley to a piece of string that goes over a pulley (at the end of the table) and is connected on the other side to a hook.
- Mark a starting line on the table so that the trolley always travels the same distance to the light gate.
- Place the trolley on the starting line, holding it so the string is taut, and release it.
- Record the acceleration measured by the light gate as the trolley passes through it.
- Repeat this twice more to get an average acceleration.
What are the
two different variations of the trolley experiment?
- Investigating the effect of mass.
Add masses to the trolley one at a time to increase the mass of the system. Record the average acceleration for each mass. - Investigating the effect of force.
Keep the total mass of the system the same, but change the mass on the hook. To do this, start with all the masses loaded onto the trolley, and transfer the masses to the hook one at a time, to increase the accelerating force. Record the average acceleration for each force.
What is an
emergency stop?
(for a car)
where maximum force is applied by the brakes in order to stop the car in the shortest possible distance
the longer it takes to do this, the higher the risk of crash
What is the general equation to find
stopping distance?
(for a car)
stopping distance = thinking distance + braking distance
What is the
thinking distance?
(in an emergency stop)
how far the car travels during the driver’s reaction time
(the time between the driver seeing a hazard and applying the brakes)
What is the
braking distance?
(in an emergency stop)
the distance taken to stop under the braking force
What is a
typical car braking distance at
30mph?
60mph?
70mph?
30 - 14m
60 - 55m
70 - 75m
What is
thinking distance
affected by? (2 things)
- Your SPEED - the faster you’re going the further you’ll travel during the time you take to react.
- Your REACTION TIME - the longer your reaction time, the longer your thinking distance.
What is
braking distance
affected by? (4 things)
- Your SPEED - for a given braking force, the faster a vehicle travels, the longer it takes to stop.
- 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.
- The CONDITION of your TYRES - if the tyres of a vehicle are bald (they don’t have any tread left) then they cannot get rid of water in wet conditions. This leads them to skidding on top of the water.
- 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 happens when the
brake pedal is pushed?
(in a car)
The brake pads are 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. The brakes increase in temperature.
Why are
very large decelerations (due to a very large braking force) dangerous?
they may cause brakes to overheat (so they don’t work as well)
or could cause the vehicle to skid
What is a
typical huamn reaction time?
between 0.2 and 0.9s
What affects
reaction time?
(4 examples)
- tiredness
- drugs
- alcohol
- distractions
Describe the
ruler drop test.
(for reaction time) (steps)
- Sit with your arm resting on the edge of a table.
- Get someone else to hold a ruler so it hangs between your thumb and forefinger, lined up with zero.
- Without giving any warning, the person holding the ruler should drop it. Close your thumn and finger to try to catch the ruler as quickly as possible.
- The measurement on the ruler at the point where it is caught is how far the ruler dropped in the time it takes you to react.
- You can calculate how long the ruler falls for (the reaction time) because acceleration due to gravity is constant.
(do a lot of repeats and calculate an average)
To calculate this you would use these two equations:
v^2 - u^2 = 2ad
a = Δv/t
What are the
typical stopping distances at
30mph?
50mph?
70mph?
30mph - 21m (9 + 14)
50mph - 53m (15 + 38)
70mph - 96m (21 + 75)
What is the relationship between
braking distance and speed?
as speed doubles, the braking distance increase 4-fold
The work done to stop the car is equal to the energy in the car’s kinetic energy store (1/2 mv^2). So, as speed doubles, the kinetic energy increases 4-fold , and so work done to stop the car also increases 4-fold. Since W = Fd and the braking force is constant, the braking distance increases 4-fold.
What type of quantity is
momentum?
a vector
What is the equation for
momentum?
momentum = mass x velocity
p = mv
p: kgm/s
m: kg
v: m/s
What is the
conservation of momentum?
in a closed system, the total momentum before an event, is the same as after the event
What equation links
time, mass, force and velocity?
(and sort of momentum)
force = (mass x change in velocity) / change in time
F = mΔv / t
momentum = mass x velocity
p = mΔv
so this can be rewritten as F = p / t
Why are
cars designed to slow people down over a longer time then they have a crash?
The longer it takes for a change in momentum, the smaller the rate of change of momentum, and so the smaller the force. Smaller forces mean the injuries are likely to be less severe.
What are some safety features of
cars?
and how do they work? (3)
- crumple zones - crumple on impact, increase the time taken for the car to stop
- seat belts - stretch slightly, increasing the time taken for the wearer to stop
- air bags - inflate before you hit the dashboard, slows you down more gradually
How does a
bike helmet reduce the severity of injury after a crash?
it contains a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash
this reduces the impact on your brain
How do
crash mats reduce the severity of injury after a crash?
they increase the time taken for you to stop if you fall on them
this is because they are made from soft, compressible materials
