forces Flashcards
examples of scalar quantities
mass, temperature,speed, energy, distance, time
scalar quantities
have a magnitude but no direction
vector quantities
-displacement
-weight
force
-acceleration
velocity
momentum
vector quantities
-have both direction and magnitude[
vectors can be presented using arrows
length of arrow represents magnitude of the vector
direction of arrow represents direction of the vector
definition of a force
a force is a push or a pull acting on an object due to the interaction with another object
all forces have both magnitude and direction- its a vector quantity
unit newtons
in contact forces, the two objects are
physically touching
contact forces:
- tension
- friction ( eg acts between water or aeroplane causing aeroplane to slow down and stop)
- air resistance
- normal contact force (an object on a table, object exerts downwards force on the table and that force is the weight of the lamp at the same time table exerts upwards force on the lamp) this can only happen if two objects are in direct contact
non contact forces two objects physically separated
- gravitational force (attracts moon to Earth and Earth to moon
- electrostatic force (force between two charged objects) objects with opposite charges experience a electrostatic force of attraction. objects with two like charges experience a electrostatic force of repulsion
- magnetic force (force experienced by certain objects in a magnetic field)
gravity defintion
force of attraction between all objects
is gravity a contact or non contact force
a non contact force because two objects are physically separated
is gravity a scalar or vector quantity
is a vector quantity because it has magnitude and direction
definition of mass
mass tells us how much matter is in an object
unit is kilogram
scalar quantity because it does not have direction
key point about mass
mass of an object does not depend on where the object is
weight of an object is the force acting on it due to gravity
unit=newtons
weight of an object does depend on where it is
weight depends on gravitational field strength
what is gravitational field strength
measure of the force of gravity in a particular location
equation to calculate weight
weight N= mass (kg) x G (N/kg)
weight of an object is directly proportional to mass
if we double the mass weight also doubles
a newton meter can be used to determine
the weight of an object
centre of mass
weight of an object(force acting on an object due to gravity) can be considered to act at a single point
definition of force
a force is a push or a pull acting on an object due to its interaction with another object
forces are vector quantities because they have both magnitude and direction
resultant force definition
single force that has the same effect of all the original forces acting together
how to calculate resultant force
subtract smaller force from larger force
what does it mean if the resultant force is 0N
the forces acting on the object are balanced
what does altitude mean
height above the ground
description of free body diagram of aeroplane
aeroplane is represented as a circle
the weight of the aeroplane is acting downwards towards the Earth
lift acting upwards
forward force provided by engine called thrust acting to the right
air resistance drag acting in reverse to thrust
Equation of work done
work done J= force N x distance M
distance must be in the line of action of the force
what is work done
work done is when a force is used to move an object
work done is an example of energy transfer
list elastic materials
- slinky
- rubber bands
- rubber gloves
- eraser
- playground surface
- tennis ball
in an elastic material
the stretching forces are equal in magnitude but opposite in direction
these forces cause the objects to stretch but when the forces are released, the object returns back to its orginal length
elastic materials will always return to their original length or shape if we take away forces acting on them
if we apply squeezing forces to the object the object compresses. if we take away the forces, the object returns to its original length
if we remove the forces acting on the material it recovers its orginal properties and returns to its orginal shape or length
elastic deformation
when we take away the forces acting on the object, the object will return to its original shape and length
how do we change an objects length or shape
we have to apply more than one force
what would happen if we only applied one force to a stationary object
forces are no longer balanced
the object would simply move rather than changing length or shape
when we stretch inelastic materials what happens
they do not return to their original shape or length when the forces are removed
example of inelastic materials
-certain polymers
force= spring constant n/m x extension m
force= spring constant n/m x extension
calculate the force required to extend a spring by 0,4 metres the spring constant is 200n/m
0.4 x 200= 8N
the same equation can be used to calculate compression how?
force (N)= k x e
e is compression in this case
when we stretch or compress an elastic object we are using a force to
do work (transfer energy) elastic potential energy is stored in the object work done= elastic potential energy but that is only true if object is not inelastically deformed
an elastic material returns to its orginal shape when forces are removed
an inelastic material….
an inelastic material does not return to its original shape when forces are removed
When a person stretches or compresses an elastic object, ———– energy stores in the person’s
muscles transfer to the ———— energy store in the elastic object. This is an example of work
done. The total work done is ——— the energy transferred as long as the object is not inelastically deformed.
when a person stretches or compresses an elastic object, chemical energy stores in the person’s muscles transfer to the elastic potential energy store in the elastic object. this is an example of work done. the total work done is equal to the energy transferred as long as the object is not inelastically deformed.
what is the definition of elastic potential energy
energy stored in an elastic material when its stretched
energy used in stretching or compressing a material is stored as…
elastic potential energy
what does hookes law state
if you apply a force in the form of a weight, to a spring it will stretch and the extension produced is directly proportional to force applied
what is the limit of proportionality
the force applied is no longer directly proportional to the extension
what equation is used to calculate pressure
pressure (pa)= force normal to surface (N) / area of surface m^2
A gas inside a container applies a force of 40,000 N the wall of the container have an area of 8m^2 calculate pressure of the gas
40,000/ 8
50,000 Pa
how does atmpspheric pressure dcrease with an icnrease in height
atmosphere is a thin layer of air around the Earth
the atmosphere is most dense at the surface of the Earth
at the Earth’s surface there is the greates number of particles for a given volume
atmosphere gets less dense as we increase altitude
pressure of atmosphere is caused by air molecules colliding with a surface
as a surface increases height above ground level, number of air molecules above surface decreases
this means that atmospheric pressure decreases with an increase in height
why do mountain climbers need an oxygen supply
air pressure on mountain may be too low to supply all oxygen they need
crips packet inflates because
at ground level, pressure inside the bag is equal to pressure outside the bag however when aeroplanes are flying the air pressure in the cabin is kept at slightly lower pressure than air pressure on the ground. air pressure inside the bag is now greater than the air pressure inside the cabin
as surface height above ground level increases
number or air molecules above the atmospehere decreases so atmospeheric pressure ddecreases with an increase in height
atmosphere gets less dense as we increase altitude
what os pressure of the atmosphere caused by
air molecules colliding with a surface
How do particles in fluids produce pressure on walls of the container
When particles in a fluid collide with the walls of the container, they exert a force. This force acts at
right angles to the surface of the wall. Scientists call this a normal force (ie acting at right angles).
How do particles in fluids produce pressure on walls of the container
When particles in a fluid collide with the walls of the container, they exert a force. This force acts at
right angles to the surface of the wall. Scientists call this a normal force (ie acting at right angles).
pressure of column of liquid equation
pressure (pa)= height of column (m)- denisty of liquid kg/m^3 x gravitational field strength N/kg
the pressure increases with the height of the column ( the depth) because
as the depth increases, there is a greater weight of liquid acting downwards
pressure of liquid also increases with density
liquids with a greater density have a greater weight acting downwards
the pressure of a liquid depends on teh depth because the greater teh depth teh greater the weight acts downwards
as pressure increases there is a greater weight of liquid acting downwards
the pressure of q liquid dpeends on teh depth
the bottom of the liquid
is a at a greater depth than at the top of the liquid
the bottom of the liquid experiences a larger pressure than at the top
because of this there is a larger force acting at the bottom of the object
there is a resultant force acting upwards
what is upthrust
the resultant force acting upwards
for an object to float
upthrust must be equal to objects weight ( force acting downwards due to gravity)
if the upthrust is less than teh objects weight then the
object sinks
when an object is lowered the level of water rises why
because the object displaces the water
the size of the upthrust acting on an object is the same as the weight of the water displaced by the object
the size of the upthrust acting on an object is teh same as the wieght of the water displaced by the object
if an object can displace its own weight of water, then the upthrust will equal teh objects weight and the object will float
an object that is less dense only needs to displace a small volume of water before the weight of the water displaced equals the wieght of the object now the upthrust is euqal to teh wieght of the objecr and the objetc can float. in this cas ethe object floats high in the water
an object that is more dense than water
object cannot displace water equal to its own weight
the weight of the object is greater than uothrust so the object sinks
why does a greater depth have a greater pressure
because bottom of teh object is at ta greater depth then at the top so it experiences a greater pressure. this means the the bottom of the object experiences a greater force than atthe top.
because of this we have a resultant force acting upwards and scientists call this upwards force upthrust an object will float if the upthrust is equal to the weight of the object if the weight is greater than the upthrust then the object will sink.
momentum
all moving objects have a momentum
if an object is not moving the momentum is always
zero
momentum equation
momentum kg/ms= mass kg x velocity m/s
conservation of momentum
in a closed system, total momentum before an event is equal to total momentum after event
conservation of momentum question: a van with a mass of 1400kg is travelling at 20m/s. the van collides with a stationary car with a mass of 1000 kg. both the van and the car continue moving together. calculate velocity of van and car together
1400x20=28000kgm/s
car has momentum of zero because its stationary
total momentum after collision =28,000kgm/s
to calculate velocity divide total momentum by mass of the car
28000/2400=
11.67m/s
rapid changes in momentum can be extremely dangerous
a huge force is placed on the passenger and this could increase the risk of injury
force(N)= mass (kg) x acceleration m/s^2
equation for acceleration
change in velocity/ time
combine the two equations to calculate force
force= mass x change in velocity/ change in time
how do you calculate change in momentum
mass x change in velocity
a man with a mass of 100kg is a passenger in the car. the car is travelling at 30m/s. the car comes to a stop in 1 s. calculate forces acting on the man
100x30/1 force= mass x change in velocity/ change in time
3000N
a force acting on a person could be fatal
rapid changes in momentum lead to huge forces applying and these are extremely dangerous
a force acting on a person could be fatal rapid changes in momentum lead to huge forces applying and tehse are extremly dangerous
how can we reduce danger
make change in momentum over a longer itme
devices that can slow down momentum change
- air bags
- safety belt
- helmet
- cushioned surface
- crashmat
how do these reduce momentum change
they reduce the forces acting and reduces risk of serious injury
when momentum is acting in a backwards direction
we present velocity ….
with a negative sign because velocity and momentum are vector quantities they have both magnitude and direction
atmospheric pressure decreases with height
explanation
the atmosphere is a layer of thin air that surrounds the Earth. atmospheric pressure is created on a surface by air molecules colliding with the surface
as the altitude (height above the ground increases) atmospheric pressure decreases because as altitude increases atmosphere gets less dense fewer air molecules that are able to collide with the surface
there are also fewer air molecules above a surface as height increases. this means that teh weight of the air above it whivh contributes to atmospheric pressure, decreases with altitude
the more dense a given liquid, the more particles it has in a certain space this means that
there are more particles able to collide so pressure is higher
as the depth of the liquid increases, number of particles above that [point increases the weight of these particles adds to pressure felt at that point so liquid pressure increases with depth.
p=hpg
stopping distance
total distance travelled from when driver spots obstruction to when the car actually stops
the stopping distance can be divided into two parts
the thinking distance
braking distance
thinking distance- distance travelled by car during drivers reaction time
reaction time- time taken for driver spot the obstruction, make a decision and move their foot to the brake
the braking distance is the distance
car travels from when driver applies breaks to when car actually stops
greater the speed of the vehicle
greater stopping distance assuming same force was applied to breaks
reaction time varies from
0.2 to 0.9 seconds
reaction time can be measured using ruler
how do we measure reaction time
one person holds the ruler and a volunteer places their fingers on either side. the ruler is dropped and the volunteer has to catch it
the further the ruler falls before its caught the longer the reaction time
by measuring distance the ruler fell we can look up reaction time in a table
should a driver have a short reaction time or a long reaction time
drivers should have shortest possible reaction time so that they can react very quickly when they see obstructions.
what factors affect the braking distance ( distance travelled when drivers apply brakes to when drivers actually stop)
terrain
wet or icy conditions reduce friction between tyres and the road and increase braking distance.
braking distance will also increase if car has worn out tyres because it reduces friction between tyre and road.
worn brakes also increase breaking distance
what factors would affect reaction time thus thinking distance
-tiredness a tired driver would have a longer reaction time
-alcohol and drugs can make the reaction time longer
-distractions in the car such as a mobile phone will also increase drivers reaction time.
because these factors increase reaction time they also increase thinking distance
what happens if a car has a greater speed
the stopping distance will increase larger force will be needed to stop the car
what are ideal conditions like
driver is well rested
no alcohol or drugs
weather is good no fog mist , sunny weather
road condtions are normal no wet icy terrain
recap stopping distance of a car consists of thinking distance + braking distance
defintions for all trhee
stopping distance- sum of thinking and braking distance. distance car travels from when driver spots obstruction to when vehicle actually stops
braking distance- distance travelled by car from when driver applies breaks to when car actually stops.
thinking distance- distance travelled by car during reaction time. reaction time varies from 0.2 to 0.9 seconds. we can measure it using a ruler. reaction time is the time taken to spot obstruction, make a decision and move foot to brakes.
any moving object has kinetic energy
1/2mv^2
kinetic energy dpeends on velocity squared if we double velocity kinetic energy of car quadruples
when a car brakes and comes to a stop
what is all the kinetic energy converted to
other forms of energy
what happens during braking
brake presses against wheel
force of friction now acts between brake and the wheel
kinetic energy of teh car is conevrted to thermal energy in the brakes
this causes temperature of the brakes to increase
at the same time, car slows down as it loses kinetic energy
the greater the speed, the greater the breaking force needed to stop the car in a certain distance.
if a car is travelling at high speeds. the driver needs to brake
they have to apply a very large breaking force
a large breaking force will cause the car to decelrate rapidly
a large amount of kinetic energy is transferred to thermal energy in the brakes
this can cuase brakes to overheat and driver could lose control of vehicle
force mass acceleration
force- mass x accerlation
‘if a car is travelling at high speeds the driver needs to brake then they have to apply a very large breaking force
a large breaking force cuases car to decelerate rapidly at the same time,a large amount of kinetic energy is transferred to thermal energy in the brakes
this can cause brakes to overheat and driver can lose control of vehicle
a car of 1000kg is travelling at speed of 3m/s on the motorway they decerlate to leave the motorway velocity decreases to 0 in ten seconds
force- mass x acceleration
1000x30= 3000N
typical speeds
walking
runnning
cycling
walking 1.5m/s
running 3m/s
cycling 6m/s
speed can depend on terrain and nature of person
an older unfit person will have a slower speed than a fit young person
people move more rapidly on flat ground than on hill
speed can also depend on distance travelled
typical vehicle speeds
car on a main road 13m/s
fast train 50m/s
cruising aeroplane 250m/s
speed of sound in air
330 m/s
sound travels faster on warmer days than cooler ones
speed of a moving object is not constant
objects moving in a circle- velocity
if an object moves at constant speed in a circle, its velocity is constnatly changing even though its speed is constant that is because velocity is speed in a given direction
forces acting on skydiver
as soon as a skydiver jumps out of the plane the only force acting is weight
this is due to gravity
it will not change during a journey
becayse of weight skydiver experiences a resultant force acting downwards so they accelerate towards the ground
as the skydiver falls, they expereince friction with air molecules ( this is called air reistsance and it acts upwards)
the weight is greater than the air resistance so the skydiver accelerates downwards
as the sky divers velocity increases the air reistance also increases
at a certain point the air reistsance balances the weight
now there is no resultant force so velocity stays constant
skydiver has reached terminal velocity
this velocity is extremely great, the sky diver would die if they touched the ground
so the skudiver opens their parachute increasing their surface area this causes air resistance to massively increase
air resistance is greater than the weight
so there is a resultant force acting upwards
this causes skydiver to decelerate (velocity decreases)
because velocity has decreased air resistance also decreases
at some point air resistance balances teh weight and teh resultant force will be 0.
so vlocity will stay constant
skydiver is falling at a lower terminal velocity
this is now safe for them to hit the ground
velocity time graph
as soon as sky diver jumps out of the plane only force acting is weight
skydiver accelerates towards teh ground
as the skydiver falls air resistance increases
but weight is still greater than air resistance so teh sky diver ocntinues to accelerate
velocity increases, air resistance also increases
air resistance balances weight and resultant force is zero so velocity stays constant
terminal velocity
this velocity is extremely great the sky diver would die if they hit the ground
at this point skudiver opens parachute
this massively increases surface area and the air resistance increases massively
air resistance is now greater than the weight
so the resultant force now acts upwards
vecause resultant force acts upwards skydiver decelerates
they are still moving towards the ground but at much lower velocity
because velocity has decreased, air resistance has also decreased
when air reistance balances the weight resultnat force is now zero
velocity is now constant and this is alower terminal velocity
tje sky diver lands
