unit one: forces and motion Flashcards
torque (turning effect) unit
newton meter (Nm)
momentum unit
kilogram metre per second (kg m/s)
acceleration unit
metre per second squared
force unit
newton (N)
gravitational field strength unit
newton per kilogram (N/kg)
average speed (m/s) = v =
distance moved (m) / time (s) s / t
v =
s / t
how do you convert seconds into hours
multiply by 60 and then multiply by 60
how do you convert m/s into km/h
(multiply by 60 twice and then divide. by 1000) multiply by 3.6
on a distance time graph, how do you calculate the speed
speed = gradient
what does a curved line on a distance time graph mean
the speed or velocity is changing
what is a vector
a quantity with magnitude and direction
if the line on a distance time graph slopes down to the right what is it
a negative slope or negative gradient.
average velocity =
increase in displacement / time taken
practical: how would you investigate motion using a toy car
roll a car down a ramp and use the equation ( v=s/t) to calculate the speed. you will need to measure the height of the raised end of the wooden track every time you change height.
acceleration def
the rate at which objects change theri velocity
acceleration (m/s squared) =
a =
(final velocity - initial velocity) / time (s)
(v - u) / t
why is u the symbol for initial velocity
it comes before v
what does a negative acceleration mean
its a deceleration
velocity =
v =
distance / time
s / t
how do you find the gradient of a velocity time graph
divide the increase in velocity by time taken to do the increasee
how do you find the acceleration of an object from a velocity time graph
calculate the gradient of the line by (v-u)/t
how do you find the distance travelled from a velocity time graph
either calculate the area underneath the the line or use the equation: (u +v) / 2
scalar
a quantity which only has magnitude and no direction
forces acting on a car
push by a man or engine
normal reaction
weight (downward pull of gravity)
friction
a force of one newton will make a mass of one kilogram accelerate at…
one metre per second
if the forces add to zero will the object move
no there has to be a resultant force
how do you work out resultant force
add all the forces (but subtract the ones going in the OTHER direction.)
what is friction
the force that causes moving objects to slow down and finally stop. the kinetic energy of the moving object is transferred to heat as work is done by the friction force
when does friction occur
when solid objects rub up against eachother
brittle
materials that break rather than stretching and do not return to their orginal shape
elastic
materials that dont break easily and stretch. they return to their original shape
plastic
materials that dont break often but change shape and dont go back to their original shape
what happens if you stretch a spring too mucuh
it wont return to its original shape and length
hookes law
the extension of a spring is proportional to the force of the spring
practical: investigate how the extension of. a spring changes with load
attach the spring to a retort stand and clamp with ruler behind it. attach weights to the end of it and plot the extension of the spring to the load on a graph. if it goes through the origin ten it obeys hooked law
do rubber bands obey hookes law
no
an object will not change its velocity (accelerate) unless
there is an unbalanced force acting on it
how would you investigate the acceleration of a trolley going down a ramp
take a series of pictures at equal time intervals using a digital video camera, or use a data logger
if force vs acceleration shows a straight line goi no through the origin what does it ean
the force is proportional to acceleration
force =
F =
mass x acceleration
m x a
what does breaking distance depend on
weight, tread of wheels, dry road
what is thinking distance
the distance travelled during the time. for the driver to respond to the new situation before taking any action
what is breaking distance
the time it takes for the vehicle to stop
what does thinking distance depend on
alchol, drugs, visibility, tiredness, distraction
weight (N) =
W =
mass (kg) x gravitational field strength (N/kg)
m x g
what is drag coefficient
a measure of how easily an object moves through air
what forces does an object falling through air experience
air resistance/drag/ weight force
opposing drag force
what is terminal velocity
when there is no longer an unbalanced force because the drag force has increased to the point where it exactly balances the weight force, so its acceleration is 0. however it is still falling, it just cant get faster
the size of drag forces in liquids are much ______ than in gases
higher
current unit
amperes (A)
charge unit
coulomb (C)
resistance unit
ohm
voltage unit
volts (V)
power unit
watts (W)
purpose of the earth wire
to protect you if an appliance develops a fault by. providing a path fo current to escape through the user
voltage supplied to homes in the UK
230 V
what is a fuse
a cylinder or cartridge whcih obtains a thin piece of wire made from a metal with a low melting point
what is the fuse there for
if there is too large a current then the fuse wire becomes very hot and melts. the circuit is now incomplete and reduces the chance of getting a shock or electrical fire
size of fuses
3A
5A
13A
why do many appliances have a metal casing
if the live wire becomes damaged or breaks and comes into contact with the casing the earth wire provides a low resistance path for the current, which would probably be big enough to blow the fuse and turn the circuit off.
where should a switch always be placed and why
in the live wire so when the switch is o pen no energy in the form of electricity can reach an appliance. if the switch is placed in the neutral wire, energy can still enter a faulty appliance and possibly cause an electric shock
if we want wires to become warm (heating elements) what do they have
high resistance
if we dont want wires to become warm what do they have
low resistance
why will a 70J bulb be brighter than a 50@ oe
it transfers 70J of electrical energy every second as opposed to 50J
what is power measured in
joules per second or watts
1kW is
1000W
power (W) =
P =
current (A) x voltage (V)
I x V
energy (J) =
E =
power (W) x time (s)
P x t
direct current
current/voltage that isi always in the same direction and have the same value. on a graph it would be a straight line
alternating current
an electric current that reverses its direction many times a second at regular intervals, typically used in power supplies.
what is an electric current
a flow of charge
electrons flow through all metals , so metals are
conductors of electricity
voltage (V) =
V =
current (A) x resistance (ohms)
I x R
what usually happens in a conductor
the electrons are free to move between the atoms. the number of electrons flowing in any one direction is equal to the number of electrons flowing in any other direction.
what happens in a conductor when a cell or battery is connected across a conductor
the electrons start flowing away from the negative terminal and to the positive terminal. now there is a net flow of charges or electric current.
how do you measure current
using an ammeter
the size of an electric current indicates…
the rate at which the charge flows
if there is a current of 1A then it means
1C of charge is passing along the wire each second
charge (C) =
Q =
current (A) x time (s)
I x t
what is a volt
a joule per coulomb
energy (j) =
charge (c) x voltage (v)
Q x V
how do you measure volts
using a voltmeter
series circuit
only one path the current can take
parallel circuit
multiple paths and junctions that the current can take
if one bulb in a series breaks,
it causes a gap int he circuit and all of the other bulbs will stop working
the energy in a series circuit is shared between all the bulbs, so the more bulbs you add,
the dimmer they all become
if one bulb in parallel breaks,
only bulbs in the same branch of the circuit break so not all of the bulbs will break
in a series circuit how is the current distributed
the current is the same in all parts and not used up
in a parallel circuit how is the current distributed
the current will be different in different parts of the circuit,
voltage (V) =
V =
current (A) x resistance (ohms)
I x R
practical: how to investigate how current varies with voltage with different components
have a circuit with a switch, variable resistor , voltmeter, ammeter and piece of wire. turn up the variable resistor to maximum and close the switch to take readings from the. ammeter and voltmeter. alter value of variable resistor or and take a new reading. repeat at least six times and plot values, drawing a graph of current against voltage
on a current vs voltage graph what does a flat slope indicate
a high resistance
what does a fixed resistor do
to control the sizes of currents and voltages. without it the voltage across the bulb may cause too large a current and the bulb will break
how is a variable resistor different to a fixed resistor
you can vary the resistor, allowing the bulb to shine more brightly or less brightly.
what is a thermistor
a resistor whose resistance changes quite.a lot even with small changes in temperature
what is a light dependent resistor
it has a resistance that changes when light is shone on it. in the dark its resistance is high
where are thermistors used
temp sensitive circuits like fire alarms
where are LDRs used
automatic lighting controls and burglar alarms
what are diodes
special resistors that allow charges to follow through them in one direction.
when diodes are used in rectifier circuits what do they do
they convert alternating current into direct current
PRACTICAL: investigate the motion of everyday objects (such as toy cars)
- Attach the bench pulley to the end of a long bench.
- Secure the mass hanger to one end of the string and attach the other to the toy car
- pass the string over the bench pulley, and pull the car back so that the mass
hanger is just resting on the floor. - Mark the car’s position with masking tape - this is the end marker.
- Pull the car back so the mass hanger is raised and touches the pulley - mark the
car’s position with tape - this is the starting marker. - Use the tape measure to record the distance between the start and end markers.
- Release the car from the start marker and start the stop clock - stop timing when
the car reaches the end marker (this will be the same time that the mass hanger
reaches the ground). - Repeat the experiment 5 times and calculate an average time.
PRACTICAL: investigate how extension varies with applied force
- Using the ruler, measure the initial length of the first spring when no force is applied.
- Set up the spring so it is hanging securely from the clamp stand.
○ You can also secure the ruler to the clamp stand to ensure it does not move at all
during the experiment. - Add one of the masses to the end of the spring and record the extension of the spring.
○ The extension is the difference between the new length and the initial length. - Continue adding masses and recording the extension each time.
- Plot a graph of force applied against the extension of the spring.
○ Force can be calculated from mass x gravitational field strength (i.e. 10 x the mass
hanging on the spring).
○ The gradient of the line of best fit will be the spring constant as . k = F/x
F - Using this value, you can calculate the work done each time the spring extends
○ Use the formula . W = 1/2kx squared
explain why a vehicles stopping distance is different to its breaking distance
stopping distances take into account the driver’s thinking distance as well as the vehicles breaking distance
car safety features are designed to…
increase the time taken for a passenger’s momentum to change
unbalanced forces acting on object makes them…
accelerate or decelerate
what is a pivot
the fixed point where the turning effect of an object takes place.
moment of a force =
force x perpendicular distance from pivot
F x d
what are moments measured in
newton metrees (Nm)
sum of anticlockwise moments =
sum of clockwise moments (if in balance)
what is the centre of gravity/mass
it is the point at which the whole weight of the object appears to act. if we support the object at this point there is no turning moment in any direction about the point and it balances.
what happens when an object is pivoted at the centre of gravity
there is no turning point
what happens when an object is not pivoted at the centre of gravity
there is a turning point
if the weight is closer to one end of a beam where will the most force be
on that end of the beam.
momentum =
p =
mass x velocity
m x v
what is momentum measured in
kg m/s
when an unbalances force acts on an object
it causes a change in the momentum of that object
inital momentum is
mu
final momentum is
mv
rate of increase of momentum is
F is
time
t
momentum before a collision =
momentum after a collision
what is the conservation of momentum principle
momentum before = momentum after
why do cars have safety features
they increase the time over which the car’s momentum changes in an accident. they protect the passengers in case of an accident.
what are crumple zones/what do they do
in a car, they collapse during a collision and increse the time during which the car is decelerating. they protect the passengers in case of an accident.
newtons third law
when you push something it pushes you back just as hard but in the opposite direction. for every action there is an equal and opposite reaction.
balanced forces act in
opposite directions on the same object
action and reaction forces act in
opposite directions but always on different objects
