Physics Forces & Energy Test

what is a scalar quantity

has a magnitude but no direction

what is a vector quantity

has both magnitude and direction

the 5 scalar quantities

• speed

• distance

• mass

• time

• energy

the 6 vector quantities

• velocity

• displacement

• weight

• force

• acceleration

• momentum

velocity definition

speed in a stated direction

equation for speed

speed (m/s) = distance (m)
time (s)

what does a straight line mean on a distance/time graph

constant speed

what does the slope of the line represent on a distance/time graph

the magnitude of the speed:

• steep slope = large speed

• shallow slope = small speed

• flat slope = stationary (not moving)

what do curves mean on distance/time graphs

changing speed

• if the slope is increasing the speed is increasing

• if the slope is decreasing the speed is decreasing

how do you work out speed from a distance/time graph

using the gradient of the line:

speed = gradient = rise / run

equation of acceleration

acceleration (m/s²) = change in velocity (m/s) / time taken (s)

a = v-u
t

equation for change in velocity using acceleration and distance

final velocity² - initial velocity² = 2 x acceleration x distance

v² - u² = 2ax

what does a straight line mean on a velocity/time graph

constant acceleration - the object is speeding up at a constant rate

what does the slope of the line represent on a velocity/time graph

the magnitude of acceleration

• steep slope = large acceleration (speed changes quickly)

• gentle slope = small acceleration (speed changes gradually)

• flat line = zero acceleration (object is at a constant velocity)

how do you calculate acceleration from a velocity/time graph

acceleration = gradient = rise / run

what does the area under a velocity/time graph represent

the displacement - the distance travelled by the object

split the area up into triangles and rectangles to work it out

what are light gates

pieces of digital equipment that accurately measure times. using it you can accurately work out speed

using 2 light gates to measure time

the moving object will block the beam of light on the light gate when it passes through, triggering the equipment

1. a timer will start when the object passes through the light gate

2. the timer is stopped when the object passes through the second light gate

the time can then be used to work out speed

using a single light gate to measure speed

1. there is a flag/object on top of the moving object

2. as the object passes through the light gate it blocks the beam of light for a set amount of time as it passes through

3. the timer measures how long the light is blocked for

4. the distance is the length of the flag

5. then you can work out speed

typical speeds table

acceleration due to gravity

10 m/s²

typical accelerations table

newton’s first law

when the resultant force on a body is zero (still or at a constant speed), it will stay at zero unless an external force acts upon it

when the resultant force on a body is not zero, the object will accelerate, deaccelerate, or change direction

no force = no change in motion // force = change in motion

newton’s second law

force (N) = mass (kg) x acceleration (m/s²)

f = m x a

---

force = change in momentum (kg m/s) / time (s)

F = (mv - mu)
t

weight definition

the force acting on an object due to gravitational attraction

weight equation

weight (N) = mass (kg) x gravitational field strength (N/kg)

W = m x g

how to measure weight

using a newton-meter, which measures force in newtons. spring fixed at one end with a hook to attach an object on the other

you can indirectly measure weight using a top pan balance (scales) to find mass and then use w=mg

gravitational field strength (g) on earth

10 N/kg

relationship between weight & gravitational field strength

mass always stays the same

weight changes depending on gravity:

• if gravity increases, weight increases

• if gravity decreases, weight decreases

equipment for investigating acceleration practical

• metre ruler: to measure distance

• car / trolley: the object for which acceleration is measured

• pencil / chalk: to mark intervals of distance

• bench pulley & string: to connect masses to the trolley

• weights & weight hanger: to provide force on the trolley

• stopwatch: to time the trolley between distance intervals

• blu tac: to attack extra weight to trolley if needed

independent variable

the variable you change, that is unaffected by other variables

dependent variable

the variable you measure, depends on other variables

control variable

something you keep the same

what happens when an object is moving in a circular orbit at a constant speed

even though speed is constant, the velocity is constantly changing because it depends on direction

centripetal force

The resultant perpendicular force towards the centre of the circle required to keep a body in uniform circular motion

inertial mass

how hard it is to change an object’s velocity (speed or direction), something with a large inertial mass resists acceleration more than one with a small inertial mass

mass = force / acceleration (comes from f = ma)

the more mass an object has, the more it resists changes in velocity

newton’s third law

if object A exerts a force on object B, then object B will exert a force on object A that’s exactly the same size, at exactly the same time, but in the opposite direction

action reaction forces

newton’s third law in equilibrium situation

if an object is at rest or moving at a constant velocity, the forces acting on it are balanced

e.g. for a book on a table, the book pushes down on the table (its weight) and the table pushes up on the book with an equal force (reaction force) meaning nothing moves.

newton’s third law in collisions with momentum conservation

if e.g. two ice skaters push apart, A pushes on B, and B pushes back on A, their combined total momentum before and after is the same

this is because they each have the same momentum but going in opposite directions so together it equals zero

law of conservation of momentum in collisions

total momentum before = total momentum after

momentum definition

how much motion an object has

equation for momentum

momentum (kg m/s) = mass (kg) x velocity (m/s)

p = m x v

elastic collsision

when objects collide and move in opposite directions

each object will have a different velocity depending on its mass and the initial momentum of the system

inelastic collisions

when objects collide and move in the same direction together

objects have a combined velocity

reaction time

how much time passes between seeing something and reacting to it

a reaction time for someone who is alert (waiting to react to something) is between 0.2 and 0.9 seconds

ruler method for measuring reaction time

1. person a holds a ruler vertically at the top end so that the bottom end hovers over the top of person b’s hand

2. person a releases the ruler unexpectedly

3. as soon as person b sees the ruler move, they close their hand to catch it

4. the ruler is marked at the point at which it was caught by person b

5. the greater distance, the longer reaction time

formula of stopping distance of a vehicle

thinking distance + braking distance

thinking distance = distance travelled in the time it takes the driver to react (reaction time) in meters
braking distance = distance travelled under the braking force in meters

the greater the speed of the vehicle, the greater the stopping distance

factors affecting stopping distance of a vehicle

• vehicle mass

• vehicle speed

• driver’s reaction time

• state of the brakes

• state of the road

• friction between the tire and the road (e.g. icy road = less friction)

factors affecting a driver’s reaction time

• tiredness

• distractions e.g. using a phone

• intoxication: alcohol / drugs

dangers of large decelerations

overheating of brakes: kinetic energy of vehicle is converted into thermal energy of the brakes, and if they get too hot they can fail, meaning they won’t work effectively

injury: when a vehicle decelerates the driver & passengers also decelerate and this can cause injuries e.g. whiplash, a neck injury when your head moves suddenly relative to your body

loss of control: it’s more difficult to control a vehicle that’s decelerating, and loss of control can cause a collision

equation for work done

work done (J) = force (N) x distance moved in direction of force (m)

E = F x d

braking distance with kinetic energy & work done

when a vehicle brakes, its kinetic energy is being transferred to thermal energy.

so work is being done on the vehicle by the braking force. the work done = the total kinetic energy transferred to heat

you can calculate braking distance and/or work done and/or braking force using E = F x d:
E = work done = KE
F = braking force
d = braking distance

because work done is the same as kinetic energy, we can say that ½ x m x v² = F x d

equation for change in GPE

change in GPE (J) = mass (kg) x gravitational field strength (n/kg) x change in vertical height (m)

ΔGPE = m x g x Δh

kinetic energy equation

kinetic energy (J) = ½ x mass (kg) x speed² (m/s)

KE = ½ x m x v²

sankey diagrams

sankey diagrams show the amount of energy transferred and where to

different thicknesses of arrows represent how much energy is transferred in that way

percentages are put in brackets

other energy transfer diagrams

principle of conservation of energy

energy can’t be created or destroyed, it can only be transferred from one store to another

so the total amount of energy in a closed system remains constant

the total energy transferred into a system must be equal to the total energy transferred out

energy transferred when a ball being thrown up

• before ball is thrown the person holding the ball has energy in their chemical store

• when the ball is thrown some of that energy is transferred to the kinetic store of the ball as it begins to move upwards

• as the height of the ball increases, energy from the kinetic store of the ball is transferred to its gravitational potential store

energy transfers with a moving object hitting an obstacle

• when a car is moving energy in the chemical store of fuel is transferred to the kinetic store

• when it hits a wall the speed and therefore the energy in the kinetic store decreases quickly

• most of this energy is dissipated

• but some is transferred mechanically to the thermal store of the wall (the force of the car on the wall)

• and some is transferred by heating to the thermal store of the air as the sound waves transfer energy away from the system

energy transfers with a vehicle being accelerated by a constant force

• when the vehicle is stationary it has energy in the chemical store of its fuel

• when it accelerates, the energy is transferred to the kinetic store of the car

energy transfers with a vehicle slowing down

• the moving vehicle has energy in its kinetic store

• as it slows energy is transferred to the thermal store of the surroundings (dissipated)

• energy is also transferred by heating due to friction between the tyres and the ground and the the brakes and the brake pads

• energy is transferred by heating as sound waves transfer energy away from the system

energy transfers with boiling water in an electric kettle

• energy is transferred electrically from the mains supply to the thermal store of the heating element in the kettle

• as the heating element gets hotter energy is transferred by heating to the thermal store of the water

energy transfers with a trampoline

elastic potential energy → kinetic energy → gravitational potential energy

• while jumping the person has energy in their kinetic store

• when the person lands on the trampoline most of that energy is transferred to the elastic potential store of the trampoline

• that energy is transferred back to the kinetic store of the person as they bounce up

• as the person gains height energy is transferred from kinetic → gravitational potential

• some of the energy is dissipated by heating to the thermal store of the surroundings (person, trampoline, air)

how do mechanical processes become wasteful (energy)

when they cause a rise in temperature (usually through friction) because energy dissipates through heating into the surroundings

useful energy definition

an energy transfer that serves an intended purpose

wasted energy definition

an energy transfer that is not useful for the intended purpose and is dissipated to the surroundings

dissipated energy

energy that spreads out into the surroundings is hard to ‘gather’ so that it can be used usefully again. this means that the energy becomes less useful

whenever a process produces unwanted heating, light, or sound, the energy is dissipated and wasted

however not all dissipated energy is wasted e.g. a tv generates light & sound energy that is useful, or a heater generates useful heat energy

reducing energy loss

producing lots of waste energy can take up lots of energy and be very expensive, so we need to find ways to reduce waste

lubrication prevents friction by lubricating the parts that rub together

insulation reduces conduction by lowering a area’s rate of cooling, meaning less energy is transferred to the surroundings. effectiveness depends on the thermal conductivity (low), the density (low), and the thickness (high)