Forces

Question 1

vector quantities

Answer 1

quantities with both magnitude (size) and direction
examples:
force, acceleration, displacement, velocity, momentum
usually represented by an arrow - length shows the magnitude, direction shows direction of the quantity

Question 2

scalar quantities

Answer 2

quantities with only magnitude (size)
examples:
speed, distance, mass, temperature, time

Question 3

contact forces

Answer 3

two objects touching for force to act
e.g. friction, air resistance, tension in ropes

Question 4

non-contact forces

Answer 4

objects do not need to be touching for the force to act
e.g. magnetic force, gravitational force, electrostatic force, nuclear forces

Question 5

gravitational force

Answer 5

the force of attraction between masses
important effects:
- makes all things fall towards the ground on the surface of a planet
- gives everything a weight

Question 6

mass

Answer 6

the amount of ‘stuff’ in an object
for any given object this will have the same value anywhere in the universe
NOT a force
- measured in Kg with a mass balance

Question 7

weight

Answer 7

the force acting on an object due to gravity (the pull of the gravitational force on the object)
changes with location as it depends on the strength of the gravitational field at the location of the object
measured in newtons
- acts from a single point on the object (the centre of mass) - a point at which you assume the whole mass is concentrated (the centre of the object for a uniform object)
- measured using a calibrated spring balance (or newtonmeter)

Question 8

gravitational field strength

Answer 8

varies with location
(9.8 on Earth)
- stronger the closer you are to the mass causing the field
- stronger for larger masses

Question 8

Weight equation

Answer 8

Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
W = m x g
For earth, g ≈ 9.8N/kg
For moon, g ≈ 1.6N/kg

Mass and Weight are directly proportional
- increasing the mass of an object increases its weight
- if you double the mass, the weight doubles too - directly proportional
- can be written using the direct proportionality symbol, as W ∝ m

Question 9

resultant force

Answer 9

the overall force on a point or object
- a single force which has the same effect as all the original forces together)
Found by adding forces going in one direction and subtracting any going in the opposite direction

Question 10

free body diagrams

Answer 10

show all the forces acting on an object
e.g. a skydiver
- arrow pointing down represents skydiver’s weight pulling him to the ground
- arrow pointing up represents air resistance acting on him in the opposite direction to his motion
The sizes of the arrows show the relative magnitudes of the forces and the directions show the directions of the forces acting on the object.
Arrows are drawn from centre of gravity of the object.

Question 11

energy transferred and work done

Answer 11

When a force moves an object through a distance, ENERGY IS TRANSFERRED and WORK IS DONE on the object

When you are pushing something along a rough surface (e.g. carpet) you are doing work against frictional forces. Energy is being transferred to the kinetic energy store of the object because it starts moving, but some is also being transferred to the thermal energy stores due to friction. This causes the overall temperature of the object to increase.

Question 12

Work done equation

Answer 12

Work done (J) = Force (N) x Distance (m)
W = Fs

Question 13

scale drawings

Answer 13

used to find resultant force
shows all forces acting on an object
1. draw all the forces acting on an object, to scale ‘tip-to-tail’
2. draw a straight line from the start of the first force to the end of the last force - this is the resultant force
3. measure the length of the resultant force on the diagram to find the magnitude and the angle to find the direction of the force

you can split a force into components
- draw the force to scale on a scale grid
- add the horizontal and vertical components along the grid lines
- measure the lines

Question 14

an object is in equilibrium if…

Answer 14

…the forces on it are balanced
e.g. on a scale diagram for three forces the scale diagram will from a triangle

Question 15

elastically deformed

Answer 15

an object can go back to the original shape and length after the force has been removed
- objects that can be elastically deformed are called elastic objects (e.g. a spring)

Question 16

inelastically deformed

Answer 16

an object doesn’t return to its original shape and length after the force has been removed

Question 17

stretching, compressing and bending transfers energy

Answer 17

Work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object. If it is elastically deformed, ALL this energy is transferred to the object’s elastic potential energy store.

Question 18

extension is … to force

Answer 18

directly proportional so F ∝ e
F = ke
Force (N) = Spring constant (N/m) x Extension (m)
The spring constant depends on the material that you are stretching - a stiffer spring has a greater spring constant
This equation also works for compression (where e is just the difference between the natural and compressed lengths - the compression)
HOWEVER this STOPS working when the force is great enough
- there’s a limit to the amount of force you can apply to an object for the extension to keep on increasing proportionally

Question 19

limit of proportionality

Answer 19

extension is no longer proportional to force

Question 20

find the work done in stretching or compressing a spring

Answer 20

as long as its not stretched past its limit of proportionality
Ee = 1/2ke^2
Elastic potential energy (J) = 1/2 x Spring constant (N/m) x extension^2 (m)
for elastic deformation, this formula can be used to calculate the energy stored in a spring’s elastic potential energy store - it’s also the energy transferred to the spring as it’s deformed (or transferred by the spring as it returns to its original shape).

Question 21

moments

Answer 21

a moment is the turning effect of a force
the size of the moment of the force is given by
M = Fd
Moment of a force (Nm) = Force (N) x Distance (m) - the perpendicular distance from the pivot to the line of action of the force
- if the total anticlockwise moments equals the total clockwise moment about a pivot, the object is balanced and won’t turn.

Question 22

Levers make it … for us to do work

Answer 22

easier
levers increase the distance from the pivot at which the force is applied. Since M= Fd this means less force is needed to get the same moment. This means levers make it easier to do work - e.g. lift a load or turn a nut.

Question 23

gears

Answer 23

gears transmit rotational effects
- gears are circular disks with ‘teeth’ around their edges
- their teeth interlock so that turning one causes another to turn, in the opposite direction
- used to transmit the rotational effect of a force from one place to another
- different sized gears can be used to change the moment of the force
a force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater
- the larger gear will turn slower than the smaller gear

Question 24

pressure is...

Answer 24

...the force per unit area

Question 25

fluids

Answer 25

substances that can 'flow' because their particles are able to move around - as these particles move around, they collide with surfaces and other particles - particles are light, but they still have a mass and exert a force on the object they collide with. - Pressure is force per unit area, so this means the particles exert a pressure - the pressure of a fluid means a force is exerted normal (at right angles) to any surface in contact with the fluid - calculate the pressure at the surface of a fluid by using: p = F/A Pressure in pascals (Pa) = Force normal to a surface (N) / Area of that surface (m^2)

Question 26

Pressure in a liquid depends on ... and ...

Answer 26

depth and density Density is a measure of the 'compactness' of a substance (how close together the particles in a substance are) - for a given liquid, the density is uniform (the same everywhere), and doesn't vary with shape or size - the more dense a given liquid is, the more particles it has in a certain space - this means there are more particles that are able to collide so the pressure is higher Depth - pressure increases with depth - the water at the bottom of the container is pushed on by the weight of the water further up, which causes it to be under higher pressure - you can calculate the pressure at a certain depth due to the column of liquid above using p = hρg Pressure (Pa) = Height of the column of liquid (the depth) (m) x Density of the liquid (kg/m^3) x Gravitational field strength (N/kg)

Question 27

upthrust

Answer 27

a resultant force that pushes objects upwards in water due to pressure increasing with depth and the force at the bottom being stronger than at the top

Question 28

objects in fluids experience upthrust

Answer 28

1. when an object is submerged in a fluid (partially or completely) the pressure of the fluid exerts a force on it from every direction 2. 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 3. this causes a resultant force upwards, known as upthrust 4. the upthrust is equal to the weight of fluid that has been displaced by the object. e.g. the upthrust on a pineapple in water is equal to the weight of a pineapple-shaped volume of water.

Question 29

upthrust is equal to

Answer 29

the weight of fluid that has been displaced by the object

Question 30

why does an object float?

Answer 30

if the upthrust on an object is equal to the object's weight, then the forces balance and the object floats - because it is less dense than the fluid it is placed in and weighs less than the equivalent volume of fluid. This means that it displaces a volume of fluid that is equal to its weight before it is completely submerged

Question 31

why does an object sink?

Answer 31

if an object's weight is more than the upthrust, the object sinks - because it is denser than the fluid it is placed in and is unable to displace enough fluid to equal its weight

Question 32

atmospheric pressure

Answer 32

is created on a surface by air molecules colliding with the surface - the atmosphere is a layer of air that surrounds Earth. it is thin compared to the size of the Earth - as the altitude increases, atmospheric pressure decreases. 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 - there are also fewer air molecules above a surface as the height increases. This means that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude.

Question 33

distance is

Answer 33

scalar - how far an object has moved - scalar quantity so doesn't involve direction

Question 34

displacement is

Answer 34

a vector - measures the distance and direction in a straight line from an objects starting point to its finishing point

Question 35

speed and velocity are both...

Answer 35

...how fast you're going speed - scalar velocity - vector - this means you can have objects travelling at a constant speed with a changing velocity - this happens when the object is changing direction whilst staying at the same speed - an object moving in a circle at a constant scene has a constantly changing velocity as the direction is always changing distance travelled = speed x time s = vt

Question 36

speed of: - person walking - person running - person cycling - car - train - plane

Answer 36

- person walking - 1.5m/s - person running - 3m/s - person cycling - 6m/s - car - 25m/s - train - 55m/s - plane - 250m/s

Question 37

uniform acceleration

Answer 37

speeding up or slowing down at a constant rate (constant acceleration) - acceleration due to gravity (g) is uniform for objects in free fall. It's roughly equal to 9.8m/s^2 near the Earth's surface and has the same value as gravitational field strength use this equation for uniform acceleration: v^2 - u^2 = 2as final velocity (m/s) - initial velocity (m/s) = 2 x acceleration (m/s^2) x distance (m)

Question 38

acceleration is...

Answer 38

...how quickly you're speeding up ...the change in velocity in a certain amount of time acceleration (m/s^2) = change in velocity (m/s) / time (s)

Question 39

deceleration is...

Answer 39

...negative acceleration (if something slows down, the change in velocity is negative)

Question 40

distance-time graphs

Answer 40

if an object moves in a straight line, its distance travelled can be plotted on a distance-time graph 1) Gradient = speed (the steeper the graph, the faster it's going). Due to speed = distance / time = change in vertical axis / change in horizontal axis 2)Flat sections are where it's stationary - it's stopped 3)Straight uphill sections mean it's travelling at a steady speed 4)Curves represent acceleration or deceleration 5)A steepening curve means it's speeding up (increasing gradient) 6)A levelling off curve means it's slowing down 7)If the object is changing speed (accelerating) you can find its speed at a point by finding the gradient of the tangent to the curve at that point

Question 41

velocity-time graphs

Answer 41

- show how an object's velocity changes as it travels 1) Gradient = acceleration 2) Flat sections represent travelling t a steady speed 3) The steeper the graph, the greater the acceleration or deceleration 4) Uphill sections are acceleration 5) Downhill sections are deceleration 6) A curve means changing acceleration 7) The area under any section of the graph (or all of it) is equal to the distance travelled in that time interval

Question 42

friction

Answer 42

the resistive force between two surfaces that are sliding - if an object has no force propelling it along it will always slow down and stop due to friction - always acts in the opposite direction to movement - to travel at a steady speed, the driving force needs to balance the frictional force - you get friction between two surfaces in contact, or when an object passes through a fluid (drag)

Question 43

drag

Answer 43

the resistance you get in a fluid (a gas or a liquid) - drag increases as speed increases - air resistance is a type of drag - the most important factor in reducing drag is keeping the shape of the object streamlined (where the object is designed to allow fluid to flow easily across it, reducing drag) - frictional forces from fluids always increase with speed

Question 44

terminal velocity

Answer 44

objects falling through fluids reach a terminal velocity (maximum speed) - when a 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

Question 45

terminal velocity depends on..

Answer 45

..shape and area - caused by air resistance - terminal velocity of any object is determined by its drag in comparison to its weight - (skydiver and parachute example)

Question 46

Newtons first law

Answer 46

known as law of inertia 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 (same speed and direction). an object... - at rest will remain at rest - in motion in a straight line will continue moving in a straight line ...unless acted on by a resultant force - a non-zero resultant force will always produce acceleration (or deceleration) in the direction of the force - this 'acceleration' can take five different forms: starting, stopping, speeding up, slowing down and changing direction. - on a free body diagram, the arrows will be unequal.

Question 47

Newton's second law

Answer 47

Acceleration is proportional to the resultant force and inversely proportional to mass F = ma Resultant force (N) = mass (kg) x acceleration (m/s^2) - the larger the resultant force acting on an object, the more the object accelerates - the force and the acceleration are directly proportional. F ∝ a - acceleration is also inversely proportional to the mass of the object - an object with a larger mass will accelerate less than an object with a smaller mass (for a fixed resultant force)

Question 48

Newton's third law

Answer 48

when two objects interact, the forces they exert on each other are equal and opposite forces act in pairs if object A exerts a force on object B, object B exerts an equal and opposite force on object A. Every action has an equal and opposite reaction.

Question 49

inertia

Answer 49

the tendency for motion to remain unchanged inertial mass is the ratio of force over acceleration

Question 50

stopping distance

Answer 50

the amount of time it takes to stop thinking distance + braking distance

Question 51

resultant force equation

Answer 51

resultant force (N)= inertial mass(kg) x acceleration (m/s^2) F = ma

Question 52

factors affecting thinking distance

Answer 52

- speed - fast speed = further you go before you stop - reaction time - the longer your reaction time, the longer your thinking distance - alcohol/drugs - driver tiredness

Question 53

factors affecting braking distance

Answer 53

- speed - faster a vehicle travels, the longer it takes to stop - weather or road surface - if it's wet/icy/leafy/oily there's less grip (so less friction) between a vehicle's tyres and the road, which can cause tyres to skid - condition of 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 to them skidding on top of water - how good your brakes are - if breaks are worn or faulty, they won't be able to apply as much force as well-maintained braes, which could be dangerous when you need to break hard

Question 54

braking relies on friction between the brakes and wheels

Answer 54

braking relies on friction between the brakes and wheels - 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

Question 55

factors affecting reaction times

Answer 55

- tiredness - drugs - alcohol - distractions

Question 56

momentum

Answer 56

the tendency of an object to continue moving momentum (kg m/s)= mass (kg) x velocity (m/s) p = mv - a vector quantity - it has size and direction - the greater the mass of an object, or the greater its velocity, the more momentum the object has.

Question 57

momentum before =

Answer 57

momentum after - this is called conservation of momentum

Question 58

forces cause a ... in momentum

Answer 58

change force = change in momentum / change in time