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motion and forces Flashcards

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1
Q

explain that a scalar quantity has magnitude (size) but no specific direction

A

a scalar quantity has magnitude but no direction

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2
Q

Explain that a vector quantity has both magnitude (size) and a
specific direction

A

a vector has both magnitude and direction

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3
Q

explain the difference between vector and scalar quantities

A

vector - magnitude and direction
scalar - only magnitude and no direction

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4
Q

examples of vector quantities

A

displacement, velocity, acceleration, force, weight, momentum

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5
Q

examples of scalar quantities

A

distance, speed, mass, energy

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6
Q

recall that velocity is speed in a stated direction

A

velocity is speed in a given direction (ex 30 mph north, 20m/s)
meaning you can have objects travelling at a constant speed with a changing velocity because the object is changing direction whilst staying at the same speed.

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7
Q

equation relating speed, distance and time

A

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

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8
Q

Analyse distance/time graphs

A

gradient = speed
flat = stopped
steeper = faster
curves = accelerating
steeper curve = speeding up (increasing gradient)
levelling off = slowing down (decreasing gradient)

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9
Q

how to determine the speed from the gradient in distance/time graphs

A
  1. if the line is straight - speed at any point on line is equal to the gradient of the line
  2. work out gradient - change in vertical/change in horizontal
  3. if the graph is curved, to find speed at certain time, you need to draw a tangent and then find the gradient of the tangent
  4. calculate average speed of an object when it has non uniform acceleration (its accelerating) by dividing distance travelled by time it takes to travel that distance
    ex speed = distance /time
    *still have to use tangent if graph is curved
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10
Q

equation relating acceleration, change in velocity and time *and when to use it

A

acceleration (m/s*2) = (final(m/s) - initial velocity(m/s)) /time(s)
*when working out average acceleration/deceleration

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11
Q

equation relating final velocity, initial velocity, acceleration, distance

A

final velocity squared - initial velocity squared = 2 * acceleration * distance

*=multiply

should use this to work out uniform acceleration/constant acceleration

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12
Q

Analyse velocity/time graphs to compare acceleration from gradients qualitatively

A

Flat sections- steady speed
Steeper graph- greater accel./decel.
Uphill- decel
Curve- changing accel

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13
Q

Analyse velocity/time graphs to calculate the acceleration from the gradient (for uniform acceleration only)

A

-area under section of the graph = distance travelled in that time interval
-for bits where accelerations constant you can split it into triangles and rectangles
- also find area under graph by counting squares under lien then multiplying the number of the value of one square

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14
Q

Analyse velocity/time graphs to determine the distance travelled using the area between the graph line and the time axis (for uniform acceleration only)

A
  • split area into a triangle and rectangle and add areas
    -area of triangle = base times height
  • or find value of one square and count total number of squares under the line and then multiply the values togther
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15
Q

Describe a range of laboratory methods for determining the speeds of objects such as the use of light gates

A
  • Ruler and stopwatch -ruler finds distance travelled -stopwatches finds time
  • light gates - size of object with ruler - light gate connected to timer which gives reading
    -video analysis - Distance moved from frame to frame observed on a ruler in the pictures - time between frames is known
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16
Q

Recall some typical speeds encountered in everyday experience for wind and sound, and for walking, running, cycling and other transportation systems

A

-wind = 5-20 m/s
-speed of sound in air = 340m/s
- walking = 1.4 m/s
-running = 3 m/s
-cycling = 5.5 m/s
-car= 13-30 m/s
-train = up to 55m/s
-aero plane= 250 m/s

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17
Q

Recall the acceleration (due to gravity) in a free fall

A

-10m/squared

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18
Q

What is newtons first law

A

If resultant force on a stationary object is 0, the object will remain stationary. If the resultant force on a loving object is 0 it’ll keep on moving at the same velocity

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19
Q

Occording to Newton’s first law what will happen where the resultant force on a body is zero

A
  • the resistive and driving forces on it must all be balance
  • velocity will only change if there’s a non-zero resultant force acting on the object
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20
Q

Occording to Newton’s first law what will happen when the resultant force is not zero

A
  • a nonzero resultant force will always produce an acceleration or deceleration in the direction of the force
  • this acceleration could be starting stopping speeding up slowing down or changing direction
21
Q

Recall and use newtons second law

A

force (newton, N) = mass (kilogram, kg) × acceleration (metre per second squared, m/s2)
F =m×a

22
Q

Define weight, recall and use the equation

A

weight (newton, N) = mass (kilogram, kg) × gravitational field
strength (newton per kilogram, N/kg)
W = m× g
- weight is the force acting on an object due to gravity

23
Q

Describe how weight is measured

A
  • calibrated spring balance (or newton meter)
24
Q

Describe the relationship between the weight of a body and the
gravitational field strength

A
  • directly proportional
25
Q

Core Practical: Investigate the relationship between force, mass
and acceleration by varying the masses added to trolleys

A
  1. measure mass of trolley, unit masses and hanging hook, measure length of piece of card (which will interrupt light gate beams). then set up apparatus *don’t attach string to trolley
  2. adjust height of ramp until trolley just starts to move
  3. mark line before first light gate, to make sure trolley travels the same distance every time, light gate records initial speed of the trolleu as it begins to move
  4. attach trolley to hanging mass by string , hold trolley still at start of line then let go so it rolls down slope
  5. each light gate will record the time when the trolley passes through it and the speed of the trolley at that time
  6. acceleration of the trolley is then found using acceleration = change in speed/time where
    a) initial speed = trolley passing through light gate (roughly 0m/s)
    b) final speed = the trolley which equals speed of trolley through second light gate
    c) time it takes trolley to travel between the two light gates
26
Q

why do you change the height of the ramp so the trolley only just begins to move

A
  • means that other forces that are applied (like force due to gravity caused by hanging mass) will be the main cause of the trolley accelerating as it travels down the ramp, the size of this acceleration depends on mass of trolley and size of accelerating force
27
Q

how to investigate effect of the trolleys mass

A
  • add masses one at a time to the trolley, keep mass on the hook constant (so accelerating force is constant - where force is equal to the mass on hook times acceleration due to gravity) repeat steps 2-5 of core practical experiment each time
28
Q

how to investigate effect of accelerating force

A
  • start with all masses loaded onto the trolley and transfer the masses to the hook one at a time
  • repeat steps 2-5 each time you move a mass
    (you transfer the masses because you need to keep the mass of the whole system (mass of trolley and mass on hook) the same, because the accelerating force causes BOTH the trolley and the hanging mass to accelerate
  • as accelerating force increases, acceleration increases (for given trolley mass)
    so force and acceleration are proportional as mass of trolley increases its acceleration decreases (for a given force) - mass and acceleration are inversely proportional
29
Q

Explain that for motion in a circle there must be a
resultant force known as a centripetal force that acts
towards the centre of the circle

A

(When two or more forces act on an object, the resultant force can be found by adding up the individual forces)
- an object constantly changing direction is constantly changing velocity so is accelerating
- this means there must be a resultant force
- this force acts towards the centre of the circle
- the force that keeps something moving in a circle is the centripetal force

29
Q

Explain that an object moving in a circular orbit at
constant speed has a changing velocity

A
  • velocity is speed and direction of an object
  • in an object travels in a circle (at as constant speed) it is constantly changing direction = constantly changing velocity = accelerating
30
Q

Explain that inertial mass is a measure of how difficult it
is to change the velocity of an object (including from
rest) and know that it is defined as the ratio of force over
acceleration

A
  • newtons first law
  • tendency to keep moving with the same velocity is the inertia
  • objects inertial mass measures how difficult it is to change the velocity of an object
  • inertial mass can be found by newtons second law (f=ma)
  • which rearranged is m=F/a so inertial mass is the ratio of force over acceleration
31
Q

what is newtons third law

A

when two objects interact the forces they exert on each other are equal and opposite

32
Q

apply Newton’s third law collision interactions and relate to the conservation of momentum

A

ex two skaters
- when skater A pushes skater B she feels an equal and opposite force
- both skaters feel the same sized force in opposite directions and so accelerate away from each other
- the skater with the smaller mass will accelerate more as a=F/m
- these equally sized forces in opposite directions explain the conservation of momentum

33
Q

apply newtons third law to equilibrium reactions and relate it to the conservation of momentum in collisions

A

ex book sat on table
- weight of book pulls it down and normal reaction force from table pushes it up - these forces are equal to each other so the book is in equilibrium and doesn’t move
- this is not newtons third law these forces are different types and are both acting on the book
- the pairs of forces due to newtons third law in this case are
a) book is pulled down by its weight due to gravity on earth and the book also pulls back up on the earth
b) normal contact force from the table pushing up on the book and the normal contact force from the book pushing down on the table

34
Q

Define momentum, recall and use the equation

A

-momentum (kilogram metre per second, kg m/s) = mass
(kilogram, kg) × velocity (metre per second, m/s)
p = m × v
- momentum = property, all moving objects have, product of all the objects mass and velocity

35
Q

Describe examples of momentum in collisions

A
  • in a closed system the total momentum before and event is the same as after the event (ex collision) = conservation of momentum
  • in snooker balls of same size and mass collide, each collision is an event where the momentum of each ball changes, but overall momentum stays the same (momentum si conserved)
  • white ball hits red so it moves, red has momentum, white continues moving but at smaller velocity (and smaller momentum)
  • combined momentum of red and white is equal to original momentum of white ball
36
Q

use newtons second law as:

A

force (newton, N) = change in momentum (kilogram
metre per second, kg m/s) ÷ time (second, s)
- F= (mv-mu)/t

37
Q

Explain methods of measuring human reaction times and recall
typical results

A
  • a computer based test
  • ruler test
    1. arms on table and someone else holds a ruler so it hangs between you thumb and forefinger lined up to zero, may need a third person to be at eye level with the ruler to check its lined up
    2. no warning, drop ruler and person needs to catch it as quick as they can
    3. measurement on ruler at point where it was caught is how far the ruler dropped in time it took you to react
    4. longer distance = longer reaction time
    5. calculate reaction time use acceleration equations as its acceleration is constant
  • repeat a lot and get an average
  • typical reaction time = 0.2-0.6 seconds
  • someone’s reactions time in a real r=situation will be longer, ex alert driver = 1 second
38
Q

recall what affects the stopping distance of a vehicle

A

thinking distance + braking distance

39
Q

Explain that the stopping distance of a vehicle is affected by a
range of factors including the mass of the vehicle

A
  • heavy car won’t stop as quickly
40
Q

Explain that the stopping distance of a vehicle is affected by a
range of factors including the speed of the vehicle

A
  • faster speed = longer time to stop
  • faster speed = further to travel during reaction time
41
Q

Explain that the stopping distance of a vehicle is affected by a
range of factors including the driver’s reaction time

A
  • reaction time can be increase by tiredness, alcohol, drugs and distractions
42
Q

Explain that the stopping distance of a vehicle is affected by a
range of factors including the state of the vehicle’s brakes

A
  • work of faulty brakes won’t be able to brake with as much force
43
Q

Explain that the stopping distance of a vehicle is affected by a
range of factors including the state of the road and amount of friction between the tyre and the road surface

A
  • amount of friction between tyres and road = more likely to skid if road is dirty, icy or wet
44
Q

Describe the factors affecting a driver’s reaction time including
drugs and distractions

A
  • reaction time is increased by tiredness, alcohol, drugs and distractions
45
Q

Explain the dangers caused by large decelerations

A
  • large decelerations = large force (F=ma) = serious injuries
  • force can be lowered by slowing object over a long time (ex decreasing its deceleration)
  • safety features in vehicles increase collision times as it reduces force
  • ex seat belts stretch and air bags slow you down gradually
  • crumple zones are areas which crumple (in a vehicle) up easily in a collision = increases time taken to stop
45
Q

define the thinking and braking distances

A
  • thinking = distance car travels in driver’s reaction time (time between noticing hazard and applying brakes)
  • braking distance = distance taken to stop once the brakes have been applied
46
Q

explain and estimate the forces involved in typical situations on a
public road

A
  • brakes of a vehicle works on its wheels = transfers energy from vehicles KE store to its thermal energy store of the brakes
  • large decelerations may make brakes overheat and a vehicle may skid

combined physics (7 decks)

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