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physics > mechanics > Flashcards

mechanics Flashcards

1
Q

displacement

A

change in position from one point to its equilibrium position

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2
Q

velocity

A

rate of change of displacement

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3
Q

acceleration

A

rate of change of velocity

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4
Q

describe how to find velocity from a linear displacement vs time graph

A

find the gradient of the graph using as much of the graph as possible

(change in displacement and in time are as big as possible to minimise percentage uncertainty)

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5
Q

describe how to find acceleration from a non-linear velocity vs time graph

A

draw a tangent to the graph at the point you want to find the acceleration and find the gradient of that tangent

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6
Q

describe how to find displacement from a velocity vs time graph

A

find the area between the graph and the axis during the period of time.

(above x-axis is positive displacement, below x-axis is negative displacement)

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7
Q

describe how to calculate the final velocity of an object from an acceleration vs time graph that had initial velocity u

A

find the area between the graph and the x-axis during the period of time using an approximation method e.g. counting squares or trapezium rule.

the area is equal to the change in velocity so add the change to the initial velocity to get the final velocity. (v = u + area)

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8
Q

describe how light gates can be used to measure velocity of a trolley

A

measure a length of card and fix it to the top of the trolley. using a datalogger, record the time taken for the card to pass through the light gate.

velocity = card length / time

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9
Q

describe how 2 light gates can be used to find average acceleration

A

measure a length of card and fix it to the top of the trolley. Using a datalogger, record the time taken for the car to pass between the 2 light gates and the time taken to pass through each

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10
Q

describe an experiment to determine g in a normal school laboratory as accurately as possible

A

an electronic trap door method should be used to eliminate any error starting and stopping the timer correctly and a ball with a small surface area use to minimise the impact of air resistance

the time should be measured for the ball to fall from heights of 20cm, 30cm up to 80cm which will be measured with a metre ruler with each measurement of time being repeated twice and an average time calculated to reduce the effects of any random error

plot a graph of height vs time squared and determine the gradient. g will be equal to the gradient x2

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11
Q

state an equation to calculate the thinking distance (distance travelled by a vehicle before the driving presses the brake)

A

s = ut

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12
Q

state an equation to calculate the braking distance (the distance travelled when the driver applies a constant driving force)

A

v^2 = u^2 + 2as
=> v = 0
=> s = - (u^2 / 2a)
=> mu^2/fm

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13
Q

state an equation for the stopping distance (total distance travelled)

A

s = ut + (mu^2/2f)

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14
Q

explain why the vertical and horizontal motion of a projectile are indpendent

A

a projectile experiences a weight force vertically downward. the component of weight force horizontally = mgcos90 = 0N

if horizontal force = 0N, according to Newtons first law, horizontal velocity must be constant and therefore be unaffected.

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15
Q

describe the condition that lead to a drag force

A

an object colliding with the particles of fluid (liquid or gas) that it is travelling through. the drag force will always act in the opposite direction to velocity

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16
Q

describe the factors that affect the size of a drag force

A

the speed at which the object is travelling and its surface area perpendicular to its velocity (F ∝ Av^2)

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17
Q

describe the conditions at terminal velocity

A

drag force is equal to weight force so the resultant force is zero. Therefore acceleration is zero and velocity remains constant

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18
Q

describe an experiment that could be performed in a school laboratory to demonstrate and measure terminal velocity

A

get a 1 litre measuring cylinder and fill it with water. Then measure the distance between the divisions using a 30cm ruler and finally set up a slow motion camera to record.

drop a ball with higher density than water and cross sectional area just less than the measuring cylinder into the water and start recording

record the location of the ball each 0.1 seconds and use the earlier measurement to calculate the total distance travelled at that time.

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19
Q

define a moment of a force

A

product of force and perinduclar distance between the line of action of the force and the pivot point

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20
Q

define a couple

A

2 forces of equal magnitude but opposite and direction acting to rate an object in the same direction

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21
Q

define a torque

A

the product of the magnitude of the 2 forces that are a couple and the perpendicular distance between them

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22
Q

state the principle of moments

A

if an object is in equilibrium, the total moment clockwise about a pivot is equal to the total moment anti - clockwise about the same pivot

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23
Q

state the principle of moments

A

if an object is in equilibrium, the total moment clockwise about a pivot is equal to the total moment anti - clockwise about the same pivot

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24
Q

define centre of mass

A

a point on an object where all the mass can be modelled to be concentrated

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25
Q

define centre of gravity

A

a point on an object where the weight force acts from

26
Q

experiment to determine centre of gravity

A

freely hang the object from a point and use a plumb line to draw a vertical line downwards from the pivot.

freely hang the object from another point and use a plumb line to draw a vertical line downwards from the pivot

if the object is 3D, repeat this step again

the centre of gravity is located at the intersection of these lines

27
Q

define work done

A

product of force and displacement parallel to the force

28
Q

conservation of energy

A

energy cannot be created or destroyed, only transferred

29
Q

facts about the equation below:
W = Fscos θ

A

θ is the angle between the force and displacement

this equation can be used when force is constant

30
Q

mechanical energy

A

the sum of an systems kinetic energy, gravitational potential energy and elastic potential energy

31
Q

conservation of mechanical energy

A

if no external force acts on a system, its total mechanical energy is constant

32
Q

calculate work done from conservation of energy

A

W = ∆KE +∆GPE + ∆EPE

33
Q

kinetic energy

A

the work done to accelerate an objects from stationary

34
Q

calculate kinetic energy

A

KE = 0.5 x m x v^2

35
Q

define and calculate gravitational potential energy

A

the work done against an objects weight force to increase the height of an object from a point defined as zero GPE

GPE = mgh

36
Q

define power

A

the rate at which work is done

37
Q

calculate power

A

W / t

38
Q

Calculate instantaneous power

A

P = Fv

39
Q

Calculate efficiency from power or energy

A

Useful energy output / total energy input = useful power output / total power input

40
Q

define tensile force

A

a force acting to increase the length of an object

41
Q

define compressive force

A

a force acting to decrease the length of an object

42
Q

state Hooke’s law

A

force is directly proportional to extension up the limit of proportionality

43
Q

describe techniques to investigate the force-extension characteristics of other materials

A

measure the original length of the material using a metre ruler and then fixed one end to a clamp with stand which has been G-clampled to the desk to prevent it toppling

hang 100g on the wire using fixed masses and measure the extended length and calculate the extension

43
Q

describe techniques to investigate the force-extension characteristics of other materials

A

measure the original length of the material using a metre ruler and then fixed one end to a clamp with stand which has been G-clamped to the desk to prevent it toppling

hang 100g on the wire using fixed masses and measure the extended length and calculate the extension

repeat this process with 200g, 300g, up to 700g and plot a graph of force vs extension for the material

44
Q

define elastic potential energy

A

the work done to change the shape of a material from undeformed

45
Q

calculate elastic potential energy from a material obeying Hooke’s law

A

EPE = 0.5 x F x X
F = kx
=> EPE = 0.5 x K x X^2

46
Q

define stress

A

the force divided by the cross-sectional area

47
Q

define strain

A

the extension divided by the original length

48
Q

define Ultimate tensile stress

A

the maximum tensile stress that a material can undergo

49
Q

define young modulus

A

the ratio of tensile stress to tensile strain

50
Q

equations to calculate Young Modulus

A

E = (F/A) / (x/L) = (FL) / (Ax)

51
Q

describe techniques to determine young modulus

A

measure the original length of the material using a metre ruler and then fixed one end to a clamp stand which has been G-clamped to the desk to prevent it toppling

hang a 1kg mass on the wire and use a Vernier scale to measure the extension of the test wire

increase the mass to 2kg, 3kg up to 7kg and measure the extension for each

to calculate the stress, first measure the diameter of the wire in several location to get an average diameter and calculate the cross-sectional area

plot a graph of stress vs strain and measure the gradient in the straight line section to find the young modulus

52
Q

define elastic deformation

A

when a material is stretched below its elastic limit and will return to its original shape once the force is removed

53
Q

define plastic deformation

A

when a material is stretched above its elastic limit and will be permanently deformed once the force is removed

54
Q

newtons 1st law

A

velocity is constant unless a resultant force acts

55
Q

newtons 2nd law

A

resultant force is equal to the rate of change of momentum

56
Q

newtons 3rd law

A

every action has an equal and opposite reaction

(the force will be equal in magnitude, opposite in direction and of the same type)

57
Q

define linear momentum

A

the product of mass and velocity

58
Q

calculate linear momentum

A

p = mv

59
Q

calculate resultant force

A

F = ∆mv / ∆t

60
Q

define impulse

A

the change in momentum of an object

61
Q

calculate impulse from graphs and equations

A

impulse is calculated by the area under a force vs time graph (if force is variable)

impulse is calculated using I = Ft (if force is constant)

physics (6 decks)

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