mechanics

Question 1

displacement

Answer 1

change in position from one point to its equilibrium position

Question 2

velocity

Answer 2

rate of change of displacement

Question 3

acceleration

Answer 3

rate of change of velocity

Question 4

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

Answer 4

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)

Question 5

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

Answer 5

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

Question 6

describe how to find displacement from a velocity vs time graph

Answer 6

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)

Question 7

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

Answer 7

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)

Question 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

s = ut

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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

Question 14

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

Answer 14

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.

Question 15

describe the condition that lead to a drag force

Answer 15

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

Question 16

describe the factors that affect the size of a drag force

Answer 16

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

Question 17

describe the conditions at terminal velocity

Answer 17

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

Question 18

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

Answer 18

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.

Question 19

define a moment of a force

Answer 19

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

Question 20

define a couple

Answer 20

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

Question 21

define a torque

Answer 21

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

Question 22

state the principle of moments

Answer 22

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

Question 23

state the principle of moments

Answer 23

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

Question 24

define centre of mass

Answer 24

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

Question 25

define centre of gravity

Answer 25

a point on an object where the weight force acts from

Question 26

experiment to determine centre of gravity

Answer 26

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

Question 27

define work done

Answer 27

product of force and displacement parallel to the force

Question 28

conservation of energy

Answer 28

energy cannot be created or destroyed, only transferred

Question 29

facts about the equation below: W = Fscos θ

Answer 29

θ is the angle between the force and displacement this equation can be used when force is constant

Question 30

mechanical energy

Answer 30

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

Question 31

conservation of mechanical energy

Answer 31

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

Question 32

calculate work done from conservation of energy

Answer 32

W = ∆KE +∆GPE + ∆EPE

Question 33

kinetic energy

Answer 33

the work done to accelerate an objects from stationary

Question 34

calculate kinetic energy

Answer 34

KE = 0.5 x m x v^2

Question 35

define and calculate gravitational potential energy

Answer 35

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

Question 36

define power

Answer 36

the rate at which work is done

Question 37

calculate power

Answer 37

W / t

Question 38

Calculate instantaneous power

Answer 38

P = Fv

Question 39

Calculate efficiency from power or energy

Answer 39

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

Question 40

define tensile force

Answer 40

a force acting to increase the length of an object

Question 41

define compressive force

Answer 41

a force acting to decrease the length of an object

Question 42

state Hooke's law

Answer 42

force is directly proportional to extension up the limit of proportionality

Question 43

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

Answer 43

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

Question 43

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

Answer 43

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

Question 44

define elastic potential energy

Answer 44

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

Question 45

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

Answer 45

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

Question 46

define stress

Answer 46

the force divided by the cross-sectional area

Question 47

define strain

Answer 47

the extension divided by the original length

Question 48

define Ultimate tensile stress

Answer 48

the maximum tensile stress that a material can undergo

Question 49

define young modulus

Answer 49

the ratio of tensile stress to tensile strain

Question 50

equations to calculate Young Modulus

Answer 50

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

Question 51

describe techniques to determine young modulus

Answer 51

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

Question 52

define elastic deformation

Answer 52

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

Question 53

define plastic deformation

Answer 53

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

Question 54

newtons 1st law

Answer 54

velocity is constant unless a resultant force acts

Question 55

newtons 2nd law

Answer 55

resultant force is equal to the rate of change of momentum

Question 56

newtons 3rd law

Answer 56

every action has an equal and opposite reaction (the force will be equal in magnitude, opposite in direction and of the same type)

Question 57

define linear momentum

Answer 57

the product of mass and velocity

Question 58

calculate linear momentum

Answer 58

p = mv

Question 59

calculate resultant force

Answer 59

F = ∆mv / ∆t

Question 60

define impulse

Answer 60

the change in momentum of an object

Question 61

calculate impulse from graphs and equations

Answer 61

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)