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Physics paper 1 > Mechanics and Materials > Flashcards

Mechanics and Materials Flashcards

1
Q

Horizontal and Vertical motion of a projectile

A

-Horizontal and vertical motion are independent.
-Gravitational force only acts vertically, so projectiles only experience acceleration due to gravity in vertical direction (down)
In absence of resistive forces:
-Horizontal motion: no force horizontally, no acceleration so constant velocity = Vcosθ
-Vertical motion: constant force due to weight, uniform acceleration down (g) = Vsinθ

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2
Q

difference between a scalar and a vector

A

vector has magnitude and direction, scalar has only magnitude

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3
Q

examples of scalars

A

speed, mass, time, energy, power

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4
Q

examples of vectors

A

displacement, velocity, acceleration, force, weight

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5
Q

adding perpendicular vectors by calculation

A
  • draw vectors as a right angled triangle and use pythagoras to find magnitude of resultant vector
  • use SOHCAHTOA for the angle
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6
Q

adding vectors by scale drawing

A
  • write down scale
  • draw vectors to correct length and angle to each other “tip to tail”
  • add the resultant vector line and measure length (convert with scale) and angle of resultant vector
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7
Q

conditions for equilibrium of two or three coplanar forces acting at a point

A

total resultant force equals zero OR if vectors representing forces are added together they will form a closed triangle

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8
Q

two conditions for a body to remain in equilibrium

A
  1. resultant force = 0
  2. resultant moment about any force is zero
    - stationary or travelling at constant velocity
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9
Q

moment

A

force x perpendicular distance from the point to the line of action of the force

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10
Q

principle of moments

A

in equilibrium, the sum of the clockwise moments about a point = sum of anticlockwise moments

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11
Q

definition of a couple

A

a pair of equal and opposite coplanar forces - only rotational motion occurs

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12
Q

definition of the moment of a couple

A

one force x perpendicular distance between the lines of actions of the two forces

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13
Q

definition of centre of mass

A

point in a body through which weight appears to act - object will balance if supported at its COM

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14
Q

stability

A

if the centre of mass of an object lies outside its base it will topple over

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15
Q

how size affects COM

A

the wider the base, the lower its COM

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16
Q

contact force

A

force exerted between two objects when they are in contact with each other (reaction force)

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17
Q

tension/tensile force

A

force applied to an object that acts to stretch it

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18
Q

stable equilibrium

A

when a body is displaced then released it will return to its equilibrium position

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19
Q

displacement

A

distance in a given direction

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20
Q

velocity

A

rate of change of displacement

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21
Q

acceleration

A

rate of change of velocity

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22
Q

accelerates / decelerates definition

A

accelerates - velocity increases with time
decelerates - velocity decreases with time

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23
Q

displacement and velocity time graph

A

uniform acceleration - gradient increasing, directly proportional
non uniform acceleration - curve upwards = increasing gradient

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24
Q

gradient of displacement and velocity time graphs

A

gradient of a s-t graph = velocity
gradient of a v-t graph = acceleration

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25
Q

area under velocity and acceleration time graphs

A

area under a v-t graph = displacement
area under an a-t graph = velocity

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26
Q

average velocity

A

total displacement/ total time

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27
Q

instantaneous velocity at a point

A

rate of change of displacement at that point - gradient at a point on a s-t graph (tangent)

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28
Q

equation of motion

A

only apply where acceleration is uniform - both the magnitude and direction remain the same

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29
Q

free fall

A

situation when gravitational force is the only force acting on an object

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30
Q

g by free fall experiment

A
  • measure height from ball to trapdoor
    -flick switch to start timer and disconnect electromagnet releasing ball bearing
    -the ball bearing falls knocking down the trapdoor and breaking the circuit stopping the timer- record the time
    -repeat 3 times and average the time taken
    -g=2xgradient of graph
  • use a small and heavy ball to ignore AR and have a computer release and time ball
    -most error occurs in the measurement of h
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31
Q

drag

A

resistive forces like AR that act to oppose motion - negligible for objects moving slowly in air

32
Q

terminal speed

A

Maximum speed of a falling object reached when the forces of weight and drag are equal

33
Q

conditions for an object falling at terminal velocity

A
  • resultant force on object is zero hence acceleration is zero
  • objects travels at a constant velocity
34
Q

factors affecting drag force on an object

A
  • the shape and speed of the object
  • the viscosity of the fluid
35
Q

explain why an object reaches terminal velocity falling through air

A
  • object dropped from rest so only force acting is weight so there is a resultant force on the object producing acceleration
  • as velocity of object increases the drag force increases, reducing the resultant force. The object still accelerates, but at a decreasing rate
  • Eventually object is falling fast enough for drag force to equal weight so no resultant force hence no acceleration → object falls at a uniform velocity (terminal speed)
36
Q

newtons 1st law of motion

A

an object will continue at rest or uniform velocity unless acted on by a resultant force

37
Q

newton’s 2nd law of motion

A

the acceleration of an object is proportional to resultant force acting on it

38
Q

newton’s 3rd law of motion

A

in an object A exerts a force on a second object B, then object B will exert an equal and opposite force on object A

39
Q

energy conversions of an object falling in presence of resistive forces

A

loss in GPE = gain in KE + WD against resistance

39
Q

principle of conservation of energy

A

energy is neither created or destroyed, only converted from one form to another

40
Q

definition of mechanical work done

A

force multiplied by distance moved in the direction of the force - units J

41
Q

power

A

rate at which energy is transferred

42
Q

units of power

A

W or Js-1

43
Q

density

A

mass per unit volume (scalar)

44
Q

units of density

A

kg m-3

45
Q

Hooke’s Law

A

extension is proportional to the tensile force applied, up to the limit of proportionality

46
Q

features of graph of force against extension confirming Hooke’s law

A

straight line through the origin

47
Q

units of spring constant

A

Nm-1

48
Q

springs in series

A

-Both springs experience the same force F
-the total extension is the sum of the extension of each spring individually

49
Q

identical springs in parallel

A
  • the force applied to the spring combination is shared across each of the springs individually
  • all springs have the same extension
  • therefore they extend less than they would normally as their spring constants add up
50
Q

limit of proportionality

A

the point beyond which force is no longer proportional to extension

51
Q

elastic limit

A

the point beyond which a material will not return to its original length/size when the forces are removed

52
Q

elastic behaviour

A

the material will return to its original length when forces are removed with no permanent extension

53
Q

plastic behaviour

A

the material will be permenantly deformed when forces are removed

54
Q

ethical transport design

A

methods to increase the time of the impact, reducing the force such as crumple zones, seat belts and air bags

55
Q

area under a force/extension graph

A
  • work done on a spring and hence the energy stored as it is loaded
    OR
    -work done by the spring, and hence the energy released as it is unloaded
56
Q

area between the loading and unloading curves of an elastic band

A

internal energy retained

57
Q

derivation of energy stored = ½ FΔL from a graph of force against extension

A
  • ΔW=FΔs, so area beneath line from origin to ΔL represents the WD to compress/extend spring.
  • work done on spring = energy stored
  • area under graph = area of triangle = ½ base x height, therefore energy stored = ½ F x ΔL.
  • OR ∆L proportional to F so produces a straight line with positive gradient hence area = area of a triangle = 0.5F∆L
58
Q

derivation of energy stored = ½ FΔL

A
  • energy stored in a stretched spring = work done stretching the spring
  • work done = force x distance
    -as string is stretched the force gets bigger
  • force is proportional to extension so average force = ½ F
    -W = average force x displacement = ½ FΔL
  • this is the area under the graph of Force against Extension
59
Q

tensile stress

A

tensile force divided by cross sectional area

60
Q

units of stress

A

Pa or Nm-2

61
Q

tensile strain

A

extension of material/original length

62
Q

units of strain

A

none

63
Q

breaking stress (ultimate tensile stress)

A

tensile stress needed to break a solid material

64
Q

description of stiffness

A

requires a large force/stress for a small deformation/extension

65
Q

description of fracture

A

non-brittle fracture: material necks which reduces the CSA so increases stress at that point until the wire breaks
brittle fracture: no plastic deformation, usually snaps suddenly without any noticeable yield through crack propagation

66
Q

brittle material

A

a material that fractures without any plastic deformation

67
Q

description of ductile

A

a material can be drawn into a wire. these are often polymers such as rubber which are used for activities such as bungee jumping which require elasticity

68
Q

description of strength

A

material with a higher breaking stress

69
Q

Young Modulus

A

tensile stress/tensile strain

70
Q

units of Young Modulus

A

Pa or Nm-2

71
Q

use of stress/strain curves to find Young Modulus

A

GRADIENT of the linear section of a graph of stress against strain

72
Q

area under a graph of stress against strain

A

energy stored per unit volume

73
Q

one simple method of measuring Young Modulus

A

measurements to make:
- original length of wire with a ruler
- diameter of a wire with a micro-meter
- mass attached to end of wire
- length of stretched wire with a ruler
Reducing uncertainty
- repeat length measurements
- repeat diameter measurements at different points of the wire
- check for zero error on micrometer and scales
Measurements used to determine Young Modulus
- F=weight=mg. Extension = ΔL
- Cross-sectional area of wire A = πd² / 4.
- Stress = F/A; Strain = ΔL/L
- Plot a graph of stress (y) against strain (x)
- Young Modulus = gradient of linear section of graph

74
Q

energy transfers in a compressed spring that is released

A

elastic strain energy in a spring is converted to KE, which in turn is converted into GPE, as spring is thrown up into the air

Physics paper 1 (5 decks)

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