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A Level Physics > Mechanics and Materials > Flashcards

Mechanics and Materials Flashcards

1
Q

Scalar vs Vector

A
  • A scalar is a quantity which only has a magnitude (size)
  • A vector is a quantity which has both a magnitude and a direction
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2
Q

Is distance/displacement vector or scalar and explain why

A
  • Distance is a scalar quantity because it describes how far an object has travelled overall, but not the direction it has travelled in
  • Displacement is a vector quantity because it describes how far an object is from where it started and in what direction
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3
Q

How can vectors be represented and how does this work

A

by an arrow:

The arrowhead indicates the direction of the vector
The length of the arrow represents the magnitude

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4
Q

What are the two methods used to add vectors

A
  • Calculation – if the vectors are perpendicular
  • Scale drawing – if the vectors are not perpendicular
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5
Q

Describe how vectors can be found using the triangle vs parallelogram method

A
  • To combine vectors using the triangle method:
    Step 1: link the vectors head-to-tail
    Step 2: the resultant vector is formed by connecting the tail of the first vector to the head of the second vector
  • To combine vectors using the parallelogram method:
    Step 1: link the vectors tail-to-tail
    Step 2: complete the resulting parallelogram
    Step 3: the resultant vector is the diagonal of the parallelogram
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6
Q

Describe the forces acting on an inclined plane

A
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7
Q

When are forces in equilibrium

A
  • At rest
  • Moving at constant velocity
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8
Q

What are coplanar forces

A

Forces that act in the same plane

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9
Q

What is a moment

A

the turning effect of a force

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10
Q

When do moments occur

A

when forces cause objects to rotate about some pivot

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11
Q

What is the equation stating the moment of a force

A

Moment (N m) = Force (N) × perpendicular distance from the pivot (m)

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12
Q

What is the unit of a moment

A

N m

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13
Q

What is the principle of moments

A

For a system to be in equilibrium, the sum of clockwise moments about a point must be equal to the sum of the anticlockwise moments (about the same point)

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14
Q

What is a couple

A

a pair of equal and opposite coplanar forces that act to produce rotation only

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15
Q

What kind of forces does a couple contain

A

Forces that are:
- Equal in magnitude
- Opposite in direction
- Perpendicular to the distance between them

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16
Q

Does an object with a couple accelerate

A

No- Couples produce a resultant force of zero, so, due to Newton’s Second law (F = ma), the object does not accelerate

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17
Q

What kind of moment system does not require a pivot

A

a couple

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18
Q

What is the moment of a couple equal to

A

Force × Perpendicular distance between the lines of action of the forces

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19
Q

Centre of mass def

A

the point at which the weight of the object may be considered to act

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20
Q

Where is the position of the centre of mass for a uniform regular solid

A

at its centre

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21
Q

Where is the position of the centre of mass for symmetrical objects with uniform density

A

at the point of symmetry

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22
Q

When is an object stable

A

when its centre of mass lies above its base

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23
Q

How is the base width, centre of mass and stability linked?

A
  • The wider base an object has, the lower its centre of mass and it is more stable
  • The narrower base an object has, the higher its centre of mass and the object is more likely to topple over if pushed
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24
Q

What is instantaneous speed (or velocity)

A

the speed (or velocity) of an object at any given point in time

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25
Q

How to find instantaneous velocity on a displacement-time graph

A

Draw a tangent at the required time
Calculate the gradient of that tangent

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26
Q

How is acceleration shown on a displacement-time graph

A

by a curved gradient

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27
Q

How to find average speed on a displacement-time graph

A

divide the total displacement (on the y-axis) by the total time (on the x-axis)

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28
Q

What do the features of a displacement-time graph mean

A
  • The gradient (or slope) equals velocity
  • The y-intercept equals the initial displacement
  • A diagonal straight line represents a constant velocity
  • A positive slope represents motion in the positive direction
  • A negative slope represents motion in the negative direction
  • A curved line represents an acceleration
  • A horizontal line (zero slope) represents a state of rest
  • The area under the curve is meaningless
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29
Q

What do the features of a velocity-time graph mean

A
  • Slope equals acceleration
  • The y-intercept equals the initial velocity
  • A straight line represents uniform acceleration
  • A positive slope represents an increase in velocity (acceleration) in the positive direction
  • A negative slope represents an increase in velocity (acceleration) in the negative direction
  • A curved line represents the non-uniform acceleration
  • A horizontal line (zero slope) represents motion with constant velocity
  • The area under the curve equals the displacement or distance travelled
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30
Q

What do the components of an acceleration-time graph mean

A
  • The slope is meaningless
  • The y-intercept equals the initial acceleration
  • A horizontal line (zero slope) represents an object undergoing constant acceleration
  • The area under the curve equals the change in velocity
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31
Q

Draw a displacement-time graph for a ball bouncing up and down

A
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32
Q

Draw a velocity-time graph for a ball bouncing up and down

A
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33
Q

Draw an acceleration-time graph for a ball bouncing up and down

A
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34
Q

Describe the mechanical features of a bouncing ball at its highest point (displacement, velocity and acceleration)

A
  • The ball is at its maximum displacement
  • The ball momentarily has zero velocity
  • The velocity changes from positive to negative as the ball changes direction
  • The acceleration, g, is still constant and directed vertically downwards
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35
Q

Describe the mechanical features of a bouncing ball at its lowest point (displacement, velocity and acceleration)

A
  • The ball is at its minimum displacement (on the ground)
  • Its velocity changes instantaneously from negative to positive, but its speed (magnitude) remains the same
  • The change in direction causes a momentary acceleration (since acceleration = change in velocity / time)
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36
Q

What are the components that make up the SUVAT equations

A

s = displacement (m)
u = initial velocity (m s-1)
v = final velocity (m s-1)
a = acceleration (m s-2)
t = time (s)

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37
Q

What are the components of the trajectory of an object undergoing projectile motion

A

a vertical component and a horizontal component

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38
Q

What is the range for projectile motion

A

the horizontal distance travelled by the projectile

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39
Q

How is the time of flight and maximum height calculated for an object in projectile motion

A
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40
Q

How is the range calculated for an object in projectile motion

A
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41
Q

What are drag forces

A

forces that oppose the motion of an object moving through a fluid (gas or liquid)

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42
Q

Examples of drag forces

A

friction and air resistance

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43
Q

What are the rules of drag forces

A
  • Are always in the opposite direction to the motion of the object
  • Never speed an object up or start them moving
  • Slow down an object or keeps them moving at a constant speed
  • Convert kinetic energy into heat and sound
  • increases with the speed of the object
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44
Q

What is ‘lift’ and explain an example of it

A
  • an upwards force on an object moving through a fluid. It is perpendicular to the fluid flow
  • For example, as an aeroplane moves through the air, it pushes down on the air to change its direction
  • This causes an equal and opposite reaction upwards on the wings (lift) due to Newton’s third law
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45
Q

In what direction is a lift force

A

in the opposite direction to the weight

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46
Q

What is air resistance

A

an example of a drag force that objects experience when moving through the air

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47
Q

What factors affect the maximum speed of an object

A
  • Cross-sectional area
  • Shape
  • Altitude
  • Temperature
  • Humidity
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48
Q

Why is there less air resistance at higher altitudes

A

because air is less dense

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49
Q

Draw a height/distance graph for an object in projectile motion with and without air resistance

A
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50
Q

Describe how terminal velocity is achieved

A
  • For a body in free fall, the only force acting is its weight and its acceleration g is only due to gravity.
  • The drag force increases as the body accelerates
  • This increase in velocity means the drag force also increases
  • Due to Newton’s Second Law, this means the resultant force and therefore acceleration decreases (recall F = ma)
  • When the drag force is equal to the gravitational pull on the body, the body will no longer accelerate and will fall at a constant velocity
  • This is the maximum velocity that the object can have and is called the terminal velocity
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51
Q

Newton’s first law

A

A body will remain at rest or move with constant velocity unless acted on by a resultant force

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52
Q

Newton’s second law

A

The resultant force on an object is equal to its rate of change in momentum.

(The resultant force on an object with constant mass is directly proportional to its acceleration - F=ma)

53
Q

What is a resultant force

A

the vector sum of all the forces acting on the body

54
Q

Newton’s third law

A

If body A exerts a force on body B, then body B will exert a force on body A of equal magnitude but in the opposite direction

(the force pairs must be the same type)

55
Q

What is, and what are the components of, the linear momentum equation

A
56
Q

Unit of momentum

A

kg m s−1

57
Q

Principle of conservation of momentum

A

The total momentum before a collision = the total momentum after a collision provided no external force acts

58
Q

What is linear momentum

A

The momentum of an object that only moves in one dimension

59
Q

External vs internal forces

A
  • External forces = forces that act on a structure from outside e.g. friction and weight
  • Internal forces = forces exchanged by the particles in the system e.g. tension in a string
  • Forces which are internal or external will depend on the system itself
60
Q

What are systems with no external forces called

A

‘closed’ or ‘isolated’

61
Q

Force def

A

the rate of change of momentum on a body

62
Q

What is the equation linking force, momentum and time

A
63
Q

What is impulse equal to

A

The change In momentum

64
Q

What do the components of the impulse equation mean

A
65
Q

In what direction does the impulse work in, in regards to force

A

In the same direction as the force

66
Q

Unit of impulse

A

N s

67
Q

What does the impulse equation say about the relationship between force over time

A

A small force acting over a long time has the same effect as a large force acting over a short time

68
Q

What does the area under a force-time graph equate to

A

Impulse

69
Q

How are impact forces reduced

A

By increasing the contact time

70
Q

Examples of where reducing impact force is important

A
  • In sport
  • In packaging
71
Q

What are the two types of collision (or explosion)

A

Elastic and inelastic

72
Q

Describe the difference between elastic and inelastic collisions

A
  • Elastic – if the kinetic energy is conserved
  • Inelastic – if the kinetic energy is not conserved
73
Q

What do elastic/inelastic collisions commonly look like

A

Elastic collisions are commonly those where objects colliding do not stick together and then move in opposite directions

Inelastic collision are commonly those where objects collide and stick together after the collision

74
Q

What is the difference between collisions and explosions

A
  • collisions are usually to do with objects striking against each other
  • explosions are usually to do with recoil
75
Q

Describe the function of the safety features of cars

A

Vehicle safety features are designed to absorb energy upon an impact by changing shape

Seat belts:
- These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop
- They are designed to stretch slightly to increase the time for the passenger’s momentum to reach zero and reduce the force on them in a collision

Airbags:
- These are deployed at the front on the dashboard and steering wheel when a collision occurs
- They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact

Crumple zones:
- These are designed into the exterior of vehicles
- They are at the front and back and are designed to crush or crumple in a controlled way in a collision
- This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions
- The crumple zones increase the time over which the vehicle comes to rest, lowering the impact force on the passengers

76
Q

Draw the differences in a time/force graph for a car collision with and without a seatbelt

A
77
Q

Work def

A

The amount of energy transferred when an external force causes an object to move over a certain distance

78
Q

What do the components of the work done equation mean

A
79
Q

When does an object gain/lose energy, in regards to the direction of the force

A
  • Usually, if a force acts in the direction that an object is moving then the object will gain energy
  • If the force acts in the opposite direction to the movement then the object will lose energy
80
Q

What do the components of the work done (at an angle) equation mean

A
81
Q

Power def

A

the rate of doing work or the rate of energy transfer

82
Q

What do the components of the power equation mean

A
83
Q

What does the power equation show

A

that the power is increased if:
- There is a greater energy transfer (work done)
- The energy is transferred (work is done) over a shorter period of time

84
Q

What do the components of the power, f and v equation mean

A
85
Q

What does the area under a force-displacement graph equal to

A

The work done

86
Q

What is the magnitude of work done, in regards to force/ displacement

A

The work done is equivalent whether there is:
- A small force over a long displacement
- A large force over a small displacement

87
Q

What is a variable force

A

A force on an object which is not always constant

88
Q

Can these equations be used when the force is not constant?

A

No

89
Q

How must work done be calculated, if the force is not constant

A

By the area under a force-displacement graph

90
Q

Efficiency def

A

the ratio of the useful power output from a system to its total power input

91
Q

What does it mean when a system has high/low efficiency

A
  • If a system has high efficiency, this means most of the energy transferred is useful
  • If a system has low efficiency, this means most of the energy transferred is wasted
92
Q

Efficiency equation

A
93
Q

Principle of conservation of energy

A

Energy cannot be created or destroyed, it can only be transferred from one form to another

94
Q

What is kinetic, gravitational potential, elastic, chemical, nuclear and internal energy

A
95
Q

What is energy dissipation

A

Ways in which energy is wasted

96
Q

Density def

A

Mass per unit volume of an object

97
Q

Units of density

A
98
Q

Hooke’s law

A

The extension of the material is directly proportional to the applied force (load) up to the limit of proportionality

(Applies to both compressions and extensions)

99
Q

What do the components of the hooke’s law equation mean

A
100
Q

What is the spring constant a measure of

A

The stiffness of a material. The larger the spring constant, the stiffer the material

101
Q

Draw the force-extension graph for a spring

A
102
Q

What do the features of the force-extension graph for a spring mean

A
  • The limit of proportionality: The point beyond which Hooke’s law is no longer true when stretching a material i.e. the extension is no longer proportional to the applied force
    The point is identified on the graph where the line starts to curve (flattens out)
  • Elastic limit: The maximum amount a material can be stretched and still return to its original length (above which the material will no longer be elastic). This point is always after the limit of proportionality
  • The gradient of this graph is equal to the spring constant k
103
Q

What are forces called that stretch an object

A

Tensile forces - they lead to tensile stress and tensile strain

104
Q

What do the components of the tensile stress equation mean

A
105
Q

Unit of tensile stress

A

Pascals (Pa)

106
Q

What do the components of the tensile strain equation mean

A
107
Q

Draw a labelled diagram of the features of a stress-strain graph

A
108
Q

Describe the key points of the features of a stress-strain graph

A
  • Yield Stress: The force per unit area at which the material extends plastically for no / a small increase in stress
  • The elastic strain energy stored per unit volume is the area under the Hooke’s Law (straight line) region of the graph
  • Breaking point: The stress at this point is the breaking stress. This is the maximum stress a material can stand before it fractures
  • Elastic region: The region of the graph up till the elastic limit. In this region, the material will return to its original shape when the applied force is removed
  • Plastic region: The region of the graph after the elastic limit. In this region, the material has deformed permanently and will not return to its original shape when the applied force is removed
109
Q

Explain what elastic strain energy is

A
  • Work has to be done to stretch a material
  • Before a material reaches its elastic limit (whilst it obeys Hooke’s Law), all the work is done is stored as elastic strain energy
110
Q

What is the area under a force-extension graph equal to

A

Elastic strain energy (true for whether the material obeys Hooke’s law or not)

111
Q

What do the components of the elastic strain energy equation mean

A
112
Q

What is the equation for elastic strain energy that is not on the formula sheet

A
113
Q

What is the breaking stress

A

the maximum stress a material can stand before it fractures (breaks)

114
Q

What is a material called when it has a high breaking stress, what does this mean and give an example

A
  • ductile, which means it can extend more before breaking because of plastic deformation
  • A common example of this is copper, as well as being a good electrical conductor, copper is ductile so it is a suitable material for making wires
115
Q

What is the ultimate tensile stress

A

the maximum stress that the material can withstand

116
Q

Describe the two types of deformation that materials can undergo

A
  • Elastic deformation:
    When the load is removed, the object will return to its original shape
    This is shown in the elastic region of the graph
  • Plastic deformation:
    The material is permanently deformed
    When the load is removed, the object will not return to its original shape or length
    This is beyond the elastic limit and is shown in the plastic region of the graph
117
Q

Draw a labelled force-load graph, showing the points of deformation

A
118
Q

Describe what brittle/ductile materials are

A
  • Brittle materials have very little to no plastic region e.g. glass, concrete
  • The material breaks with little elastic and insignificant plastic deformation
  • Ductile materials have a larger plastic region e.g. rubber, copper
  • The material stretches into a new shape before breaking
119
Q

Draw a stress-strain graph for a brittle/ductile material

A
120
Q

Draw a force-extension graph for a material that has undergone plastic deformation

A
121
Q

Draw a force-extension graph for loading and unloading a rubber band (elastic material)

A
122
Q

Describe what the features of a force-extension graph for loading and unloading a rubber band

A
  • The graph shows the rubber band stores a greater amount of strain energy when it is loaded (stretched) than when it is being unloaded (contracted)
  • The curve for contraction is always below the curve for stretching
  • However, due to the conservation of energy, the difference in strain energy when loading and unloading must be accounted for
  • A rubber band becomes warm when it is stretched and contracted hence some energy is transferred to heat energy
123
Q

Describe how shock absorbers in a car wheel work

A
  • Impact energy is absorbed by shock absorbers
  • These are elastic objects designed to absorb or dampen the compression and rebound of the springs above a vehicle’s tires
  • They help keep the tires on the road at all times
  • When a vehicle hits a bump in a road, the shock absorbers dampen the movement of the springs in the suspension system
  • They do this by converting kinetic energy, from the movement of the car, into thermal energy which is dissipated
  • The faster the springs in the suspension system move (say, if a vehicle hits a bump at a high velocity), the more resistance the shock absorber provides
124
Q

What is the Young’s modulus a measure of

A
  • Of the ability of a material to withstand changes in length with an added load
  • This gives information about the stiffness of a material
125
Q

What is the Young’s modulus

A

the ratio of a material’s tensile stress and tensile strain

126
Q

Units of Young’s modulus

A

Pascals (Pa)

127
Q

What is the gradient of a stress-strain graph (when it is linear) equal to

A

the Young’s modulus

128
Q

What is the moment of a force equal to

A

A Level Physics (33 decks)

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