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Physics > Mechanics > Flashcards

Mechanics Flashcards

1
Q

What is a scalar

A

A quantity which only has magnitude (size)

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2
Q

What is a vector

A

A quantity which has both a magnitude and a direction

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3
Q

Displacement

A

Describes how far an object is from where it started and in what direction

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4
Q

Examples of scalars

A

Distance
Speed
Mass
Time
Energy
Temperature

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5
Q

Examples of vectors

A

Displacement
Velocity
Acceleration
Force
Momentum

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6
Q

What are vectors represented by

A

An arrow

Length represents magnitude and arrowhead indicated direction

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7
Q

When can you use calculation to add vectors

A

IF the vectors are perpendicular, so you can use pythagoras theorem

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8
Q

When can you use scale drawings to add vectors

A

When the vectors are not perpendicular

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9
Q

How can coplanar forces be represented

A

By vector triangles

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10
Q

When are forces in equilibrium

A

If the object is at rest or moving at constant velocity

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11
Q

Moment

A

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

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12
Q

Couple

A

Pair of equal and opposite coplanar forces

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13
Q

Moment of couple

A

Force x perpendicular distance between the lines of action of the forces

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14
Q

SI unit for moment

A

Nm

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15
Q

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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16
Q

Forces up =

A

Forces down

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17
Q

Coplanar forces

A

Pair of forces that are :

Equal in magnitude
Opposite in direction
Perpendicular to the distance between them
NOT IN THE SAME LINE OF ACTION !!

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18
Q

Why do objects with a couple not accelerate

A

Couples produce a resultant force of zero, so, due to Newton’s second law, the object does not accelerate

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19
Q

Centre of mass

A

The point at which the 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 of a uniform regular solid

A

At the centre e.g for a person standing up it is behind the navel

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21
Q

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

A

At the point of symmetry

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22
Q

What does the position of the centre of mass of an object affect

A

Its stability

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23
Q

What happens to the centre of mass and the stability of an object if the base is wider

A

Lower centre of mass
More stable

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24
Q

What happens to the centre of mass and stability of an object if the base is narrower

A

Higher centre of mass
Less stable (more likely to topple over)

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25
Q

What does the centre of GRAVITY depend on and why

A

The gravitational field because weight = mass x acceleration due to gravity.
Therefore an object in spaces’ centre of gravity will be more towards object with larger gravitational field

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26
Q

Instantaneous velocity/speed

A

The speed/velocity of an object at any given point in time

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27
Q

How to find instantaneous velocity on a displacement-time graph

A

Draw a tangent to the curve and calculate its gradient

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28
Q

Average speed

A

The total distance divided by the total time

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29
Q

Gradient of a displacement-time graph

A

Velocity

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30
Q

Y intercept on displacement-time graph

A

Initial diplacement

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31
Q

What does a diagonal straight line represent on a displacement-time graph

A

Constant velocity

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32
Q

What does a curved line represent on a displacement-time graph

A

Acceleration

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33
Q

What does the horizontal slope represent on a displacement-time graph

A

State of rest

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34
Q

What does the slope on a velocity-time graph represent

A

Acceleration

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35
Q

What does the y-intercept on a velocity-time graph represent

A

Initial velocity

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36
Q

What does a straight line on a velocity-time graph represent

A

Uniform acceleration

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37
Q

What does a curved line on a velocity-time graph represent

A

Non-uniform acceleration

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38
Q

What does a horizontal line on a velocity-time graph represent

A

Motion with constant velocity

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39
Q

What does the area under a velocity-time graph represent

A

Displacement/distance travelled

40
Q

What does the slope on an acceleration-time graph represent

A

Nothing

41
Q

What does the y-intercept on an acceleration-time graph represent

A

Initial acceleration

42
Q

What does a horizontal line on an acceleration-time graph represent

A

An object undergoing constant acceleration

43
Q

What does the area under an acceleration-time graph represent

A

Change in velocity

44
Q

Time of flight

A

How long the projectile is in the air for

45
Q

Range

A

The horizontal distance travelled by the projectile

46
Q

What is the v when the projectile is at max height

A

0

47
Q

What displacement is used when calculating time of flight when the projectile leaves and stops at the same height

A

0

48
Q

How do we calculate the vertical and horizontal components of velocity

A

vertical - SUVAT
horizontal - distance = speed x time

49
Q

Drag forces

A

Forces that oppose the motion of an object moving through a fluid e.g. friction and air resistance

50
Q

Properties of drag forces

A

Always in OPPOSITE direction to motion of the object
Never speed an object up or start them moving
Slow down an object or keep them moving at constant speed
Convert KE in heat and sound

51
Q

What happens to drag forces as speed increases

A

Drag forces also increase

52
Q

What happens to the range and maximum height of a projectile when there is air resistance

A

It decreases

53
Q

Describe how terminal velocity is achieved

A

For a body in free fall, the only force acting is its weight and its acceleration is only due to gravity.

Weight is initially greater than drag/friction and the resultant force causes acceleration

Drag force increases as velocity increases.

Due to Newton’s Second Law, this means the resultant force and therefore acceleration decreases. 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 constant velocity - the terminal velocity.

54
Q

What is the aim of the experiment

A

To calculate the value of the acceleration due to gravity, g

55
Q

What is the independent and dependent variable in the determination of g practical

A

Independent - Height
Dependent - Time

56
Q

What are the control variables in the determination of g practical

A

Same steel ball-bearing
Same electromagnet
Distance between ball-bearing and top of glass tube

57
Q

What is the resolution of the measuring equipment in the determination of g practical

A

Metre ruler - 1mm
Timer - 0.01s

58
Q

Describe the method in the determination of g practical

A

Set up the apparatus by attaching the electromagnet to the top of a tall clamp stand. Do not switch on the current till everything is set up.

Place the glass tube directly underneath the electromagnet, leaving space for the ball-bearing. Make sure it faces directly downwards and not at an angle.

Attach both light gates around the glass tube at a starting distance of around 10 cm.

Measure this distance between the two light gates as the height, h with a metre ruler.

Place the cushion directly underneath the end of the glass tube to catch the ball-bearing when it falls through.

Switch the current on the electromagnet and place the ball-bearing directly underneath so it is attracted to it.

Turn the current to the electromagnet off. The ball should drop.

When the ball drops through the first light gate, the timer starts.

When the ball drops through the second light gate, the timer stops.

Read the time on the timer and record this as time, t

Increase h (eg. by 5 cm) and repeat the experiment. At least 5 – 10 values for h should be used

Repeat this method at least 3 times for each value of h and calculate an average t for each

59
Q

What is the equation in the determination of g practical

A

2h/t = gt + 2u

60
Q

What are the systematic errors in the determination of g practical

A

Residue magnetism after the electromagnet is switched off may cause t to be recorded as longer than it should be

61
Q

What are the random errors in the determination of g practical

A

Large uncertainty in h from using a metre ruler with precision of 1mm

Ball may not fall accurately down the centre of each light gate

Random errors are reduced through repeating the experiment for each value of h at least 3-5 times and finding an average time

62
Q

Safety considerations in the determination of g practical

A

Only switch current on the electromagnet once everything is set-up to reduce risk of electrocution

A cushion must be used to catch the ball-bearing so it doesn’t damage the surface

The tall clamp stand needs to be attached to a surface with a G clamp so it stays rigid

63
Q

Newtons first law

A

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

64
Q

Newtons second law

A

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

F=ma

65
Q

What happens if the resultant force is along the direction of motion

A

It will speed up or slow down the body

66
Q

What happens if the resultant force is at an angle

A

It will change the direction of the body

67
Q

Newtons 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

68
Q

Conditions for 2 forces to be a newtons’ third law pair

A

Forces must act on 2 DIFFERENT objects
Forces must be of the same type
Forces must be the same size
Forces must act in opposite directions

69
Q

momentum

A

mass x velocity

70
Q

Unit for momentum

A

kg ms^-1

71
Q

Principal of conservation of linear momentum

A

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

72
Q

What are closes or isolated systems

A

Systems with no external forces

73
Q

Change in momentum =

A

Force x (change in)Time where F is constant

74
Q

Impulse

A

Change in momentum (Ns)

75
Q

What does the area under a force- time graph represent

A

The impulse

76
Q

How can impact forces be reduced

A

Increasing the contact time

77
Q

What happens to change in momentum as the contact time increases

A

Change in momentum is greater

78
Q

What happens to KE in an elastic collision

A

Kinetic energy IS conserved

79
Q

What happens to KE in an inelastic collision

A

Kinetic energy IS NOT conserved

80
Q

What happens to the objects in an elastic collision

A

Objects collide and DO NOT stick together and then move in opposite directions

81
Q

What happens to the objects in an inelastic collision

A

Objects collide and stick together after the collision

82
Q

How do seatbelts, crumple zones and airbags generally work as safety features

A

They absorb the energy from the impact and increase the time over which the force takes place.

This, in turn, increases the time taken for the change in momentum of the passenger and the vehicle to come to rest.

The increased time reduces the force and risk of injury on a passenger

83
Q

How do seatbelts reduce the force in a collision

A

Seat belts are supposed to keep people fixed to their seat in an abrupt stop.

They stretch slightly to increase the time for the passenger’s momentum to reach zero and therefore reduce the force

84
Q

How do airbags reduce the force in a collision

A

They act as a soft cushion

85
Q

How are crumple zones used to reduce the force in a collision

A

They increase the time over which the vehicle comes to rest, lowering the impact force on the passengers

86
Q

Work

A

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

87
Q

Work done =

A

Average force applied parallel to direction of displacement x Displacement

88
Q

How is work done when a force is used to move an object over a distance

A

When pushing a block, work is done against friction to give the box KE to move.

The KE is then transferred to other forms of energy e.g. sound, heat

89
Q

Work done when direction of motion is not parallel to direction of force =

A

Force x displacement x cos/sin0

90
Q

Power

A

Rate of energy transfer

91
Q

Power =

A

change in work done / change in time

92
Q

Power if an object is moving at constant velocity with constant force

A

P = F x V with the force acting in the direction of the velocity

93
Q

What does the area under the force-displacement graph represent

A

Work done

94
Q

How to calculate work done when there is a variable force

A

Using the area under the graph

95
Q

Loss/change in KE

A

work done

96
Q

Conservation of energy

A

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

Physics (10 decks)

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