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Physics > Mechanics and Materials > Flashcards

Mechanics and Materials Flashcards

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1
Q

Equilibrium

A

At rest/constant velocity
No acceleration
No resultant force

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2
Q

Moment

A

Force multiplied by perpendicular distance from pivot in line of action of force

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3
Q

Principle of Moments

A

At equilibrium - sum of clockwise moments = sum of anti-clockwise moments

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4
Q

Describe what is meant by centre of mass and state where the centre of mass of a uniform object lies

A

Point at which weight of an object acts and for uniform objects that is directly in the middle

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5
Q

Moment of a couple

A

Force multiplied by perpendicular distance between lines of action of pair of equal forces acting in opposite directions

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6
Q

Explain why objects topple over if tilted beyond their critical angle

A

Objects topple over when the line of action of their weight passes beyond the pivot and a moment is produced about the pivot causing it to topple over

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7
Q

Suggest why a vehicle full of people is more likely to topple over a cliff than an empty vehicle

A

Centre of mass will increase producing a greater moment beyond the pivot of the vehicle

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8
Q

Describe how to calculate the maximum angle an object can be tilted before it topples over

A

Align line of action of weight with pivot and using the dimensions of the object calculate the angle between the object and the ground

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9
Q

State the assumptions made for calculations involving projectile motion

A

No air resistance

Only gravity is acting

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10
Q

Explain why maximum range is reached at an angle of 45 degrees to the ground

A

At 45 degrees the vertical component for velocity equals the horizontal component. This ensures the longest time of flight for the highest horizontal velocity in order to achieve maximum range.

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11
Q

Suggest why objects of similar shape and size hit the ground at the same time regardless of their mass

A

Acceleration due to gravity is constant

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12
Q

Explain why driving force must increase to ensure a constant velocity of a vehicle at higher speeds

A

Air resistance increases at higher velocities so driving force must increase to ensure a constant resultant force

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13
Q

Suggest why air resistance increases when velocity increases

A

Air particles hit object with a greater force so exert a greater force against it and more air particles hit per second

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14
Q

Suggest how a parachute protects skydivers

A
  • increases drag force so velocity decreases
  • reduces change in momentum when skydiver hits ground
  • results in smaller impact force
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15
Q

Terminal velocity

A

Maximum velocity reached by an object when drag force equals force causing motion of object

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16
Q

Newton’s First Law of Motion

A

Objects remain at rest or move at a constant velocity unless acted upon by a resultant force

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17
Q

Newton’s Second Law of Motion

A

The rate of change of momentum is proportional to the resultant force acting on an object

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18
Q

Newton’s Third Law of Motion

A

If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A

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19
Q

Explain how crumple zones in vehicles work

A

Increase collision time so increase time taken for change of momentum so impact force is reduced

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20
Q

Explain how air bags in vehicles work

A

Spread impact force over a greater area so less pressure on body

21
Q

Impulse

A

Change of momentum of an object when the object is acted upon by a force for an interval of time

22
Q

Conservation of Momentum

A

Momentum before = Momentum after if no external forces are acting

23
Q

State one similarity and one difference between elastic and inelastic collisions

A
  • momentum is conserved in both
  • kinetic energy is conserved in elastic but not in inelastic (kinetic energy of object colliding is same before and after collision)
24
Q

Energy

A

Ability to do work

25
Q

Work

A

Product of force and distance moved in direction of force

26
Q

Hooke’s Law

A
  • extension of a spring is proportional to the force needed to extend it
  • up to limit of proportionality
27
Q

Power

A

Rate of energy transfer

28
Q

Density

A

Mass per unit volume

29
Q

Elastic Limit

A

Maximum amount a material can be stretched and still return to its original length without being plastically deformed

30
Q

State the difference between calculating total spring constant of springs in series and parallel

A

Parallel - sum of spring constants

Series - reciprocal of sums of reciprocals of spring constants

31
Q

Stress

A

Force per unit CROSS SECTIONAL area, where line of action of force is perpendicular to cross section

32
Q

Strain

A

Extension per unit length of a solid when deformed

33
Q

Young’s Modulus

A

Ratio of stress to strain (measure of stiffness)

34
Q

Yield Point

A

Point where material weakens temporarily and stress decreases for as strain increases

35
Q

Ultimate Tensile Stress

A

Stress needed to break a solid material

36
Q

Suggest what happens to kinetic energy in an elastic collision between a stationary ball and a ball with velocity

A

All kinetic energy from one ball is transferred to the other so the ball with velocity initially comes to rest and the other ball moves off with the same velocity

37
Q

A ball is bounced against the wall - compare its velocity and change in momentum as it rebounds off the wall for an elastic and inelastic collision

A
  • rebounds with same velocity for elastic and less for inelastic
  • change in momentum is -2mv for elastic and -m(u+v) for inelastic
38
Q

A ball moves towards the boot of a footballer with some velocity and he kicks it. Discuss the consequences if the ball had approached the boot at a higher velocity but received the same impulse.

A
  • initial momentum of ball greater
  • same change in momentum
  • initial and final momentum in opposite directions so final velocity of ball is lower since mv–mu = m(v+u)
39
Q

Describe an experiment to test if speed is constant

A
  • mark on equal distances along height object is dropped from
  • measure time taken to travel each distance
  • should be equal if constant speed
    Or
  • place card on trolley
  • card obscures light gate
  • light gate attached to data logger and there should be zero acceleration between gates
40
Q

Describe how to draw unloading line for a material that has been plastically deformed

A
  • parallel line that originates from last point on curve

- touches x axis after 0

41
Q

Suggest why horizontal and vertical motion are independent of each other

A
  • gravity only acts downwards

- assuming air resistance is negligible

42
Q

Suggest why air resistance in negligible in the vertical direction but not horizontally

A
  • air resistance increases with speed

- speed is low vertically but high horizontally

43
Q

Explain why air resistance decreases protection range

A
  • projectile decelerates horizontally

- less distance travelled

44
Q

Describe method by which mass of an object can be determined using a spring and suggest ways in which accuracy/safety of experiment could be improved

A
  • measure initial length of spring with a metre rule
  • apply different standard masses to spring one at a time and measure new length of spring
  • extension = new length - original length of spring
  • plot graph of mass (or F but relationship is proportional so either is accepted) against extension
  • hang object of unknown mass from spring and measure extension
  • interpolate mass of spring from graph
  • clamp stand to bench/counterweight
  • use set square to measure length of spring parallel
  • confirm mass of standards with scale
45
Q

Describe how to calculate Young’s Modulus of a wire and suggest ways to ensure accuracy of data

A
  • measure original length with metre rule
  • measure diameter of wire using micrometer
  • measure extension with vernier calliper
  • extension = new length - original length of wire
  • suspend from wire from a height and apply standard masses then determine extension
  • plot graph of extension against length and calculate gradient= e/F
  • divide L/A by e/F to find Young’s Modulus of wire
  • repeat
  • measure diameter of wire at several places
  • length of wire greater than a metre
  • use set square to measure extension
  • Searles control wire to negate temp change
46
Q

Suggest what area under stress-strain graph represents

A

energy stored per unit volume in material

47
Q

Explain why using an inclined plane rather than free fall produces valid data when investigating motion of a falling object

A
  • free fall is too quick

- no accurate method to time free fall

48
Q

Describe what is meant by limit of proportionality for force-extension graph

A

point beyond which graph is no longer linear

Physics (5 decks)

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