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Physics > 3.4 Mechanics and materials > Flashcards

3.4 Mechanics and materials Flashcards

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1
Q

What is a scalar quantity?

A

A quantity with only a magnitude

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2
Q

What is a vector quantity?

A

A quantity with both magnitude and direction

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3
Q

What are some examples of scalars?

A

Speed, mass, distance, temperature

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4
Q

What are some examples of vectors?

A

Velocity, force/weight, acceleration, displacement

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5
Q

What is the moment of a force about a point?

A

The force multiplied by the perpendicular distance from the line of action of the force to the point

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6
Q

What is a couple?

A

A pair of coplanar forces (forces in the same plane) where the forces are equal in magnitude but act in opposite directions

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7
Q

What is the principle of moments?

A

For an object in equilibrium, the sum of anticlockwise moments about a pivot is equal to the sum of clockwise moments

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8
Q

What is the centre of mass of an object?

A

The point at which an object’s mass acts

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9
Q

What does it mean if an object is uniform?

A

Its centre of mass is exactly at its centre

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10
Q

What is speed?

A

The scalar quantity describing how quickly an object is travelling

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11
Q

What is displacement?

A

The vector quantity describing the overall distance travelled from the starting position (with direction)

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12
Q

What is velocity?

A

The rate of change of displacement

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13
Q

What is acceleration?

A

The rate of change of velocity

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14
Q

What is instantaneous velocity?

A

The velocity of an object at a specific point of time

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15
Q

What is uniform acceleration?

A

Where the acceleration of an object is constant

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16
Q

What does the area under an acceleration-time graph represent?

A

The change in velocity

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17
Q

What does the gradient of a velocity-time graph represent?

A

Acceleration

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18
Q

What does the area under a velocity-time graph represent?

A

Displacement

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19
Q

What does the gradient of a displacement-time graph represent?

A

Velocity

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20
Q

When can you use SUVAT equations?

A

When an object is moving at uniform acceleration

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21
Q

What does it mean if an object is in free fall?

A

It is experiencing an acceleration of g (9.81ms^-2)

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22
Q

What is friction?

A

A contact force that opposes the motion of an object

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23
Q

What do frictional forces do in terms of energy transfer?

A

Convert kinetic energy into other forms (eg heat, sound)

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24
Q

When does terminal velocity occur?

A

When the frictional forces acting on an object and driving forces are equal (no resultant force/ acceleration)

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25
Q

What components of a projectile’s motion are affected by air resistance?

A

Both the vertical and horizontal components

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26
Q

What is Newton’s 1st law?

A

An object with no resultant force will remain at rest or will be travelling at a constant velocity

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27
Q

What is Newton’s 2nd law?

A

The acceleration of an object is proportional to the resultant force experienced by the object
F=ma

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28
Q

What is Newton’s 3rd law?

A

For each force experienced by an object, the object exerts a force of equal and opposite magnitude

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29
Q

Why are free-body diagrams called ‘free-body’?

A

The diagram concerns only the object and nothing else

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30
Q

What is momentum?

A

The product of the mass and velocity of an object

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31
Q

What does the conservation of linear momentum state?

A

Momentum is always conserved in any interaction where no external forces act, so the momentum before and after an event is equal

32
Q

What is force?

A

The rate of change of momentum

33
Q

What is impulse?

A

The change in momentum

34
Q

What does the area under a force-time graph represent?

A

The change in momentum

35
Q

What is the purpose of crumple zones, seat belts and air bags?

A

To decrease the force exerted on passengers, by increasing the time of the collision, reducing the change in momentum

36
Q

What is conserved in an elastic collision?

A

Momentum and kinetic energy

37
Q

What is conserved in an inelastic collision?

A

Only momentum

38
Q

Why is kinetic energy not conserved in inelastic collisions?

A

It is converted into other forms of energy, and may be larger/smaller after a collision

39
Q

What type of collision is it when the objects stick together after the collision?

A

Inelastic

40
Q

What type of collision is an explosion?

A

Inelastic

41
Q

What is work done defined as?

A

The force causing a motion multiplied by the distance travelled in the direction of the force

42
Q

What is the rate of doing work the same as?

A

The rate of energy transfer

43
Q

What does the area under a force-displacement represent?

A

Work done

44
Q

What is efficiency?

A

A measure of how efficiently a system transfers energy

45
Q

What is the principle of conservation of energy?

A

Energy cannot be created or destroyed, but can be converted from one form into another

46
Q

What is Hooke’s Law?

A

The force needed to extend/compress a spring is proportional to the extension, provided the elastic limit has not been exceeded

F = ke

47
Q

What is the elastic limit?

A

The maximum extent a material can be stretched without permanent alteration of its form

48
Q

What is tensile force?

A

The force applied to a material that makes it stretch

49
Q

What is tensile stress?
What are the units?

A

The tensile force per unit cross-sectional area
Units: Nm^-2 or Pa

50
Q

What is tensile strength?

A

The tensile stress at which the material breaks

51
Q

What is tensile strain?
What are the units?

A

How much a material stretches per unit length
It does not have units as it is a ratio of length

52
Q

What is the Young Modulus of a material?
What are the units?

A

The ratio of stress to strain
Units: Nm^-2 or Pa

53
Q

What is the gradient of a stress-strain graph?

A

The Young Modulus of the material

54
Q

On a stress-strain graph, what is are the 2 points (one followed by another) called where the extension is no longer proportional to the force (where Hooke’s Law is no longer obeyed)?

A

The proportionality limit, followed by the elastic limit

55
Q

What happens to the material at the elastic limit?

A

The material becomes permanently deformed (plastic, no longer elastic)

56
Q

What is the difference between elastic deformation and plastic deformation?

A

Elastic deformation - after the load is removed, the material can return to its original form (reversible)

Plastic deformation - after the load is removed, the material cannot return to its original form (irreversible)

57
Q

What is the point after the elastic limit on a stress-strain curve?

A

The yield point

58
Q

What is the yield point?

A

Where there is a large increase in extension for a small increase in force (steeper gradient)

59
Q

What is the region after the yield point on a stress-strain curve?

A

The plastic region

60
Q

What is happening in the plastic region?

A

Extension is rapidly increasing
Makes wires for ductile materials

61
Q

What is happening in the region after the plastic region?

A

The material is necking

62
Q

What is the point at which the curve ends on a stress-strain graph?

A

The ultimate tensile strength - the material’s fail point

63
Q

What is the area under the linear region of a stress-strain curve?

A

Stress energy density

64
Q

What is the point at which the line is no longer linear on a force-extension graph?

A

The elastic limit, where the material becomes plastic

65
Q

What type of materials have a large plastic region on a stress-strain curve?

A

Ductile materials - form wires after plastic deformation

66
Q

What type of materials have no plastic region on a stress-strain curve?

A

Materials with a high tensile strength
Strong but brittle

67
Q

What do highly elastic materials look like on a stress-strain curve?

A

Difficult to stretch at first, but then easier before becoming difficult again (hysteresis curve)
Steep -> shallow -> steep

68
Q

What is the area under a force-extension graph?

A

Work done

69
Q

What is the gradient of a force-extension graph?

A

Spring constant

70
Q

What is a ductile material?

A

Forms wire after plastic deformation

71
Q

What is a brittle material?

A

Strong but not stretchy (opposite to ductile)

72
Q

What is a tough material?

A

Resists high stresses

73
Q

What is a stiff material?

A

Stretches a small amount for a large stress

74
Q

What are the 2 equations for EPE?

A

EPE = 1/2 F e
EPE = 1/2 k e^2 (NOT ON FS)

75
Q

What is spring energy transformed to?

A

Kinetic and gravitational potential energy

Physics (16 decks)

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