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Physics A Level > 3.4 Mechanics > Flashcards

3.4 Mechanics Flashcards

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1
Q

Define vector and scalar.

A 
Vector = direction and magnitude
Scalar = magnitude
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2
Q

Give examples of vectors.

A

Velocity, acceleration, force.

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3
Q

Give examples of scalars.

A

Mass, density, volume, energy, distance.

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4
Q

What are the two methods for the addition of vectors?

A

Scale drawing or calculation.

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5
Q

How do you add vectors by scale drawing?

A
  • Decide on suitable scale (1N=1cm)
  • Draw 1st vector in correct direction and length (protracted and ruler)
  • Draw 2nd vector with its tail on the nose of the 1st (correct direction and length)
  • Result is found by drawing a line from the tail of 1st to nose of 2nd
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6
Q

Does the order you draw vectors for scale drawing matter?

A

Nah.

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7
Q

For what vectors can you calculate instead of drawing?

A

Vectors at right angles.

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8
Q

How do we calculate the addition of vectors?

A

Pythagoras.

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9
Q

How do we resolve vectors into two components?

A

Use SohCahToa.

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10
Q

What is equilibrium?

A

Forces are balanced, stationary or constant velocity. Resultant force = 0.

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11
Q

Define moment.

A

The force x the perpendicular distance from its line of action to the pivot

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12
Q

For an object in equilibrium the sum of what around a point = the sum of what around the point?

A

Clockwise and anti-clockwise movements.

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13
Q

What is a couple?

A

Consists of a pair of coplanar forces equal in magnitude and opposite in direction who do not act along the same line of action.

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14
Q

What is the moment of a couple?

A

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

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15
Q

Define centre of mass.

A

The point where all off the mass can be considered to be concentrated. A force applied through this point won’t cause rotation, or a resultant movement.

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16
Q

Define centre of gravity.

A

The point where all of the weight of an object can be considered to act through.

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17
Q

What do the COM and COG share?

A

The same point.

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18
Q

Define uniform.

A

Mass is evenly distributed.

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19
Q

How can the COM be found for uniform objects?

A

Using lines of symmetry.

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20
Q

How can the COM be found for non uniform objects?

A

Balance it on a knife’s edge, COM would be directly above the support.

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21
Q

How do you find the COM for an irregular lamina?

A
  • Push a pin through the sheet and allow it to swing freely
  • It comes to rest with the COM directly below the point of suspension
  • Draw a plumb line, the COM is somewhere along this line
  • Repeat from a different position
  • Where lines cross is COM
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22
Q

v = ?

A

∆s/∆t

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23
Q

a = ?

A

∆v/∆t

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24
Q

On a s/t graph what is the gradient? What do straight lines mean?

A

Velocity, constant velocity.

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25
Q

What does it mean if an s/t graph goes into the negative quadrant? What is the x axis?

A

Moving in opposite direction to starting point. The starting point.

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26
Q

What are the two velocities you can find from a curve an s/t graph? How do we find them?

A

Instantaneous velocity of a point (draw tangent), average velocity (take gradient between top and bottom point).

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27
Q

On a v/t graph what is the gradient? What do straight lines mean?

A

Acceleration, constant acceleration.

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28
Q

What does a negative gradient mean on a v/t graph?

A

Deceleration.

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29
Q

What does it mean when a v/t graph goes into the negative quadrant?

A

Negative velocity moving in the opposite direction (and getting faster).

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30
Q

What are the two accelerations you can find from a curve on a v/t graph? How do we find them?

A

Instantaneous acceleration (tangent), average acceleration (top and bottom points).

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31
Q

What is the area of a v/t graph?

A

Displacement.

32
Q

What happens on an s/t graph for a bouncing ball?

A

Starts at s, slopes down to x axis, slopes back up but does not reach original height.

33
Q

What happens on a v/t graph for a bouncing ball?

A

Starts at x axis, straight line down into negative quadrant, vertical line up to positive quadrant, straight line (with same grad. as first) goes down reaching x axis.

34
Q

What happens on an a/t graph for a bouncing ball?

A

Straight, horizontal line at -9.81.

35
Q

If 0 air res. what happens to Vx and Vy in projectile motion? What effect does this have?

A

Vx is constant. Vy increases due to acceleration due to gravity. Angle to the horizontal increases.

36
Q

Horizontal and vertical motion are ………..?

A

Independent of each other.

37
Q

If there is air res. what happens to Vx and Vy in projectile motion?

A

Vx decreases over time (eventually reaching 0). Vy increases over time (however acceleration isn’t constant it decreases eventually reaching 0 - terminal velocity).

38
Q

What is terminal velocity?

A

When an object moves at a steady speed in a constant direction because the resultant force acting on it is zero.

39
Q

How do we change terminal velocity?

A

Increasing SA, eg. parachute.

40
Q

What is drag?

A

A resistive force that acts parallel, but in the opposite direction, to the direction of motion.

41
Q

What factors effect drag?

A

Size and shape of the object.
Properties of the fluid (density, viscosity).
Velocity of the object.

42
Q

What is lift?

A

A force that acts perpendicular to the direction of motion. When an object moves through a fluid.

43
Q

What three factors effect lift?

A

Size and shape of the object.
Properties of the fluid (density, viscosity).
Velocity of the object.

44
Q

What is friction?

A

A force that acts against the direction of motion or resultant force on a stationary object.

45
Q

What is friction controlled by?

A

Type of surfaces in contact (eg. ice vs carpet).

Perpendicular forces between the surfaces (normal forces).

46
Q

How do we increase max speed (I knew a lad called Max Speed) of a vehicle?

A 
Changing thrust (more/bigger engines).
Changing drag (changing the size and where in the atmosphere).
47
Q

What is Newton’s 1st law of motion?

A

An object at rest, remains at rest, and a moving object continues with a uniform velocity (same speed straight line), unless acted on by a resultant force.

48
Q

If a resultant force acts on an object which what does it do and in which direction?

A

Accelerates in the direction of the force.

49
Q

What is Newton’s 2nd law?

A

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

50
Q

What is the equation for Newt’s 2nd law?

A

F = ∆(mv) / ∆t

51
Q

What is the equations for Newt’s 2nd law where the mass is a constant?

A

F = ma

52
Q

What is Newton’s 3rd law?

A

If object A exerts a force on object B, object B exerts a force equal in magnitude and opposite in direction back onto object A.

53
Q

What is the condition of Newt’s 3rd law?

A

Must be two different objects.
Must be the same type of force (eg. Both gravitational).
Same line of action.

54
Q

Momentum = ?

A

Mass x velocity

55
Q

In a collision or explosion between 2 objects, if no external forces act, the linear momentum before the event is what?

A

Equal to the momentum after the event.

56
Q

Define linear momentum.

A

Momentum from an object moving in a straight line with any orientation.

57
Q

What is impulse equation?

A

The force x the time taken for the collision. Ft = ∆p

58
Q

Define impulse.

A

Impulse is the change in momentum during a collision or explosion.

59
Q

On a force time graph what is the area under the graph?

A

Impulse.

60
Q

What happens in an elastic collision?

A

The Ek before and after a collision is =, in an isolated system.

61
Q

What happens in an inelastic collision?

A

The Ek is not equal before an after a collision, it will be lower in an isolated system.

62
Q

How to prove inelastic collision?

A

Calculate Ek (0.5mv^2) before and after to show not =.

63
Q

What are examples of elastic collisions?

A

Ball with velocity v hits stationary ball what the same mass, ball 1 stops and ball 2 continues with velocity v.
Two balls come at each other with different masses and speeds, collide, move away in opposite direction with negative velocity.

64
Q

Why do we want to reduce the force experienced in a collision, what do we do?

A

Reduce risk of death and injury, we make the collision longer - reducing impulse.

65
Q

How do we reduce impulse? What are these things designed to do?

A

Crumple zones, steel motorway barriers, tyre walls (not just for the aesthetics).
Designed to deform to elongate time taken.

66
Q

What is work done, W?

A

The energy transferred to an object by the components of the force parallel to the direction of motion when it’s moved across a distance.

67
Q

What is the equation for work done?

A

W = Fscosθ

68
Q

Power = ?

A

∆Energy / ∆t

69
Q

Define power.

A

Rate of energy transfer or rate of work done.

70
Q

What is the area under a force displacement graph?

A

Work done.

71
Q

Efficiency =

A

Useful energy / total energy
OR
Useful power / input power

72
Q

What is conservation of energy?

A

Energy cannot be created or destroyed, only transferred between sources. Energy before = energy after.

73
Q

For the mark scheme what do you need to do to equations before subbing in values?

A

Rearrange.

74
Q

Why won’t real value reach calculated value?

A

Work done by friction/ resistive forces. Energy converted to internal/thermal energy.

75
Q

Define gravitational field strength, scalar or vector?

A

Force per unit mass, vector.

76
Q

State and explain one use for satellite travelling in orbit.

A

GPS, several satellites needed to fix position on earth.

77
Q

Show magnitude of rf acting on a object.

A

Find differences between ups and downs, find magnitude of these two values.

Physics A Level (15 decks)

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