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mechanics Flashcards

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1
Q

newtons third law pairs

A

-same size
-same line of action but in opposite directions
-same time of action
- same type of force
-acts on diff bodies
FAB = -FBA

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2
Q

2 relationships between forces for object in equilibrium

A
  • resultant force 0N

- resultant Torque 0 (ACW m =CWm)

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3
Q

in moments - talking about why a force / distance must be greater / lesser to balance

A
  • subject of questions relative distance (e.g. W is a greater distance)
  • subjects __ must be > or < for moment to be equal / resultant moment 0N (e.g. W force must be less)
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4
Q

if an object moves along the system in equilibrium (e.g. girl walks along diveboard) - how to describe moment

A

-moment of object’s weight increases

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5
Q

moment definition

A
  • force x perpendicular distance

- from line of action of force to pivot

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6
Q

principle of moments (3)

A
  • for body in equilibrium

- SUM of ACW moments = SUM of CW moments about a POINT/PIVOT

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7
Q

comparing forces

A
  • distance pivot (1)

- gives large enough / small enough moment

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8
Q

two conditions for object in equilibrium

A
  • resultant force = 0N

- resultant Torque =0Nm ABOUT ANY POINT

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9
Q

consequence if object is not in equilibirum

A
  • accelerate

- rotate w/ angular direction

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10
Q

direction of tension

–> force case of hanging mass

A
  • the tension will point away from the mass in the direction of the string/rope.
  • In case of the hanging mass, the string pulls it upwards, so the string/rope exerts an upper force on the mass and the tension will be in the upper side.
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11
Q

always add what on angles in vector diagrams?

A

-directions NSEW

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12
Q

what will happen to forward force to maintain constant acceleration?

A
  • forward force increase
  • air resistance increases w/ speed
  • driving forward force > drag force ( for constant acceleration)
  • otherwise, the net resultant force would decrease
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13
Q

what is constant acceleration?

A

velocity changes at a constant rate (net force must stay the same)

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14
Q

when doing distance under graph

A

-always do the WHOLE under graph even if it looks as if it starts mid y axis

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15
Q

when describing motion always

A

-include figures even if it looks as though you shouldn’t;

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16
Q

when cars start at different points on graph (3) to explain why one reaches a maximum distance ahead of other

A
  • car (B) initially slower
  • therefore distance apart increases
  • cars reach same speed after (time)
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17
Q

how to collect data for sprinter velocity time graph

A
  • record time to reach set distance

- d/t graph measure gradient

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18
Q

on elevated plane vector questions the angle is in between

A

-given W and vertical component

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19
Q

gravitational potential energy (or any energy lost) points (3)

A
  • lost as heat energy to the surroundings
  • frictional forces / air resistance / drag between (2 surfaces)
  • WORK DONE against or by resistive force
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20
Q

if there’s a delay in distance time graph e.g. sprinter w/ pistol reason is

A

reaction time

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21
Q

why does changing mass not affect g (2)

A
  • weight and mass directly proportional w=mg

- -> double weight double mass therefore factor of 2 cancels and g remains the same

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22
Q

when talking about difference between distance and displacement in graph

A

say d is scalar and s is vector the graph

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23
Q

when describing motion

A

-always comment on direction (e.g. opposite direction) to other half of graph

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24
Q

when talking about why an object might not travel as far in one direction (e.g. horizontally)

A

talk about air resistance causing deceleration in HORIZONTAL

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25
Q

for s = ut +1/2 at^2 when t is unknown

A

-create a quadratic + solve for unknown t

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26
Q

for object thrown from cliff initial velocity assumed as

A

0 ms

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27
Q

if not given acceleration and says constant acceleration assume acceleration to be

A

0ms-2

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28
Q
  1. A ball is thrown vertically upwards at 25 m s−1. Find the length of time for which the ball is above 3 m from the point of projection.

(why two solutions for t ?)

A

There are two solutions because there are two times when the ball is at a height of 3 m - once on the way up and once
on the way down. The difference between these times is the time spent above 3 m

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29
Q

Relationship displacement y and time x in free fall

A

s = ut +at^2 /2
S=(0)t + at^2
S=t^2
Y is proportional to square of x

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30
Q

3 marks

–> explaining why for smaller horizontal velocity == smaller distance travelled

A

(vertical motion independent horizontal)

(1) same acceleration downwards due to gravity = same vertical speed ( as same mass)
(1) in air for same time
(1) s=ut where u is horizontal speed

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31
Q

energy changes in cannonball being fired

A
  • GPE- Ke

- to heat / thermal

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32
Q

why if velocity diver independent mass if air resistance insignificant

A
  • all gpe transfer to KE
  • no transferred to heat
  • mass cancels
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33
Q

if the object in question is not stated to be at rest final velocity must not be

A

0

–> measuring velocity BEFORE impact

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34
Q

six marker - ball bearing in oil

A

-ball decelerates down in the fluid - force due to friction acts upwards + > downwards force on weight of ball
-resistive force = upwards + decreases
{]deceleration decreases to 0:
-drag force decreases as speed decreases
-until equal
-grad graph = acceleration + ball moves at terminal velocity

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35
Q

why acceleration decreases as move downslope(2)

A
  • component weight downwards = acceleration
  • move further down component decreases
  • air resistance increases as speed increases
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36
Q

Galileo’s experiment - frictionless track why ball returns to same height (3)

A

ALL -GPE –> KE –> GPE (1)

  • -> no work done against friction / resistive forces (1)
  • -> as initial GPE= final GPE + mg is constant h same (1)
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37
Q

what to say when air resistance is negligible in terms of energy

A

no !WORK DONE ! against resistive forces

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38
Q

Key words for Newton first law of motion

A
  • no external unbalanced resultant force acting on it

- constant velocity in a straight line

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39
Q

gravitational field strength

A

vector

force per unit mass

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40
Q

remember when horizontal component of vertical force increases the vertical force increases in magnitude at

A

an angle (infleunced by angle e.g. V sinx)

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41
Q

moments in relation to what is the centre of mass

A

point at which resultant torque / moment is 0

42
Q

3 marker on moments + toppling

A
  • feature
  • what it does (eg. sufficient ACW moment)
  • prevents what ( toppling !CW!)
43
Q

FOR conservation moments

A

momentS (!need S!)

44
Q

in moments for example girls weight consider as

A

moment of girls weight

45
Q

The figure below shows a stationary metal block hanging from the middle of a stretched wire
which is suspended from a horizontal beam. The tension in each half of the wire is 15 N.
–> horizontal component =

A

0N – as horizontal component equal magnitudes but diff directions cancel out
(vertical is sum of vertical components)

46
Q

be wary of difference between

A

constant acceleration and constant speed

47
Q

elastic collision molecules gas change in momentum

A

2mv

48
Q

Use Newton’s laws of motion to explain how the molecules of a gas exert a force on the
wall of a container.
You may be awarded additional marks to those shown in brackets for the quality of written
communication in your answer.

A

force is exerted on molecule by wall (1)
to change its momentum (1)
molecule must exert an equal but opposite force on wall (1)
in accordance with Newton’s second or third law (1)

49
Q

don’t refer to as initial velocity in momentum questions always refer to as

A

momentum (e.g. initial monetum greater)

50
Q

when saying impulse is same also say

A

change in momentum is the same

51
Q

when saying ke lost also say

A

change in ke

52
Q

explain with ref to momentum why test would specifiy whihc surface plinth is on

A

Impact time can be increased if the plinth material is not stiff✓
Increased impact time would reduce the force of the impact. ✓

53
Q

why acceleration decreases as skateboarder moves a to b

A
  • component weight down = acceleration
  • component decreases as skateboarder moves further down
  • air resistance / friction increases w/ spee
54
Q

why in fluid ball decelerates

A
  • friction force acts upwards
  • friction force due to fluid > weight ball
  • resistive forc decreases
55
Q

why deceleration decreases to 0 in fluid ball

A

-balls speed decreases friction force decreases-until equal force - balls at constant terminal v

56
Q

what is gravitational field strength ?

A

force per unit mass ✓

a vector quantity ✓

57
Q

ALWAYS REMEMBER

A

normal reaction force on pivot upwards

58
Q

newtons first law 2 terms

A

0 resultant force

0 acceleration

59
Q

always remmeber on draw on force

A

include weight + normal reaction

60
Q

F = ma when mass is

A

Constant

61
Q

suggest w/ reason how resultant force changes as speed increases

A
  • decreases

- air resistance directly proportional to speed

62
Q

use newtons laws of motion to explain why the vehicle has max speed (3)

A
  • two forces are equal (in magnitude) (1)
    -the resultant force is zero (1)
    hence constant/terminal velocity (zero acceleration)
    in accordance with Newton’s first law (1)
63
Q

effect on forwarding + sideways speed of the object when acted on by force at right angles to the motion

A
  • forward speed unchanged

- sideways speed increases

64
Q

3 ways newtons third law pairs are same

A

they have the same line of action
they have the same magnitude (not size)
the forces are of the same kind

65
Q

four thrusters on a rocket fired simultaneously in same direction - explain how propellant produces thrust (3)

A

==> Chemical combustion of propellant/fuel or gases produced at high pressure

  • Gas is expelled / expands through nozzle
  • Change in momentum of gases escaping
  • equal and opposite to change in momentum of the spacecraft
  • Thrust = rate of change of change in momentum
66
Q

Q13.
An object falls freely from rest. After falling a distance d its velocity is v. What is its velocity after it has fallen a distance 2d?

A

root 2 v

-equate

67
Q

explaining newtosn first law from graph

A

(first law) ball travels in a straight line at a constant speed / constant velocity
/ (maintains) uniform / no change in motion / zero acceleration
there is no (external) unbalanced / resultant force acting on it

68
Q

always draw reaction force up at

A

pivot

69
Q

velocity-time graph used to find MAXIMUM acceleration

A

steepest gradient

70
Q

at start of projectiles questions always state

A

vertical + horizontal velocities seperate

71
Q

why air bag is safer (3)

A

moving driver has momentum (1)
in sudden impact momentum must be lost in v. short time (1)
F = Δ(mv) / Δt (or F = ma) (1)
air bag increases stopping contact time (1)
hence reduces force (or reduces force by decreasing the deceleration) (1)

72
Q

on show that questions

A

(to 4 sf minimum-no up)

73
Q

to describe how pivot placed at a position that means moments are balanced

A
  • the center of mass of beam + box is the pivot

- CW=ACW Moments

74
Q

for saying range will be greater when projectile given vertical w/o horizontal (4)

A

component of velocity upwards ✔
rock will spend longer in the air ✔
greater t ✔
therefore the range is greater ✔

75
Q

speed of air same why does momentum change (2)

A
  • momentum vector

- direction changes

76
Q

why would decelerating force on air by deflector planes (that change momentum of air) not be constant (2)

A
  • vol air per second decreases

- smaller rate of change of momentum F = change in momentum / t

77
Q

Describe and explain what happens when a moving particle collides elastically with a stationary particle of equal mass.

A
  • the incident particle stops (loses all its momentum or KE)
  • the stationary particle moves off with the same velocity (speed) as the incident particle (gains same momentum or KE as incident particle had)
  • original KE = ½mv2 = final KE
78
Q

Collisions between neutrons and uranium nuclei can also be elastic. State, and explain briefly, how the speed of the uranium nucleus after impact would be different in the case of an elastic collision.
(uranium stationary initially + greater mass)

A
  • neutron will rebound / have velocity / momentum to the left
  • momentum transferred to the uranium will be greater
  • velocity of uranium will be greater
79
Q

projectile has max pe at

A

the top of its path

80
Q

higher Young modulus =

A

stiffer material

81
Q

what are couples NOT

A

perpendicular

82
Q

RF of a couple

A

0

83
Q

d for moment of couple

A

distance between lines of actions of forces

84
Q

object fall over if

A

centre of mass not above its base

85
Q

what’s an explosion

A

collision in reverse

86
Q

what processes are explosions

–> what does ke do and where does ke of fragments come from

A

inelastic
increases
form internal energy of system

87
Q

eqn for no acceleration

A

speed distance time

88
Q

how to show directly proportional method 1

A

Read-off points (of line of best fit for) x1 and x2 compare with corresponding y1 and y2 , compares the ratio of the x terms to the ratio of the y terms; if equal then directly proportional

89
Q

how to show directly proportional method 2

A

Determine the constant of proportionality for at least two points (on line of best fit) and compare, where constant exists then directly proportional ✔

90
Q

how does data show distance greater refer to what

A

!EQUATION!

  • how is time greater
  • how is speed greater
  • speed and time directly proportional to distance
91
Q

always give

A

decimals not fractions !

92
Q

remember

A

Can’t use suvat in km/h only m/s

93
Q

in newtons second law - why would gas exert force is displaced by parachute

A
  • gas momentum is changing - accelerated - change in direction /vecloity
  • therefore F=MA so force SO required to change momentum
94
Q

in newtons second law resultant force on object equal to rate of change in momentum IF

A

change in momentum in same direction as resultant force therefore acceleration in direction (ALWAYS SAY SAME DIRECTION)

95
Q

larger angle of projectile =

A

smaller horizontal v
bigger vertical v
DUE TO COSINE AND SINE GRAPHS 0-90

96
Q

2 marker on explaining why you drew projectile

A
  • angle in relation to velocity

- where it ends up

97
Q

energy changes when block slowing down

A

ke to heat

98
Q

measuring speed on Galileo track

A

equate ke = mgh
cancel m
v^2= gh

99
Q

efficiency

A

useful output / total input - don’t minus

100
Q

2 marker on how graph of velocity time shows acceleration constant

A

GARDIENT = ACCELERATION

constant gradient

101
Q

for increasing acceleration force has to be

A

LARGER can’t just say increase this increase that

physics specific vocabulary (15 decks)

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