Topic 2: Mechanics Flashcards
(121 cards)
1
Q
What is a scalar quantity?
A
A quantity with magnitude only, for example: speed, distance, energy, time, power, charge, current and voltage.
2
Q
What is a vector quantity?
A
A quantity with magnitude and direction, for example: velocity, displacement, forces and acceleration.
3
Q
Define speed, v in ms⁻¹.
A
The distance an object travels per unit time.
4
Q
Define velocity, v in ms⁻¹.
A
The rate of change of displacement with respect to time.
5
Q
Define distance , d in m.
A
How far an object has travelled.
6
Q
Define Dispacement, s in m.
A
How far away an object is from a given point, in a given direction.
7
Q
Define acceleration, a in ms⁻².
A
The rate of change of velocity with respect to time.
8
Q
What does the gradient of a distance-time graph represent?
A
Speed
9
Q
What does the gradient of a displacement-time graph represent?
A
Velocity
10
Q
What does the gradient of a velocity-time graph represent?
A
Acceleration.
11
Q
Describe the shape of a velocity-time
graph for an object travelling with
uniform acceleration.
A
If acceleration is constant, the gradient of the velocity-time graph will be constant and so it will form a straight line graph.
12
Q
Describe the shape of a velocity-time
graph for an object travelling with
uniform acceleration.
A
If acceleration is not constant, the
gradient of the velocity-time graph will
change and so it will form a curve.
13
Q
Compare the velocity-time graphs for a uniformly accelerating and a uniformly decelerating object.
A
● For an accelerating object, the gradient will be positive
● For a decelerating object, the gradient
will be negative
14
Q
What must always be true when using
the SUVAT equations of motion?
A
The acceleration must be uniform - if the
acceleration is changing, the equations
are not valid.
15
Q
What is the SUVAT equation, where: X = X + XX?
A
v = u + at
16
Q
What is the SUVAT equation, where: X^n = X^n + nXX?
A
v² = u² + 2as
17
Q
What is the SUVAT equation, where: X = XX + nXX^n?
A
s = ut + ½at²
18
Q
What is the SUVAT equation, where: X = n(X +X)X?
A
s = ½(u + v)t
19
Q
What is the SUVAT equation, where: X = XX - nXX^n?
A
s = vt - ½at²
20
Q
With the equations for uniformly accelerated motion, what is the symbol and units for displacement?
A
Symbol:
s
Units:
m
21
Q
With the equations for uniformly accelerated motion, what is the symbol and what are the units for initial velocity?
A
Symbol:
u
Units:
ms⁻¹
22
Q
With the equations for uniformly accelerated motion, what is the symbol and what are the units for final velocity?
A
Symbol:
v
Units:
ms⁻¹
23
Q
With the equations for uniformly accelerated motion, what is the symbol and what are the units for acceleration?
A
Symbol:
a
Units:
ms⁻²
24
Q
With the equations for uniformly accelerated motion, what is the symbol and what are the units for time?
A
Symbol:
t
Units:
s
25
If I drop something, what is it's acceleration immediately after I let go?
9.81ms⁻²
26
If I throw a something into the air, what is its acceleration immediately after I let go?
-9.81ms⁻²
27
What is **free fall**?
When the only force acting on the object is its weight.
28
How is free fall related to gravity?
The acceleration of an object in free-fall is equal to the gravitional field strength, g=9.81ms⁻², This is also known as the **acceleration due to gravity**.
29
Define a projectile
A projectile is an airborne particle that only experineces weight and no other forces while following a parabolic path.
30
How could I calculate the horizontal distance of a projectile **t** seconds after being launched?
Multiply the horizontal component of the initial velocity by **t**.
31
what is meant by an object falling at terminal velocity?
A falling object accelerates downwards due to gravity while also experiencing upward drag. the drag = weight.
terminal velocity can be different speeds i.e. a skydiver will have two different terminal velocities depending on if the parachute is open.
32
What is terminal speed?
The speed at which th frictional forces acting on an object are equal to its constant drving force, making it travel at a constant velocity.
33
(core practical) What equipment is used to determine the acceleration of a freely falling object?
Two light gates, a metre rule and a long, thin object to drop through them
34
(core practical) I have the displacement and time for an object to fall between two light gates. How do I calculate acceleration due to gravity?
Plot displacement against time squared. Multiply the gradient by 2 to get the acceleration due to gravity.
35
Define Force, ***F*** in **N**.
A pushing or pulling effect in a given direction that acts on an object.
36
Define Mass, ***m*** in **kg**.
How resistant a body is to the action of a force.
37
What is weight?
A downward force due to the effect gravity has on a body's mass.
38
State the equation for magnitude of weight for object of mass ***m***
W = mg
Weight = Mass x Gravitational Field
Strength
39
Describe friction.
A frictional force opposes the motion of an object by acting in the opposite direction to its velocity.
40
These forces ________ cause an object to start moving or speed up, only ____________ or ________ .
These forces **cannot** cause an object to start moving or **slow down** or **stop**.
41
What causes dry frictional forces?
Contact between an object and a solid surface.
42
________________ are assumed to cause no friction.
**Smooth surfaces** are assumed to cause no friction.
43
Frictional forces cause objects to lose ________________ as heat or sound.
Frictional forces cause objects to lose **kinectic energy** as heat or sound
44
Define Drag.
A frictional force that acts on an object moving throght a fluid, due to collisions with particles in the fliud.
45
Factors that increase with drag.
1. Viscosity
2. Thickness of the fluid
3. The speed an object moves the fluid
46
What dos does drag depend on?
The objects shape.
47
Define Lift.
An upward force that acts on an object oving horizontally through a fluid.
48
What causes lift?
The fluid flowig over the object changing direct.
49
How can lift be increased?
The object's shape, such a with an aeroplane wing.
50
Define normal contact force.
A force perpendicular to a surface stopping two objects crushing together.
51
Define Tension.
A force which can be due to compression of stretching of a material.
It always comes in pairs.
52
What does the size of a frictional force depend on?
type of surface
| - how hard the surfaces are pressed together
53
State Newton’s first law of motion in
words.
An object at rest will remain at rest, and an object moving with a given velocity will continue moving with that same velocity, unless acted on by an external resultant force.
54
State Newton’s second law of motion in words.
The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass.
55
State the defining equation of Newton’s second law of motion.
ΣF = ma
Resultant Force (N) = Mass (kg) x
Acceleration (ms⁻²)
56
Can an object travelling with constant speed change direction, without an external force acting?
For an object to change direction, its velocity must change. This means there must be an acceleration. An acceleration cannot occur without an external resultant force acting.
57
State Newton’s third law of motion in words.
If object *A* exerts a force on object *B*, then object *B* will exert an equal and opposite force on object *A*.
58
Name the conditions for a newtons third law pair.
* forces must have the same magnitude
* forces act along the same line but in opposite directions
* forces act for/at the same time
* forces act on a different object
* forces are the same type
59
If there is no resultant force, then there is no ____________ and the system is in ________________, as stated by the first law.
If there is no resultant force, then there is no **acceleration** and the system is in **equilibrium**, as stated by the first law.
60
What is a moment?
The turning effect of a force about a point.
## Footnote
Given as the product of a force, ***F*** and its perpendicular distance from its line of action to the point, ***d***.
61
State the equation used to calculate the
moment of a force.
Moment of a Force = Force x Perpendicular distance from the pivot
62
What distance must be used when calculating a
moment?
The perpendicular distance between the
line of action of the force and the axis of
rotation.
63
What is the unit of a moment?
Nm
64
What is the principle of moments?
For an object to be in equilibrium, the
sum of the clockwise moments about a
point must be equal to the sum of the
anticlockwise moments.
65
The moment of a force about the point it acts from will always be ________.
The moment of a force about the point it acts from will always be **zero**.
66
Moments will be directed either ____________ or ________________ around a point.
Moments will be directed either **clockwise** or **anticlockwise** around a point.
67
What is a couple?
A par of coplanar forces of equal magnitude, F which act on an object perpendicular distane d apart from each other.
68
A couple causes no ________________ but will cause a body to ____________.
A couple causes **no translational motion** but will cause a body to **rotate**.
69
State the equation for a moment of a couple.
M = Fd
70
The moment from a couple is ____________ for any point of reference.
The moment from a couple is **the** **same** for any point of reference.
71
Explain Equilibrium.
When the resultant force on an object is zero, meaning it remains stationary or travels with constant velocity.
72
Equilibrium means the resultant force in any direction must also be ________.
Equilibrium means the resultant force in any direction must also be **zero**.
73
What will occur if the line of action an object’s centre of gravity lies outside its base?
The object’s weight will produce a resultant moment and so if is unsupported, the object may topple.
74
If a 2m long seesaw has a 10kg child on one end, what is the moment around the pivot?
The child is 1.0m from the pivot, so x =1.0m. The force from the child acting on the seesaw is
10kg × g =98.1. So the moment towards the child is
Fx=98.1Nm.
75
If an object on a pivot is stationary, what can be said about the clockwise and counter-clockwise moments?
They are equal
76
Define and give units for:
momentum
The **momentum**, **p**, of an object is its mass times its current velocity. Momentum can also be thought of as the inertia of an object, or the mass times change in distance per unit time.
The SI unit of momentum is **kg m/s**.
77
Describe the principle of conservation of momentum
If there are no external forces acting on a system, then the total momentum in the system must remain constant.
## Footnote
This means the total momentum before and after a collision must be constant.
**Σ p-before =Σ p- after**
78
What is the momentum of an object if it has:
m = 2kg, v = 15 m/s?
m = 4kg, v = 20 m/s?
m = 10kg, v = 0 m/s?
30 kg*m/s (mv = 2 (15) = 30)
80 kg*m/s (mv = 4 (20) = 80)
0 kg*m/s (mv = 10 (0) = 0)
| Note: momentum can be +ve or -ve, depending on direction of velocity.
79
Use Newton's Second Law, to show that fore is equal to the rate of change momentum when is mass is constant.
F=ma
=m (Δv/Δt)
=Δmv/Δt
F =Δp/Δt
80
What is the momentum change for an object which experiences a force that doubles its velocity?
Momentum will be doubled.
## Footnote
ince Δp = mΔv, a change in momentum is directly proportional to a change in velocity. Doubling one will double the other.
81
Explain work done.
Work is done when the point of application of a force is moved in the direction of the force.
82
What is the work done by a force F which acts on an object as it moves through a distance d?
W = F × d cos(θ)
## Footnote
Where θ is the angle between the force and the direction of motion.
Since work is proportional to cos(θ), only the component of force parallel to the motion contributes to the work; any forces perpendicular to motion do no work since cos(90)=0.
83
What are the units of work, as defined in physics?
Work has units of Joules (J).
84
What force is used when calculating the work done by a force over a distance?
The component of the force that is in the
direction of the object’s motion.
85
What is the unit of work?
Joules, J
86
Describe the relationship between work and energy.
Energy is transffered to an object when work is done on it. When energy is transferred from the object to another system we say that the object does work.
87
Define Energy
Energy is the ability to do work.
88
What is potential energy?
Potential energy is the ability of an object to do work by virtue of its position or state.
89
State the equation for potential energy.
Work done to lift = force x distance moved
## Footnote
ΔW = mg x Δh
90
What form of energy does a raised object have?
Gravitational Potential Energy
91
# Define:
the **gravitational potential energy** of an object in a gravitational field
An object’s **gravitational potential energy *Ugr*** is defined as the work that was needed to raise it to its present height above an arbitrary reference point.
92
What is its gravitational potential energy of an object of mass m that is a height h above the surface of the Earth?
Ugr = mgh
## Footnote
This is identical to the work needed to raise the object that height, and is also equal to the kinetic energy the object will gain if it is allowed to fall freely to the ground.
93
How does the gravitational potential energy of a mass change, if its height above the surface of the Earth doubles?
The gravitational potential energy of the mass doubles.
## Footnote
Since gravitational potential energy of an object near Earth’s surface Ugr = mgh is proportional to height above the ground, changing the height changes the energy by the same amount.
94
What is the force on a mass m, attached to a stretched spring, at a distance x from equilibrium?
F = -kx
## Footnote
Where k is the spring constant (or force constant) of the spring (in N/m) and x is the distance from equilibrium. Notice that the negative sign means force will always act in the opposite direction of net displacement.
95
If spring 1 has twice the value of k that spring 2 has, which spring will need to be moved the greater distance in order to create the same force?
Spring 2 (the lower k) will need to be displaced further.
## Footnote
Since F=-kx and spring 2 has ½ the k value of spring 1, spring 2 will need to move twice the distance in order to create the same value of F.
96
# Define:
the potential energy of a spring which has been stretched
A spring's potential energy Usp is defined as the work needed to stretch or compress the spring from its equilibrium length to the current length.
97
What is a spring's potential energy, if a mass m is attached and the spring is stretched to a distance x from equilibrium?
Usp = ½kx²
## Footnote
Where x is the distance from equilibrium and k is the spring constant. There is no difference between sign for compression and expansion, since x² will always be positive.
98
How does the potential energy of a spring change, if its extension from equilibrium doubles?
The potential energy of the spring increases by a factor of 4.
## Footnote
Since the potential energy of the spring Usp = ½kx² is proportional to the square of the displacement of the spring, stretching the spring increases the potential energy by the square of the proportional length change.
99
# Define:
the **kinetic energy** of an object
An object’s **kinetic energy** is the energy resulting from its motion.
Kinetic energy is defined as:
KE = ½mv²
## Footnote
where m is the object’s mass and v is its speed. The units of kinetic energy are Joules, just like all other forms of energy.
100
If objects 1 and 2 are moving at the same speed, but object 2 has twice the mass of object 1, how do their kinetic energies compare?
KE₂ = 2 × KE₁
## Footnote
Kinetic energy is defined as ½mv², so it is directly proportional to the mass. If the objects’ speeds are the same, kinetic energy will increase proportionally with the mass.
101
If objects 1 and 2 are the same mass, but object 2 is moving at twice the speed of object 1, how do their kinetic energies compare?
KE₂ = 4 × KE₁
## Footnote
Kinetic energy is defined as ½mv², so it is directly proportional to the square of the velocity. If the objects’ masses are the same, kinetic energy will increase with the square of the velocity.
102
What form of energy do all moving objects have?
All moving objects have kinetic energy.All moving objects have kinetic energy.
103
What is the conservation of energy?
The law of conservation of energy states
that energy cannot be created or
destroyed - it can only be transferred
between different forms.
## Footnote
ΣE-Before = ΣE-After
104
At the bottom of the initial hill, a roller coaster car is moving with speed v. Ignoring friction, what determines whether it can glide to the top of the next hill?
The car will reach the top of the next hill if its kinetic energy at the bottom Ki is greater than the gravitational potential energy will be at the top of the hill Uf.
## Footnote
Energy conservation states that total energy K+Ugr remains constant. At the bottom of the hill, potential energy is zero. If Ki > Uf, then the initial kinetic energy is sufficient to overcome the force of gravity and climb the hill. If Ki \< Uf, the car would not make it to the top of the next hill.
105
Define:
power, as used in physics
**Power** is a measure of the rate of energy flow.
Power is defined as energy divided by time:
**P = E/t** or **P = W/t**
## Footnote
The units of power are Watts, where 1 W = 1 J/s.
106
What is the power flowing through a wire, if 1,000 J of energy flow through in 0.1 s.
P = 10,000 W = 10 kW
Power is energy divided by time;
1000 J / 0.1 s = 10,000 W
107
What is power developed?
The rate at which work is done on an object.
108
Derive the equation for power developed.
P=ΔW/Δt
=FΔs/Δt
P=Fv
109
During an energy transfer, the ________________ is usually transferred into multiple different types of energy.
During an energy transfer, the **input energy** is usually transferred into multiple different types of energy.
110
What is efficiency?
The ratio between the useful output energy and the total input energy of a transfer.
## Footnote
Efficiency can be given as a number between 0 and 1 or as a percentage.
111
Define useful energy.
The desired type of energy from an energy transfer.
112
What is the waste energy of an electrical appliance?
The waste energy is the energy that is not used for the appliance’s intended purpose.
113
State the equation relating energy and efficiency.
Efficiency = Useful Energy Output/
Total Energy Input
114
State the equation relating power and
efficiency.
Efficiency = Useful Power Output /
Total Power Input
115
What is the unit of efficiency?
Efficiency is a ratio and so doesn’t have an associated unit.
116
Name other forms of energy.
* Chemical Energy
* Sound Energy
* Electrical Potential Energy
* Thermal Energy
* Light Energy
117
Define Chemical Energy.
The energy supplied to systems by food or batteries.
118
Define Sound Energy.
The energy transferred by sound waves.
119
Define Electrical Potential Energy.
The energy stored by an object after doing work against electrostatic forces.
120
Define thermal energy.
The energy transferred to particles as heat, due to work done against frictional forces.
121
Define light energy.
The energy transffered by electromagnetic waves or photons.
