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Physics > Mechanics > Flashcards

Mechanics Flashcards

1
Q

What is a scalar quantity?

A

Any physical quantity that is not directional - only magnitude

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2
Q

Examples of scalar quantities

A

Mass, temperature, time, length/distance, speed, energy

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3
Q

What is a vector quantity?

A

Any physical quantity that has direction and magnitude

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4
Q

Examples of vector quantities

A

Displacement, velocity, force, weight, acceleration, momentum

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5
Q

Two ways of finding resultant vector?

A
  1. Scale drawings:
    -tip to tail (remember scales)
    -draw the resultant vector (from the tail of the first to the tip of the last)
    • measure its length and angle
  2. Pythagoras and Trigonometry:
    • when vectors are at right angles
    • use bearings to calculate direction from starting point (remember three digits)
    • SOHCAHTOA / Pythagoras theorem
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6
Q

How do you resolve a vector?

A

Split into components at right angles to each other

Horizontal and vertical components

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7
Q

What are free body diagrams and how do you draw them?

A

-They show all the forces that act on a a single body (not the forces it exerts)
- its a force so the arrows should represent size and direction
- if body is in equilibrium (not accelerating) the forces in each direction are balanced

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8
Q

What is equilibrium of an object?

A

An object at rest of moving at constant velocity

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9
Q

What does coplanar mean?

A

In the same plane

(Coplanar objects are objects in the same plane)

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10
Q

Types of forces on free body diagrams

A

-weight (gravity pushes object down)
-air resistance/ drag (pushes object up)
-friction (acts in opposite direction object is moving/ when surface is rough)
-pushes (acts in direction object is moving)
-reaction (when earth is pushing an object up)

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11
Q

How do you draw three coplanar forces in equilibrium?

A

-draw the forces as a triangle , forming a closed loop
-all of them are tip-to-tail (the third force is not a resultant force!), has to be in opposite
direction to balance the other two forces
-if its right angled, use Pythagoras to find missing force
-if not right angled, might have to resolve forces in each direction

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12
Q

How do you resolve forces on an inclined plane?

A

-resolve forces perpendicular and parallel TO THE SLOPE
-solve the perpendicular and parallel forces separately

-weight acts vertically down FROM THE OBJECT
-reaction force of object on slope acts PERPENDICULAR to slope
-friction (if surface is rough) or pull and push forces

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13
Q

When is an object in equilibrium?

A

-the sum of the forces is equal to zero (no resultant force)
-the sum of the moments (about any point) is zero (no resultant moment)

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14
Q

What is a moment of a force and what does it depend on?

A

Moment (Nm) = Force (N) x perpendicular distance from the point to the line of action of
the force (m)

Depends on the size of the force and how far the force is applied from the turning point

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15
Q

State the principle of moments

A

For a body to be in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments about the same point

(If moments are not balanced, the object will turn)

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16
Q

What is a couple and what does it produce?

A

A couple is a pair of forces of equal size which act parallel to each other, but in opposite directions. The forces are coplanar.

EASIER DEFINITION: a pair of equal and opposite coplanar forces

A couple doesn’t cause any resultant linear force, but does produce a turning effect

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17
Q

What is the moment of a couple equal to?

A

Moment of couple (Nm) = size of one of the forces (N) x perpendicular distance between the lines of action of the forces (m)

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18
Q

What is a support force?

A

The normal reaction force on an object exerted at the point of contact with a support or pivot

(If moments are taken about the pivot, the support force can be ignored as it does not cause rotation)

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19
Q

What is the mass of an object and weight of an object?

A

-The amount of matter in it (kg)
-The greater an objects mass, the greater its resistance to a change in velocity (its inertia)
-Weight is a force (N)
-It is the force experienced by a mass due to a gravitational field
-W=mg

20
Q

What is the centre of mass of an object?

A

The single point that you consider its whole weight to act through (whatever its orientation)

The object will always balance around this point

The COM of an object is not always its centre, that is only for a uniform, symmetrical object.

A resultant force directed through the centre of mass of an object will not give it a moment/ will not cause it to rotate

21
Q

How do you find the centre of mass of an object? (2 ways)

A

SYMMETRY:
-the COM of any regular object is at its centre- where all the lines of symmetry will cross

BY EXPERIMENT:
-hang the object freely from a point, draw a vertical line downwards from the point of -suspension (use a plumb bob to get your line exactly vertical)
-hang the object from a different point
-draw another vertical line down
-the centre of mass is where the two lines cross

22
Q

How does the stability of an object depend on its COM and base area?

A

An object will topple over if a vertical line drawn downwards from its COM falls outside its base area

An object will be nice and stable if it has a low COM and a wide base area

23
Q

Definition of inertia?

A

the tendency of objects in motion to stay in motion, and objects at rest to stay at rest, unless a force causes its speed or direction to change

24
Q

Definition of Speed, Displacement, Velocity, Acceleration

Formulas for velocity and acceleration

A

-how fast something is moving, regardless of direction
-how far an objects travelled from its starting point in a given direction
-the rate of change of an objects displacement (its speed in a given direction)
-the rate of change of an objects velocity

V=displacement/time
A=velocity/time

25
Q

What is instantaneous speed/velocity?

A

The speed or velocity of an object at any point in time

26
Q

Features of displacement time graphs
(Acceleration, velocity, instantaneous velocity)

A

-for an accelerating object, the graph is a curve
-if object is accelerating at a constant rate, the rate of change of the gradient will be constant
-different accelerations have different gradients

-when the velocity is constant, the graph is a straight line
-if the gradient isn’t constant, the object is accelerating
-average velocity over a period of time, divide the total change in displacement by the total change in time (irregardless if curved)

-to find instantaneous velocity at a certain point you need to draw a tangent to the curve at that point and find its gradient

27
Q

Features of velocity time graph
(Acceleration, displacement)

A

-acceleration is the gradient of a velocity-time graph
-uniform acceleration is always a straight line (the steeper the gradient the greater the acceleration)
-a curved graph shows changing acceleration, (increasing gradient= increasing acceleration, decreasing gradient= decreasing acceleration)

-the area under a v-t graph is the displacement of the object
-the magnitude of this displacement is the distance that object has travelled

28
Q

Features of acceleration-time graphs
(Velocity, acceleration)

A

-tells you how the object acceleration changes over time
-the height of the graph give the objects acceleration at that time
-the area under the graph give the objects change in velocity
-if a=0, then the object is moving with constant velocity
-a negative acceleration is a deceleration

29
Q

Advantages of data-loggers over traditional methods

A

-the data is more accurate- doesn’t allow for human reaction times

-automatic systems have a much higher sampling rate than humans- most ultrasound position detectors can take a reading ten times every second

-you can see the data displaced in real time

30
Q

Meaning of uniform / SUVAT equations

A

Uniform= constant

v= u + at
s=((u+v)/2)t
s = ut + 1/2(at^2)
v^2 = u^2 + 2as
S= vt- 1/2(at^2)

31
Q

Define free fall, and the forces that act on object undergoing free fall

A

Free fall is defined as the motion of an object undergoing an acceleration of ‘g’

-acceleration is a vector quantity: ‘g’ acts vertically down
-‘g’ = 9.81
-the only force acting on it is weight

-objects can have an initial velocity and still undergo free fall as long as the force providing the initial velocity is no longer acting

-all free falling objects have the same acceleration

32
Q

Define a projectile

A

A projectile is an object which is falling freely under gravity. A projected object in motion acted upon only by the force of gravity.

33
Q

Define Friction, state the two types of friction and how they work
Describe the energy changes that occurs with frictional forces

A

Friction is a force that opposes motion
-2 types: dry friction between solid surfaces and fluid friction (or drag/ air resistance)

Fluid Friction:
-force depends on viscosity of fluid
-increases as speed increases
-depends on the shape of the moving object- larger area= greater resistive force)
-a projectile is slowed down by air resistance

Frictional forces:
-always act in opposite direction to motion of object
-can never speed things up
-they convert kinetic energy into heat and sound

34
Q

What is lift?

A

-its an upward force on an object moving through a liquid
-happens when the shape of an object causes the fluid flowing over it to change direction
-the force acts perpendicular to the direction the fluid flows in

35
Q

What is terminal speed and when will you reach maximum (terminal) speed?

Give ways of increasing maximum speed

A

When the friction force is equal to the driving force

Will be reached when:
-you have a driving force that stays the same all the time
-a frictional or drag force (or collection of forces) that increases with speed

Ways of increasing maximum speed:
-increasing the driving force (e.g increasing the engine size)
-reducing the frictional force (e.g making the body more streamlined)

36
Q

Describe an object through air or water to reach terminal speed
(Eg a parachutist falling through air)

A

-as skydiver leaves the plane : will accelerate until, air resistance equals his weight
-he will then be travelling at a terminal speed
-before reaching the ground, the skydiver opens his parachute, which immediately increases air resistance so it is not bigger than his weight
-this slows his down until his speed has dropped enough for the air resistance to be equal to his weight again
-this new terminal speed is enough for his to land safely

37
Q

State Newtons 3 Laws

A

1st Law: the velocity of an object will not change unless acted on by a resultant force

2nd Law: resultant force (N) = mass(kg) x acceleration (ms^-2)
-acceleration is independent of mass

3rd Law: if object A exerts a force on object B, then object B exerts an equal but opposite force on object A

38
Q

Give the formula for momentum and why it is always conserved

A

Momentum= mass x velocity

-assuming no external forces act on it, momentum is always conserved
-total momentum before= total momentum after

39
Q

Define elastic and inelastic collisions

A

An elastic collision is one where momentum is conserved and kinetic energy is conserved (i.e no energy dissipated as heat/sound)

An inelastic collision means that some of the kinetic energy is converted into other forms during the collision. But momentum is always conserved

40
Q

Define impulse

A

Impulse= change in momentum
Impulse (Ft) = mv - mu

-the area under a force-time graph

41
Q

Describe how to reduce the impact of a car crash

A

-the force of an impact is increased by reducing the impact time
-the force of an impact can be reduced by increasing the time of the impact

Crumple zones- this causes the car to take longer to stop, increasing the time the impact time and decreasing the force on passengers

Seat belts- these stretch slightly, increasing the time taken for the wearer to stop. This reduces the forces acting on the chest

Air bags- these slow down the passengers gradually, also prevent them from hitting hard surfaces in a car

42
Q

Define work done and when is occurs

A

Work done= force causing motion x distance moved
Work done= Fscosθ

-occurs when energy is transferred

THINGS TO REMEMBER:
- work is the energy that’s been changed from one form to another (not the total energy)
-the force will be a fixed value in any calculations, either because its constant or because its the average force
-the equation assumes that the direction of the force is the same as the direction of movement

43
Q

Define a joule and what does the area under a force-displacement graph tell you

A

One joule is the work done when a force of 1 newton moves through a distance of 1 metre

Area under force-displacement graph tells you the work done

44
Q

Define power and state its equation
Define a watt

A

-is the rate of doing work (amount of energy transferred from one form to another per s)

  • Power = change in energy (work done) / change in time
  • watt is defined as a rate of energy transfer equal to 1 joule per second
  • Power = force x velocity
45
Q

What does the principle of the conservation of energy state

A

Energy cannot be created or destroyed. It can be transferred from one form to another but the total amount of energy in a closed system will not change

Total energy in= total energy out

Efficiency = useful output power / input power

46
Q

Describe kinetic energy and both types of potential energies

A

-KE is the energy of anything moving
-Potential energy has 2 types:
- Gravitational potential energy= energy something gains if you lift it up
-Elastic strain energy= energy you get in E=1/2kL^2

Physics (14 decks)

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