Mechanics

Question 1

What is a scalar quantity?

Answer 1

Any physical quantity that is not directional - only magnitude

Question 2

Examples of scalar quantities

Answer 2

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

Question 3

What is a vector quantity?

Answer 3

Any physical quantity that has direction and magnitude

Question 4

Examples of vector quantities

Answer 4

Displacement, velocity, force, weight, acceleration, momentum

Question 5

Two ways of finding resultant vector?

Answer 5

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

Question 6

How do you resolve a vector?

Answer 6

Split into components at right angles to each other

Horizontal and vertical components

Question 7

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

Answer 7

-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

Question 8

What is equilibrium of an object?

Answer 8

An object at rest of moving at constant velocity

Question 9

What does coplanar mean?

Answer 9

In the same plane

(Coplanar objects are objects in the same plane)

Question 10

Types of forces on free body diagrams

Answer 10

-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)

Question 11

How do you draw three coplanar forces in equilibrium?

Answer 11

-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

Question 12

How do you resolve forces on an inclined plane?

Answer 12

-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

Question 13

When is an object in equilibrium?

Answer 13

-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)

Question 14

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

Answer 14

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

Question 15

State the principle of moments

Answer 15

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)

Question 16

What is a couple and what does it produce?

Answer 16

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

Question 17

What is the moment of a couple equal to?

Answer 17

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

Question 18

What is a support force?

Answer 18

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)

Question 19

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

Answer 19

-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

Question 20

What is the centre of mass of an object?

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

Definition of inertia?

Answer 23

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

Question 24

Definition of Speed, Displacement, Velocity, Acceleration

Formulas for velocity and acceleration

Answer 24

-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

Question 25

What is instantaneous speed/velocity?

Answer 25

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

Question 26

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

Answer 26

-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

Question 27

Features of velocity time graph (Acceleration, displacement)

Answer 27

-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

Question 28

Features of acceleration-time graphs (Velocity, acceleration)

Answer 28

-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

Question 29

Advantages of data-loggers over traditional methods

Answer 29

-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

Question 30

Meaning of uniform / SUVAT equations

Answer 30

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)

Question 31

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

Answer 31

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

Question 32

Define a projectile

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What is lift?

Answer 34

-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

Question 35

What is terminal speed and when will you reach maximum (terminal) speed? Give ways of increasing maximum speed

Answer 35

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)

Question 36

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

Answer 36

-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

Question 37

State Newtons 3 Laws

Answer 37

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

Question 38

Give the formula for momentum and why it is always conserved

Answer 38

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

Question 39

Define elastic and inelastic collisions

Answer 39

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

Question 40

Define impulse

Answer 40

Impulse= change in momentum Impulse (Ft) = mv - mu -the area under a force-time graph

Question 41

Describe how to reduce the impact of a car crash

Answer 41

-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

Question 42

Define work done and when is occurs

Answer 42

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

Question 43

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

Answer 43

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

Question 44

Define power and state its equation Define a watt

Answer 44

-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

Question 45

What does the principle of the conservation of energy state

Answer 45

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

Question 46

Describe kinetic energy and both types of potential energies

Answer 46

-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