AS-Level Mechanics OCR G481

Question 1

define scalar and vector quantities and give examples;

Answer 1

Scalar: Magnitude without direction
Density, volume, temperature etc
Vector: A quantity that has (both) magnitude / size and direction
acceleration, displacement and weight etc

Question 2

define displacement

Answer 2

Displacement = (net) distance moved in a particular direction.

Question 3

define instantaneous speed

Answer 3

Instantaneous speed = speed of a body at a specific instance or at a specific point

Question 4

define average speed

Answer 4

distance travelled / time taken

Question 5

define velocity

Answer 5

speed in a given direction

Question 6

define acceleration

Answer 6

Acceleration is the rate of change of velocity (gradient of a velocity vs time graph, change in velocity/time taken)

Question 7

Define average velocity

Answer 7

(net) displacement / time taken

Question 8

define the newton;

Answer 8

The (net) force which gives a mass of 1kg an acceleration of 1 ms^-2

Question 9

define the torque of a couple;

Answer 9

**one of forces × perpendicular distance (between forces) **

(Not force x perpendicular distance)

Question 10

define the moment of force;

Answer 10

moment = force x perpendicular distance from pivot / axis / point

Question 11

define thinking distance

Answer 11

Thinking distance: The distance travelled (by the car) from when the driver sees a problem and the brakes are applied

Question 12

Define braking **distance **

Answer 12

The distance travelled (by the car) whilst the brakes are applied and the car stops

Question 13

define stopping distance

Answer 13

stopping distance: Thinking distance + braking distance. The total distance travelled in the interval between a driver spotting a hazard, and the vehicle coming to a complete stop.

Question 14

define work done by a force;

Answer 14

work done = force x distance moved in the direction of the force

Question 15

define the joule;

Answer 15

Energy required to move a weight of 1N (through) a distance of 1 m

Question 16

define power

Answer 16

The rate at which work is done:

Power = Work done / Time

OR

Power = Energy / Time

Question 17

Define the Watt

Answer 17

Power required to move 1N through a distance of 1m in 1 sec (Rate of doing work)

Question 18

Define Stress

Answer 18

force/(cross-sectional) area

Question 19

Define Strain

Answer 19

extension/original length

Question 20

Define Young’s modulus

Answer 20

Young modulus = stress/strain / Young modulus is equal to the gradient from stress-strain graph (in the linear region)

Question 21

Define ultimate tensile strength

Answer 21

Ultimate tensile strength = Maximum stress material can withstand (before fracture)

Question 22

define the term elastic deformation

Answer 22

Elastic extension (or compression) is proportional to force (as long as elastic limit is not exceeded)

Question 23

define plastic deformation of a material

Answer 23

Plastic: Material does not return to original length / shape/ size (is permanently deformed / longer) when the force / stress is removed.

Question 24

Define density

Answer 24

Density = mass/volume or mass per (unit) volume

Question 25

Derive the equations of motion for constant
acceleration in a straight line from a velocity
against time graph;

Answer 25

Area of triangle = ½ (v-u) t [(v-u) = at]
= ½ at²
Area of rectangle = ut add the two together

Question 26

Apply the definition of work done to derive the equation for the change in gravitational
potential energy;

Answer 26

Work done = force x distance
Force = mass x acceleration
Weight = mass x gravitational field strength
G.P.E. = m x g x h

Question 27

apply the equations of constant acceleration to describe and explain the motion of an object due to a uniform velocity in one direction and a constant acceleration in a perpendicular direction

Answer 27

With a uniform velocity in one direction the distance travelled per unit time will remain constant

s = ut + ½at²

For a = 0

s = ut which is linearly proportional to “t”

With a constant acceleration in another direction the distance travelled per unit time in that direction increases.

s = ut + ½at² which is not linearly proportional to “t”

This means that the object traces a parabollic path.

Question 28

If work is done on a system, what can we say about transfer of energy in that system.

Answer 28

If work is done energy must be transfered from one type to another. The total amount of energy transferred into other forms is equal to the work done

Question 29

describe an experiment to determine the
acceleration of free fall g using a falling body;

Answer 29

Measurements:

1) Height (distance)
2) Time (of fall)

Equipment

1) Ruler/tape (measure)
2) Stop watch/timer/clock/video

Calculation:

g = 2s/t² or g = twice the gradient of s-t² graph

Why is this not accurate?

air resistance / drag
parallax (landing time)
starting/stopping the clock

Derivation of g = 2s/t²

s=ut+½at² (ut=0),

s = ½at²,

a=2s/t²

For a falling body a = g, so g = 2s/t2

Question 30

Describe the motion of bodies falling in a
uniform gravitational field with drag;

Answer 30

1) Initially the body has no speed. The resultant force acting on it is 9.81N/kg and it is accelerating at 9.81m/s². Because the body has no velocity, air resistance is zero.
2) Then as speed increases, air resistance increases, so the resultant force is now less than 9.81N/kg and therefore its acceleration is decreasing.
3) Eventually the object will reach a speed where the force of air resistance balances the force of gravity. At this point there is no acceleration, we say the object has reached terminal velocity.

Question 31

describe a simple experiment to determine the centre of gravity of an object;

Answer 31

Suspend object from a point and then mark a vertical line on the object
Plumb line / ‘pendulum’ (used to find the vertical line)

Hang from another point / place (and draw another vertical line)

Where the lines intersect gives position of centre of gravity (wtte)

Question 32

Describe the factors that affect thinking
distance

Answer 32

Thinking distance is the distance travelled from the moment when you see a hazard, to the moment when your foot touches the pedal.

Age: older people have slower reactions

Drugs/Alcohol/Tiredness: slow reactions (some drugs like cocaine may actually reduce thinking distance)

Speed: if you are travelling faster even though it takes the same time to react, you will travel further in the same time.

Question 33

Describe factors that affect braking distance

Answer 33

Speed, mass, condition of tyres/tread, condition of brakes, condition of road (surface), gradient(steepness) of road.

You should also know how two of these factors affect breaking distance:

braking distance ∝ speed²

(if you double the speed you quadrouple the braking distance, this is because if you double the speed you quadrouple the Kinetic Energy because E_k=½mv²)

Braking distance ∝ mass

(if you double the mass you double the braking distance)

Question 34

Know the difference between braking distance stopping distance and thinking distance

Answer 34

Thinking distance: The distance travelled from the moment when you first spot a hazard to when your foot first touches the brake.

Braking distance: The distance travelled between the moment when you start braking and the moment when you come to a complete stop.

Stopping Distance: The total distance travelled between the moment when you first spot a hazard and the moment when you come to a complete stop

Stopping distance = thinking distance + braking distance

Question 35

describe and explain how air bags, seat belts and crumple zones in cars reduce impact forces in accidents

Answer 35

Seat belts, airbags, and crumple zones are “soft” and therefore deform under stress. This means that when decelerating a body, the contact time is greater, so the acceleration is smaller, so the force is smaller.

A smaller force is less likely to cause serious injury.

Question 36

describe how air bags work, including the
triggering mechanism;

Answer 36

Sensor known as an accelerometer triggers a chemical reaction when it detects a deceleration of about 10g. This size deceleration would only happen in an accident.

Spark starts violent chemical reaction between sodium nitride and potassium nitrate to produce nitrogen gas.

This will inflate the airbag within 0.05s.

The airbag will then deform (squash) when the persons head hits it, increasing the contact time, decreasing the acceleration and the force.

Question 37

describe how the trilateration technique is
used in GPS

Answer 37

(Several) satellites used

Distance from (each) satellite is determined.

Time taken for signal to travel from satellite to car/‘delay’ time for signal is determined.

Distance = c x ‘delay’ time

Once the distance from each satellite is calculated, you can (virtually) draw a sphere of radius equal to this distance, and centred on the satellite.

The car’s position must be somewhere on the surface of the sphere.

If you do this for 3 satellites, the point of intersection of the three different spheres will tell you the exact location of the car

Question 38

State the law of conservation of Energy

Answer 38

Energy cannot be created or destroyed, only changed from one form into another

Question 39

Describe the behaviour of springs and wires in terms of force, extension, elastic limit, Hooke’s law and the force constant (ie force per unit extension or compression);

Answer 39

As the force applied to a spring increases, so does its extension.

Within the “elastic limit” this change is proportional, and we say that the spring obeys “Hooke’s law”.

The ratio of force to extension is called the spring constant and is measured in Newtons per meter.

Past the elastic limit the spring will deform and will not return to its original length.

Question 40

describe an experiment to determine the
Young modulus of a metal in the form of a
wire;

Answer 40

Describe the experiment below, but if I were you I would measure the extension at the end of the string not in the middle as they suggest below.

Question 41

describe the shapes of the stress against
strain graphs for typical materials

Answer 41

Question 42

How do you determine the acceleration and distance travelled from a velocity against time graph

Answer 42

Displacement from the area under a velocity against time graph.

Acceleration from the gradient of a velocity against time graph.

Question 43

How do you find the “work done” from a force against extension graph?

Answer 43

The area under the graph is equal to the work done/energy transferred.

Question 44

explain that some physical quantities consist of a numerical magnitude and a unit

Answer 44

Physical quantities such as length (m) Weight (N) and Mass (kg) consist of a numerical magnitude and a unit.

Question 45

Explain how experiments carried out by
Galileo overturned Aristotle’s ideas of motion

Answer 45

heavy’ and ‘light’ objects / different weights / different masses dropped (from leaning tower of Pisa) / rolled down incline plane
Objects have the same acceleration (of free fall)
Objects hit ground at same time

Question 46

explain that an object travelling in a fluid
experiences a resistive or a frictional force
known as drag

Answer 46

Drag/air resistance/air friction (makes the time of “flight” longer)
drag is proportional to speed²

It decreases the velocity in both the horizontal AND vertical directions.

Question 47

explain that a couple is a pair of forces that tends to produce rotation only

Answer 47

In both the horizontal and vertical directions the forces are balanced so there is no movement in either direction.

However there is a moment of force so the object rotates

.

Question 48

What is meant by “an object in equilibrium”?

Answer 48

both the net force and net
moment on an object in equilibrium
is zero

This results in no movement of any kind

Question 49

Why can F=ma not be used for very large speeds

Answer 49

F = ma cannot be used for a particle travelling at very high speeds because its mass increases due to relativistic effects

Question 50

state the factors that affect the magnitude of the drag force

Answer 50

Area
Speed/velocity
(surface) texture/aerodynamic (shape)
Viscosity (of air)/temperature/density

Question 51

What is meant by “the centre of gravity”

Answer 51

The point about which all the weight of an object can be considered to act.

Question 52

state the principle of conservation of energy

Answer 52

Energy cannot be created or destroyed; it can only be transferred/transformed into other forms
or
The (total) energy of a system remains constant
or
(total) initial energy = (total) final energy

Question 53

State Hooke’s Law

Answer 53

Extension is proportional to force (applied) (as long as the elastic limit is not exceeded)

Question 54

Why is the efficiency of a device never 100%

Answer 54

The efficiency of a device is always less than 100% because of heat losses, this is always the case.