AS-Level Mechanics OCR G481 Flashcards
define scalar and vector quantities and give examples;
Scalar: Magnitude without direction
Density, volume, temperature etc
Vector: A quantity that has (both) magnitude / size and direction
acceleration, displacement and weight etc
define displacement
Displacement = (net) distance moved in a particular direction.
define instantaneous speed
Instantaneous speed = speed of a body at a specific instance or at a specific point
define average speed
distance travelled / time taken
define velocity
speed in a given direction
define acceleration
Acceleration is the rate of change of velocity (gradient of a velocity vs time graph, change in velocity/time taken)
Define average velocity
(net) displacement / time taken
define the newton;
The (net) force which gives a mass of 1kg an acceleration of 1 ms-2
define the torque of a couple;
**one of forces × perpendicular distance (between forces) **
(Not force x perpendicular distance)
define the moment of force;
moment = force x perpendicular distance from pivot / axis / point
define thinking distance
Thinking distance: The distance travelled (by the car) from when the driver sees a problem and the brakes are applied
Define braking **distance **
The distance travelled (by the car) whilst the brakes are applied and the car stops
define stopping distance
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.
define work done by a force;
work done = force x distance moved in the direction of the force
define the joule;
Energy required to move a weight of 1N (through) a distance of 1 m
define power
The rate at which work is done:
Power = Work done / Time
OR
Power = Energy / Time
Define the Watt
Power required to move 1N through a distance of 1m in 1 sec (Rate of doing work)
Define Stress
force/(cross-sectional) area
Define Strain
extension/original length
Define Young’s modulus
Young modulus = stress/strain / Young modulus is equal to the gradient from stress-strain graph (in the linear region)
Define ultimate tensile strength
Ultimate tensile strength = Maximum stress material can withstand (before fracture)
define the term elastic deformation
Elastic extension (or compression) is proportional to force (as long as elastic limit is not exceeded)
define plastic deformation of a material
Plastic: Material does not return to original length / shape/ size (is permanently deformed / longer) when the force / stress is removed.
Define density
Density = mass/volume or mass per (unit) volume
Derive the equations of motion for constant
acceleration in a straight line from a velocity
against time graph;
Area of triangle = ½ (v-u) t [(v-u) = at]
= ½ at2
Area of rectangle = ut add the two together
Apply the definition of work done to derive the equation for the change in gravitational
potential energy;
Work done = force x distance
Force = mass x acceleration
Weight = mass x gravitational field strength
G.P.E. = m x g x h
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
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.
If work is done on a system, what can we say about transfer of energy in that system.
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
describe an experiment to determine the
acceleration of free fall g using a falling body;
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
Describe the motion of bodies falling in a
uniform gravitational field with drag;
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.
describe a simple experiment to determine the centre of gravity of an object;
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)
Describe the factors that affect thinking
distance
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.
Describe factors that affect braking distance
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 Ek=½mv2)
Braking distance ∝ mass
(if you double the mass you double the braking distance)
Know the difference between braking distance stopping distance and thinking distance
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
describe and explain how air bags, seat belts and crumple zones in cars reduce impact forces in accidents
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.
describe how air bags work, including the
triggering mechanism;
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.
describe how the trilateration technique is
used in GPS
(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
State the law of conservation of Energy
Energy cannot be created or destroyed, only changed from one form into another
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);
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.
describe an experiment to determine the
Young modulus of a metal in the form of a
wire;
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.
describe the shapes of the stress against
strain graphs for typical materials
How do you determine the acceleration and distance travelled from a velocity against time graph
Displacement from the area under a velocity against time graph.
Acceleration from the gradient of a velocity against time graph.
How do you find the “work done” from a force against extension graph?
The area under the graph is equal to the work done/energy transferred.
explain that some physical quantities consist of a numerical magnitude and a unit
Physical quantities such as length (m) Weight (N) and Mass (kg) consist of a numerical magnitude and a unit.
Explain how experiments carried out by
Galileo overturned Aristotle’s ideas of motion
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
explain that an object travelling in a fluid
experiences a resistive or a frictional force
known as drag
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.
explain that a couple is a pair of forces that tends to produce rotation only
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
.
What is meant by “an object in equilibrium”?
both the net force and net
moment on an object in equilibrium
is zero
This results in no movement of any kind
Why can F=ma not be used for very large speeds
F = ma cannot be used for a particle travelling at very high speeds because its mass increases due to relativistic effects
state the factors that affect the magnitude of the drag force
Area
Speed/velocity
(surface) texture/aerodynamic (shape)
Viscosity (of air)/temperature/density
What is meant by “the centre of gravity”
The point about which all the weight of an object can be considered to act.
state the principle of conservation of energy
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
State Hooke’s Law
Extension is proportional to force (applied) (as long as the elastic limit is not exceeded)
Why is the efficiency of a device never 100%
The efficiency of a device is always less than 100% because of heat losses, this is always the case.
