G481 - Mechanics Flashcards
Unit of Length
metres
m
Unit of Time
seconds
s
Unit of Temperature
Kelvin
K
Unit of Electrical Current
Ampere
A
10^-12
Pico
10^-9
Nano
10^-6
Micro
10^-3
Milli
10^-2
Centi
10^3
Kilo
10^6
Mega
10^9
Giga
10^12
Tera
Scalar Quantity
Definition
A quantity that has magnitude but not direction
Vector Quantity
Definition
A quantity that has both direction and magnitude
Displacement
Definition
Distance moved in a certain direction from a point
m
Instantaneous Speed
Definition
Speed at a given moment in time
m/s
Average Speed
Definition
Total distance travelled / total time
m/s
Velocity
Definition
Rate of change of displacement
m/s
Acceleration
Definition
Rate of change of velocity
Velocity Time Graph
Grafient
Acceleration
Velocity Time Graph
Area
Displacement
Displacement Time Graph
Gradient
Velocity
Aristotle
Assumed without experimentation that heavier objects fall faster than lighter ones
Galileo
Dropped balls of different mass
Balls hit the ground at the same time
Acceleration of free fall is the same regardless of mass
Experiment to Find g
Measure the height the ball is dropped from
Time how long it takes for the ball to fall that distance
Use s=ut+0.5at² where u=0, s=height, t=time to fall
Force
Definition
An interaction between two objects involving a push it a pull
N
Newtons First Law
Every object continues in its state of rest or uniform motion in a straight line unless acted on by an external force
Newtons Second Law
F∝a
F=ma
Newtons Third Law
For every action there is an equal and opposite reaction
One Newton
Definition
Force required to give a mass of 1kg an acceleration of 1m/s²
Weight
Formula
F=ma
W=mg
Why can F=ma not be applied to objects travelling close to the speed of light?
Because as an object approaches the speed of light its mass increases
Weight
Definition
The gravitational force on a body
What is a fluid?
A liquid or a gas
Drag
Definition
The restive force that acts on an object when it moves through a fluid
Factors that Affect Drag
Velocity Cross Sectional Area Roughness of Surface Shape / Streamlining Viscosity of Fluid
Terminal Velocity
Definition
The maximum velocity reached when drag forces are equal to the motive forces acting on an object
Centre of Gravity
Definition
The point at which the weight of an object appears to act
Finding the Centre of Gravity
Pin object to wall Hang a plumb line form the pin Mark where the line hangs Rotate the object and repeat The centre of gravity is where the lines cross
Couple
Definition
A pair of equal but opposite forces
Cannot cause linear acceleration
Tend to produce rotation
Torque
Definition
A force that tends to cause rotation
Can be applied to a couple
Torque of a Couple
Definition / Formula
Torque = OneOfTheForces x PerpendicularDistanceBetweenForces
Moment
Definition and Formula
The turning effect of a single force
Force x PerpendicularDistanceFromPivot
The Principle of Moments
For a body in rotational equilibrium the sum of the clockwise moments is equal to the sun of the anticlockwise moments
The resultant force on the object is 0
Density
Definition and Formula
Mass per unit volume
ρ = m/V
Measured in kg/m³
Pressure
Definition and Formula
Force per unit area applied to a surface
P = F/A
Measured in Pa
Thinking Distance
Definition
The distance travelled from the driver seeing the obstacle to pressing the brake
Braking Distance
Definition
Distance travelled from when the driver presses the brake to when the car stops
Stopping Distance
Definition and Formula
Distance traveled from driver seeing obstacle to when the car stops
StoppingDistance = ThinkingDistance + BrakingDistance
Factors Affecting Thinking Distance
Alcohol and drugs
Velocity of car
Reaction time of driver
Distractions
Factors Affecting Braking Distance
Velocity of car Road surface / condition Tyres Brake condition Weather Mass of car
Air Bags
Increase the time over which the driver loses velocity therefore reducing deceleration
F=ma, mass is constant so the force acting on he driver reduces
Seat belts
Stretch in a collision
Increases the distance over which the driver loses velocity, reducing the deceleration
F=ma, so the force on the driver is reduced
Stop the driver from being thrown forwards and hitting the dashboard or window
Crumple Zones
Increase the time of the impact
Reduces the dece,le ration of the car
F=ma, reduces the force on the car and the passengers
Roll Cage
Strengthens the structure of the car to prevent it from collapsing and harming passengers during a collision
GPS
Trilateration
Satellite sends a radio signal to the car
There is a time delay from, the length of time taken for the signal to travel from the satellite to the GPS
Distance between the GPS and the satellite is calculated using distance = time delay x c
Work Done
Definition / Formula
force x distance moved in the direction of the force
One Joule
Definition
The energy needed to lift a weight in 1N through a height of 1m
Conservation of Energy
Energy is never created or destroyed
Energy can only be transformed form one for to another
Work Done and Energy
The work done on an object is equal to the energy transferred to the object
Kinetic Energy
Formula
KE = 0.5mv²
Gravitational Potential Energy
Formula
GPE = mgh
Power
Definition
The rate of doing work
Energy transferred per unit time
Power
Formula
P = W/t
One Watt
Definition
Noel joule of energy transferred per second
Efficiency
Formula
UsefulOutputEnergy/TotalInputEnergy x 100
Elastic Behaviour / Deformation
When a force is removed form an object and the object returns to its original shape
Plastic Behaviour / Deformation
When a force is removed from an object but the object remains permanently deformed
A force only causes deformation in the direction that it acts
Compressive Forces
Two or more forces that reduce the volume of the object on which they are acting or reduces the length of a spring
Tensile Forces
Usually two equal and opposite forces acting on an object / string to stretch it
Elastic Region
A force applied within this region causes elastic deformation
Plastic Deformation
A force applied within this region causes plastic deformation
Elastic Limit
The point at which elastic deformation becomes plastic
Hooke’s Law
Force ∝ Extension
As long as the elastic limit is not exceeded
Load Extension Graph
Area
Area = work done
For an object that obeys hookes law:
Area = 0.5Fx = 0.5kx²
Elastic Potential Energy
EPE in Spring = work done compressing it
Stress
Formula
Stress = F/A
Measured in Pa
Ultimate Tensile Stress
Definition
Maximum stress an object can withstand before it fractures
Strain
Formula
Strain = x/L
Youngs Modulus
Stress / Strain
As long as the elastic limit is not exceeded
Brittle Materials
High value of Youngs Modulus
No plastics region
e.g. glass
Ductile Materials
Small value of Youngs Modulus
Large plastic region
e.g. copper
Polymeric Materials
Do not obey Hookes Law
Elastic
Unit of Mass
kilograms
kg
Elastic Potential Energy
1/2 x Force x Extension
