# Physics AS unit G481 (Mechanics)

Why aren't I playing video games.

- Created by: Alex Hegarty
- Created on: 04-01-13 12:57

## Prefixes

These when placed in front of a unit denote its denomination.

pico (p) = 10^-12

nano (n) = 10^-9

micro (This is represented by the symbol mu in the greek alphabet) = 10^-6

milli (m) = 10^-3

centi (c) = 10^-2

kilo (k) = 10^3

mega (M) = 10^6

giga (G) = 10^9

tera (T) 10^12

## Scalar and Vector quantities

A **Scalar** quantity consists of only a magnitude. i.e Volume

A **Vector** quantity consists of both a magnitude and a direction i.e Velocity

## Vector triangle

Vector triangles can be used in order to find the resultant force of two other forces. This is because since vectors have both a magnitude and a direction we can add them and calculate there distance from the starting point this is the resultant force.

## Kinematics definitions

Displacement - The distance travelled by an object.

Instantaneous speed - The speed at a given moment

Average speed - The distance divided by the time

Velocity - The speed and direction of a given object

Acceleration - The rate at which an objects speed changes

## Speed and velocity

Average speed = Distance / Time

Acceleration = Change in velocity / Time

## SUVAT Equations

S = Displacement

U = Starting Velocity

V = Ending Velocity

A = Acceleration

T = Time

v = u +at

s = 0.5(u+v)t

s = ut +0.5at^2

v^2 = u^2 +2as

## Force

Net force = Mass x Acceleration

For particles travelling at very high speeds this is not true though because since they cannot surpass the speed of light their mass increases.

## Newton

**Newton** - A newton is the force required to give a mass of 1 kg acceleration of 1 metre per second.

## Drag

When a man wears Womens clothing this is known as drag.

Drag is also the frictional force experienced by any object moving through a liquid.

**Factorst which affect drag..**

- Shape and size of object

- The velocity of the object

- The mass, viscosity and compressibility of the fluid it is travelling through.

## Weight

Weight and mass are different....

The weight of an object unlike mass takes into account gravity.

Weight = Mass x Gravity

So weight will change depending on the planet you're on whereas mass will not.

## Terminal velocity

**Terminal Velocity** - This is where an object can no longer accelerate because the drag forces placed on it are so great, because it is moving so fast.

## Centre of Gravity

The centre of gravity is the point on an object where all forces placed on the object seem to act.

## Couples

**A couple** - A couple is a pair of forces that are equal and opposite, but are not in the same line.

**Torque of a couple** - This is the measurement of the turning effect produced by a couple.

## Moments

**Moment of a force** - The moment of a force is the turning force produced around a pivot.

Moment = Force x Distance

## Equilibrium

**Equilibrium** - This is where the total force acting on an object is 0. Be warned this is not exclusive to stationary objects, objects experiencing terminal velocity is also experiencing equilibrium.

## Density and pressure

Density = Mass / Volume

Pressure = Force / Area

## Stopping distance

**Stopping distance** = Thinking distance + Braking distance

**Thinking distance**, this is the distance inbetween seeing the need to break and pressing on the brakes.

It is affected by..

- Level of intoxication

- Sight

**Braking distance,** this is the distance travelled while your car is breaking.

It is affected by..

- Quality of brakes

- Quality of road

- Weather

## Impact Force

**Crumple zones** - These act by increasing the distance over which the force has to act, this decreases the average force.

**Seat belts -** These stop you from flying out of the car via the windscreen, they also increase the distance over which the force acts.

**Airbags -** An airbag is designed to slow you down and decrease the force.

## More on air bags!!!!

**How do they work?**

An acceleromator in the dashboard measures whether the acceleration is 10g this should only occur during an accident. It is a weight on a spring, which during a crash should continue compressing the spring until it presses a button which should set off a chemical reaction.

**Why do they inflate?**

Well I'm glad you asked because coincidentally that leads directly on from my last point. When this chemical reaction (Usually inbetween sodium nitrate and potassium nitrate) it creates nitrogen gas very rapidly, and this causes the bag to inflate.

**Where do they come from?**

Uhhhhh a factory I guess.... Oh you mean which part of the car, they are a nylon bag which is stuffed inside the dashboard or steering wheel.

## GPS and trilateration

**Trilateration** - This is the system by which we use three satellites to locate a moving object on the earth.

**Process...**

1. A signal is sent from the satellite to the car, using the response from the car the distance to the car. This produces a sphere around the satellite where the distance to the car is the radius of the circle.

2. We repeat this for two other satellites, which also produce possible spheres where the car could be.

3. Where these three spheres overlap is the position of the car.

## Work done

work done, implies that the force is having an active effect on the body.

Work done = Force x distance

## Joule

The energy needed to supply an energy of one watt for one second.

## Conservation of energy

The total amount of an energy in an isolated system remains constant over time.

(Basically energy cannot be created or destroyed)

## Kinetic energy and Gravitational potential energy

**Kinetic energy** = 0.5 x Mass x Velocity^2

**Gravitational potential energy** = Mass x Gravity x Height

## POWER!!!

Power = Rate of work done

## Watt's going on here?

Watt - The unit of energy defined as one Joule per second.

## Efficiency

Efficiency - This is the percentage of energy output process which we define as useful, so for example a fires useful output would be heat.

Efficiency is never truly 100% because of the heat produced.

We can summarise it in this equation....

Efficiency (%) = ( Useful output energy / Total input energy ) x 100

## Tensile and Compresive forces

Tensile - A stretching force. <- W ->

Compressive - A compressing force -> W <-

## Hookes Law

Hookes law - The extension of an elastic body is proportional to the force which caused it.

The equation for this is expressed....

Force = Force constant x Extension

F = kx

**What the heck is a force constant?**

The force constant is what tells how much force is required per unit of extension. So 6N/mm says that 6 Newtons are required per mm of extension.

## Finding k (Force constant)

In order to find K we need a graph showing the springs extension compared to the force applied to it.

So we know the area under this graph shows work done.

Work done = 0.5 (Its a triangle) Force x Distance (Extension)

We can then sub in F = kx

Work done = 0.5 x (Force constant x Extension) x Extension

Work done = 0.5 kx^2

## Elastic potential energy

Elastic potential energy = 0.5 x Force x Extension

Elastic potential energy = 0.5 x Force constant x Extension^2

## Spring terms

Stress = Force / Cross sectional area

Strain = extension / length

Youngs modulus = Stress / Strain

Youngs modulus = ( Force x Length ) / ( Extension x area )

Ultimate tensile strength, this is the maximum tensile force which can be applied to an object before it breaks.

## Deformation

**Elastic deformation**, this is the deformation experienced by an object, after which it will still return to its original shape.

**Plastic deformation**, when an object is deformed this way it will no longer return to its original shape.

## Ductile, brittle and polymeric materials

**Ductile** - These objects are easily plastically deformed, for example copper which can be drawn out into wires. This means that it has a very large **plastic region**. It will be steep at first, then curve out until it eventually breaks.

**Brittle** - These materials deform very little but will break easily if subjected to sufficient stress. They have little to no plastic region. The area beneath the graph will be little because no elastic potential energy has been stored in it. It should be a very steep line that stops abruptly.

**Polymeric** - The molecules in these materials are arranged in very long squashed chains, this means that these materials are very easy to stretch for a short time and then become very hard to stretch, the graph of these materials should be an S-shaped curve.

## Stiff and strong

**Stiff** - These materials have a very high youns modulus but exhibit little extension, most likely to be brittle.

**Strong** - These objects can take a lot of force before they break. For example steel.

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