# Physics 2.1&2

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• Created by: Becish
• Created on: 13-01-14 15:24

## Distance and Velocity Time graphs

Distance - Time graphs:

- Shows distance(m) travelled in how much time(s)

- Horizonal line if stationary

Velocity - Time graphs:

- Shows velocity(m/s) of travel in how much time(s)

- Horizonal line if not accelerating, could still be moving

- Area under the graph = distance

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## Velocity and Acceleration

Velocity = speed in a specified direction

Velocity changes if the direction changes even if the speed does not change

Acceleration = change in velocity         Negative acceleration value = Decceleration

The acceleration equation is:

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## Force and Resultant Force

Objects exert equal and opposite force(N) on each other

Forces have: size & direction

- Weight is a force that can only act downwards                  Some forces cancel each other out

Resultant force = the single force that has the same effect as all original forces acting together, the overall force on the object

If resultant force = zero, the object will:

- remain at rest         OR        - continue moving in the same speed and direction

If resultant force = not zero, the object will:

- begin to accelerate in the direction of the force       OR       - continue in resultant force direction

OR       - decelerate (if it was orginally moving in a different direction to the resultant force)

FOR AN OBJECT TO MOVE THERE MUST BE A RESULTANT FORCE ACTING UPON IT

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## Resultant Force effect on Acceleration&Direction

Resultant force causes acceleration (can be negative)

No acceleration = no resultant force present

A resultant force is needed for an object to change direction

Resultant force decrease = acceleration decrease    and so...

A larger resultant force = larger acceleration

The higher the mass, the bigger the force required to reach a given acceleration

Resultant force equation:

F = M x A

(Force = resultant force(N), Mass = (Kg), Acceleration = (m/s^2))

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## Forces in cars and stopping distances

If travelling at a constant speed resultant force = zero as:

driving forces = frictional forces

- Friction and air resistance oppose the driving force of a moving vehicle

The faster the speed the more decceleration needed to stop in a particular distance

Braking in a vehicle:

Thinking distance = Time from spotting an obsticle to applying the brakes

Braking distance = Time taken for brakes to stop vehicles once engaged

Stopping distance = Overall time taken for car to stop (thinking + braking distance)

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## Factors effecting thinking and braking distance

Thinking distance effected by:

- Car speed (more/less distance covered in the time it takes to think)

- Drugs

- Tiredness

- Age (these all effect reaction times)

Braking distance effected by:

- Car speed  (more/less distance covered in the time taken for brakes to slow vehicle)

- Weather (less contact with road so reduced friction)

- Worn tyres / brakes

- Condition of road (e.g. potholes / bumpy = less friction)

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## Falling and Terminal Velocity

Mass = quantity of matter               Weight = force of gravity acting on the mass

Resultant force of a falling object is gravity, on earth gravity causes an acceleration of 10m/s^2

F = M x A becomes W = M x G

(F = resultant force in N, M = mass in kg, A = acceleration in m/s^2)

(w = weight in N, M = mass in kg, G = gravity's acceleration in m/s^2)

Terminal Velocity and Equal forces in falling through gas or liquid

Terminal velocity = constant speed of a falling object where resistant stops further acceleration

- When falling, drag or air / fluid resistance slow the body, the faster the fall, the bigger this force

- Eventually the weight (downwards force) will equal the upwards force so the resultant force is zero and the object is no longer acelerating, it has reached terminal velocity.

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## Springs, Extension, Elasticity and Hooke's Law

Elastic = object / material which regains original shape after forces on it are removed

Inelastic = object / material which will not regain its original shape once forces are removed

Extension = increase in length from the spring's original length

- A graph showing spring extension is straight and passes the origin

- The extension is directly proportional to the force applied

- If force is too big line will curve as the limit of proportionality has been exceeded

- Hooke's Law states extension is directly proportional to force applied. The equation for this is:

F = K x E

(F = force in N, K = spring constant in N/m (newtons per meter), E = extension in M)

- Stiffer spring = greater spring constant             - When force removed, stored energy released

- Streching an elastic object means work is done, this is stored as elastic potential energy

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## Force and Speed Issues

Reducing speed reduces fuel consumption, this is fuel economy.

Reducing air resistance (force) by streamlining improves fuel economy.

Speed cameras discourage speeding, deterred by           - Fines         &          - Losing licence

They can be used to determine speed at a particular point.

They can be used in pairs to calcualte an average speed between the two points.

Skidding = brakes applied fast, wheels lock and slide along surface (increasing stopping distance)

Anti-skid surfaces reduce skidding as they are rougher than normal roads so increase friction.

Used at traffic lights, lights and junctions where braking is likely.

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