# Physics 2

• Created by: 10jgorman
• Created on: 14-02-15 13:19

## Distance-Time Graphs

y axis - distance
x axis - time

If stationary, the line is horizontal

Constant speed - straightsloping line

speed= distance / time

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## Velocity and Acceleration

Velocity = speed in a given direction

Change in direction = change in velocity even if speed stays the same

If velocity changes, we say it accelerates.

a = (v-u) / t

a- acceleration (m/s^2)
v- final velocity
u- initial velocity
t- time

If the answer calculated is negative, it is decelerating.

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## Velocity-Time Graphs

y axis- velocity
x axis- time

If gradient of the line is negative, the object is decelerating.

Area under the graph represents distance travelled in a given time.

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## Forces Between Objects

Measured in N(ewtons)

Objects always exert equal and opposite forces on each other. (can be called action/reaction forces)

Both forces have both size and direction.

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## Resultant Force

Resultant force- single force that would have the same effect on the object as all the original forces acting together

If the resultant force is 0:

- if the object is at rest, and will stay at rest

- if the object is moving, it will continue moving at the same speed in the same direction

If the resultant force is not 0:

- If the object is at rest, it will accelerate in the direction of the resultant force

- If the object is moving in the same direction as the force, it will accelerate in the same direction

- If the object is moving in the opposite direction to the force, it will decelerate.

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## Force and Acceleration

Resultant force always causes acceleration

f = m x a

Force (N)
Mass (kg)
Acceleration (m/s^2)

The greater the resultant force, the greater its acceleration.

The greater the mass of an object, greater the force needed to give it a particular acceleration

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Vehicle at steady speed, the resultant force is zero. Driving forces are equal and opposite to frictional forces.

Stopping Distance- distance travelled during drivers reaction time + distance travelled under breaking force

Thinking Distance- Distance travelled in between the driver realising he needs to brake, and actually braking.

- can be increased by:

- tiredness

- alcohol/drugs

Braking Distance- Distance taken to stop after applying the brakes.

- can be increased by:

- cars condition eg worn tyers or breaks.

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## Falling Objects

Weight - force of gravity on an object (N)

An object acted on by only gravity accelerates at 9.8/10m/s^2

f = m x a becomes w = m x g

w - weight (N)
m - mass (kg)
g - acceleration due to gravity m/s^2

If an object is on Earth but not falling, g = gravitational field strength (N/kg)

When an object falls in a fluid, the fluid exerts a drag force, resisting its motion.
The faster an object force, the greater the drag force becomes, eventually becoming equal and opposite to the weight. Resultant force is now 0 and it stops accelerating, this is terminal velocity.

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## Stretching and Squashing

Extention- Increase in length from its original size.

Objects that stretch and return to their original shape are called elastic.

Hooke's Law

Extention of a spring is directly proportional to the force applied. (graph is a straight diagonal line)
If there is too much force, the limit of proportionality is exceeded and the object won't return to its original shape (graph begins to curve).

F = k x e

Force (N)
K - Spring constant (N/m)
e - extention (m)

The stiffer the spring, the greater its constant.

When an elastic object is stretched, elastic potential energy is stored in the object.
When the stretching force is removed, this energy is released.

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## Force and Speed Issues

Fuel Economy - reducing speed reduces fuel used to travel a certain distance.
- making an object more streamlines also improves fuel economy

Speed Cameras - discourage speeding

Skidding - happens when brakes are applied to harshly. The wheels lock and slide along the surface of the road, increasing stopping distance.

Anti-skid surface - reduce/prevent skidding, they're rougher than normal road surfaces, increasing friction between tyers and the road.

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