# Physics 2.1

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• Created by: Laura
• Created on: 02-01-13 17:49

## What is the resultant force?

The resultant force is a single force which has the same effect on the object as all the forces together. For example if the air resistance acting on a moving object is 2N, the friction acting on it is 1N and the driving force acting on it is 5N, the resultant force would be 2N forwards. This is because the air resistance and friction would be slowing the car down, so that is 3N drag, this is opposite to the driving force which is 5N, so 3N of the driving force balances out the 3N of drag, leaving just 2N of driving force which is the resultant force.

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## What do you do to find the resultant force?

To find the resultant force you need to find the difference between all the forces and in what direction, this will be the resultant force. On a diagram you can do this by adding up all of the driving forces and resistant forces, then finding the difference between them.

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## What effect does the resultant force have?

If and object is stationary and the resultant force is 0N than the object will stay still, if the resultant force is anything above 0N the object will start to move in the direction of the force.

If an object Is moving and the resultant force is 0N it will stay at a constant speed, if it is above 0N then the object will accelerate or decelerate depending on the direction of the resultant force.

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## How do you use the equation F= m x a?

F = Force (N/Newtons)                                 Equation Pyramid---->          F
m = Mass (kg/kilograms)                                                                         m  |  a
a = Acceleration (m/s² /metres per second squared)

To calculate the force you use... mass x acceleration.        F = m x a
To calculate the mass you use... force / acceleration.       m = F / a
To calculate the acceleration you use... force / mass.        a = F / m

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## How do you see velocity on a distance-time graph?

The changes in velocity are the changes of the line. Theses are shown below, as well as any lines which have a positive correlation are moving away from the start and any with a negative correlation and moving towards the start, also horizontal lines the velocity is nothing as the object is stationary.

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## Calculate velocity using a distance-time graph?

Velocity = distance / time in a certain direction

To calculate velocity from a distance-time graph you use the gradient, as this is distance / time, then you have to state which direction the object is moving in, as velocity is in a certain direction. If the line has a positive correlation then the velocity will be the gradient in m/s away from the start and if it has a negative correlation its direction will be towards the start.

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## What is the difference between speed and velocity?

Speed is the distance travelled over time (m/s) and velocity is the same except it is in a certain direction.

For example if someone walked forwards and north at 2m/s and then walked backwards at 2m/s the speed would stay the same as the person is moving at 2m/s in both directions, however the velocity would change because the direction changes, so as the person is walking forwards the velocity would be 2m/s north, but when they walk backwards the velocity is 2m/s south.

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## What is the equation for acceleration?

Acceleration = Final Velocity - Initial Velocity          OR        a = v - u
Time Taken                                               t

Velocity is measured in metres per second, m/s.

Time is measured in seconds, s.

This means that acceleration is measured in metres per second per second, or metres per second squared, m/s/s or m/s².

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## Where is acceleration on a velocity-time graph?

Acceleration can be identified on a velocity-time graph as the gradient of the line is the acceleration. A constant gradient line is a constant acceleration, a horizontal line is no acceleration the the object is moving at a steady speed, a positive gradient is acceleration, a negative gradient is decceleration and a curve is increasing or decreasing acceleration.

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## Calculating acceleration using a velocity-time gra

To find the acceleration you use the final velocity, initial velocity and the time taken. On a graph the final velocity is the velocity at the end of the line and the initial velocity is the velocity at the start of the line, the time taken is the length of the line along the X or time axis.

You then sub these pieces of data into the equation... a = v - u
Where... v is the final velocity,                                                t
u is the initial velocity,
t is the time taken.
And then calculate the equation to get the acceleration (a).

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## State what parts make up total stopping distance.

The thinking distance (the tidistance travelled during the driver's reaction time) and the braking distance ( the distance travelled under the braking force).

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## How does a cars speed effect its braking distance?

The higher speed the greater resultant force, so a greater braking force is needed to stop the car, so the longer braking distance.

The slower speed the lower resultant force, so less braking force is needed to stop the car, so there is a shorter braking distance.

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## What factors could effect a cars braking distance?

The speed the car is travelling at.

The condition of the car, either good (shorter braking distance) or poor (longer braking distance).

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## What factors could effect a drivers reaction time?

They are tired.

They are under the influence of drugs or alcohol.

They are distracted, e.g. by a mobile phone.

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## What energy changes occur when a car brakes?

When the friction forces between the brakes and wheels, and the wheels and the road surface, they reduce the kinetic energy of the car. This kinetic energy is transformed into heat energy in the brakes, causing an increase in brake temperature and if overheating occurs it can result in a brake failure. If during the brake the cars wheels lock the car skids.

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## How does speed effect frictional force?

If an object is moving faster through a fluid or gas the frictional forces increase as the object is passing through more of the fluid molecules in less time, causing more friction.

If an object is moving slower through a fluid or gas the frictional forces decrease as the object is passing through less of the fluids molecules in more time, so they can move around the object, so there us less friction.

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## What two forces act on a falling object?

Weight - which is measured in newtons, is a downward force and remains the same (in air
the weight of an object is the force exerted on its mass by gravity)

An Upward Frictional Force - which is either air resistance in air or drag in a fluid, and the
faster an object is moving through air or a fluid the greater the
frictional force.

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## Why will an object reach terminal velocity?

An object will reach terminal velocity because its resistant forces will become equal to its driving forces, so the resultant force will be 0N and the object will be travelling at a constant speed.

This is becuase as an object accelerates its driving forces are greater than its resistive forces, but as the object moves faster the resistive forces increase and this happens until the resistive forces are equal to the driving forces, and terminal velocity is reached.

This can either happen as an object is falling (for example a sky diver) through air or a fluid as initially it accelerates as the weight of the object is greater than its air resistance or drag, but as its speed increases so does the air resistance or drag until both the weight and air resistance/drag are balanced and the object is moving at terminal velocity.

Or as an object is moving (for example a ball rolling down a hill) as again it will accelerate as its driving forces are greater than its resistive forces, but as its resistive forces increase (due to the objects increase in speed) it will become equal to the driving forces and terminal velocity will be reached.

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## On a velocity-time graph what's terminal velocity?

The graph below shows the acceleration of a parachutist jumping from an aeroplane.On it, and on other velocity-time graphs, terminal velocity is the part of the line which is horizontal, as this is where the velocity is constant, meaning that the resultant force is 0N, so terminal velocity has been reached. On this graph it shows how a parachutist accelerates then reaches terminal velocity during freefall, then deccelerates quickly as thier parachute is released before reaching a new terminal velocity as the weight becomes equal to the new increased air resistance.

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## What is the equation for weight?

Weight = Mass x Gravitational Field Strength             W = m x g                   W
m | g

Weight is measured in newtons, N.

Mass is measured in kilograms, kg.

Gravitational Field Strength is measured in newtons per kilogram, N/kg.

The weight of an object is the force exerted on its mass by gravitational field strength.

The mass of an object is how much matter is in an object.

The gravitational field strength is the amount offorce of gravity acting on a mass per kg.

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## How do forces cause some objects to change shape?

For an object to change shape the forces have to be balanced, because when there is no resultant force there can be no movement, so the object is forced to change shape if the pressure is too much to withstand or if the object is malleable.

For example if a piece of blue tac is placed on a table and the frictional force from the table is 20N and you press your finger into the blue tac with an equal force of 20N then the blue tac can't move because there is no resultant force, but the force is too much for the blue tac to withstand so the blue tac changes shape.

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## Which equation which links extension and force?

Force = Spring Constant x Extention          F = k x e                                             F
k | e
Extention = Force / Spring Constant           e = F
k

Force is measured in newtons, N.

Spring Constant is measured in newtons per metre, N/m.

Extention is measured in metres, m.

The Force is the force applied to create the extension.

The Spring Constant is the stiffness of the spring.

The Extension is the length the spring has stetched by.

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## How does an object gain elastic potential energy?

An object is elastic if it has the abitlity to return to its original shape after the force which caused it to change shape is removed. The force which is applied to the object to cause it to change shape is stored in the object as elastic potential energy, and it is released when the force is released. For example when you stretch a spring the force you use to stretch it is stored as elastic potential energy in the spring, when you release the force applied to the spring you also release the elastic potential energy, so the spring returns to its original shape.

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