# FORCES IN MOTION

PHYSICS

• Created by: katie
• Created on: 07-05-12 13:08

## velocity and acceleration

• speed and velocity are both how fast you're going
• both measured in m/s (or km/h or mph)
• speed = how fast your going, no regard to the direction
• velocity = must also have a direction specified
• SPEED = DISTANCE/TIME
• acceleration = how quickly your speeding up
• not the same as velocity or speed
• how quickly the velocity is changing
• ACCELERATION = CHANGE IN VELOCITY/TIME TAKEN

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## D-T and V-T graphs

distance - time graphs

• flat sections are where its stopped
• the steeper the graph, the faster its going
• downhill section mean its going back toward its starting point
• curves represent acceleration or deceleration
• a steepening curve means its speeding up (increasing gradient)
• a levelling off curve means its slowing down (decreasing gradient)

velocity - time graphs

• flat sections represent steady speed
• the steeper the graph, the greater the acceleration or deceleration
• unhill sections are acceleration (/)
• downhill sections are deceleration (\)
• a curve means changing acceleration
• ﻿
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## mass, weight and gravity

gravity is the force of all attraction between all masses:

- gravity attracts all masses, you only notice if the mass is really big (like a planet). On the surface of a planet, it makes all things accelerate towards the ground, it gives everything a weight, it keeps planets, moons and satellites in their obits.

weight and mas are not the same:

- mass is the 'amount of stuff' in an object, weight is caused by the pull of gravity and is measured in newtons. Mass is not a force, its measured in kilograms with a mass balance

WEIGHT = MASS X GRAVITATIONAL FIELD STRENGTH

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## the three laws of motion

first law - balanced forces mean no change in velocity

- so long as the forces on an object are all balanced, then it'll just stay still, or else if its already moving it'll just carry on at the same velocity - so long as the forces are all balanced.

second law - a resultant force means acceleration

- if there is an unbalanced force, then the object will accelerate in that direction.

third law - reactant forces

-﻿ if object A exerts a force on object B then object B exerts the exact opposite force on object A.

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## drag and terminal velocity

1) friction is always there to slow thing down: if an object has no force, propelling it along it will always slow down and stop because of friction, friction always acts in the oppsoite direction to movement, to travel at a steady speed, the driving forces needs to balance the frictional forces, you get friction between two surfaces in contact or when a object passes through a fluid (DRAG).

resistance or 'drag' from fluids (air or liquid) = in reducing drag in fluids is keeping the shape of the object streamlined.

2) drag increases as the speed increases: resistance from fluids always increase with speed.

3) cars and free-fallers both reach a terminal velocity: when cars and free-falling objects first set off they have much more force accelerations them than resistance slowing them down. As the speed increases the resistance builds up. This reduces the acceleration until the resistance force is equal to the accelerating force. Reached is max speed or terminal velocity (the terminal velocity of falling obejects depends of their shape and area)

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## stopping distances

MANY FACTORS AFFECT YOUR TOTAL STOPPING DISTANCE:

1) thinking distance - a) how fast your going (the faster your going the further you'll go) and b) how dopey you are (affected by tiredness, drugs, alcohol, old age and careless attitude)

1) braking distance - a) how fast your going (the faster your going, the further it takes to stop) b)how heavily loaded the vehicle is (with the same brakes, a heavily laden vehicle takes longer to stop) c) how good your brakes are (they must be checked regulary and maintained) and d) how good the grip is (this depends on ROAD SURFACE, WEATHER CONDTIONS AND TYRES.

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## work done

WORK DONE is just 'energy transferred'

- when a force moves an object, energy is transferred and work is done.

(for example - whenever something moves, something else if providing some sort of 'effort' to move it and the thing putting in the effort needs a supply of energy such as fuel, food, electricity etc. It does work by moving the object and it transfers the energy it receives into other forms)

it doesnt matter if the energy transferred is useful or waste, it is still 'work done'

when you do work against friction, most of the energy gets transformed into heat and usually some into sound. (generally wasted energy)

WORK DONE = FORCE X DISTANCE

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## kinetic and potentional energy

kinetic energy is energy of MOVEMENT: anything which is moving has kinetic energy, and it depends on both its mass and speed. The greater the mass and the faster its going the bigger the kinetic energy will be.

KINETIC ENERGY = 1/2 X MASS X VELOCITY²

energy can be stored as potention energy:

elastic potential energy - things like springs and elastic bands are elastic - if you stretch them then let go, they go back to their original shape. elastic potential energy is the energy STORED when work is done on an elastic object to change its shape.

gravitational potential energy - is the energy stored in an object when you raise it to a height against gravity

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## momentum and collisions

MOMENTUM = MASS X VELOCITY   The greater the mass of an object and the greater the velocity, the more momentum the object has. It has a 'vector quantity' - it has size and direction.

MOMENTUM BEFORE = MOMENTUM AFTER   Momentum is conserved when no external forces act. The total momentum after is the same as it was before.

FORCES CAUSE THE CHANGES IN MOMENTUM

1) when a force acts on a object, it causes a change in momentum

FORCE ACTING (N) = CHANGE IN MOMENTUM (kg m/s)  / TIME TAKEN FOR CHANGE TO HAPPEN (s)

2) a larger force means a faster change of momentum (and so a greater acceleration)

3) if someones momentum changes very quickly (like in a car crash), the forces on the body will be very large, and more likely to cause injury -> CRUMPLE ZONES, SEAT BELTS, AIR BAGS.

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