# P4 - FORCES

P4 topic - standard

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• Created by: Beatrice
• Created on: 14-05-12 17:47

## Forces

• Forces act in pairs
• forces act against each other
• If forces are equal we get: no movement OR constant speed
• If friction is low then the object moves less by not having much to put a force against
• overall the upwards force is equal to the downward force
• action and reaction are equal and opposite
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## WEIGHT & REACTION

mass - g/kg - 1000g = 1kg

weight is a force that is measured in newtons, it is a measure of effect of gravitational field strength on a mass

weight = mass x gravitational field strength

(N) = (kg) x (N/kg)

Gravity is an attractive force, all objects in the universe have gravity, it is the mass of an object that determines it gravitational pull.

GFS on Earth = 10N/kg

weight/mass = gravity

weight/gravity = mass

mass x gravity = weight

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## SPEED & MOMENTUM etc

average speed = total distance/total time

momentum = velocity x mass

velocity = speed in a particular direction

momentum (kg m/s) = mass(kg) x velocity(m/s)

-----> = positive     <------ = negative

change in momentum = force x time it lasts

momentum / force = time

momentume / time = force

force x time = momentum

two objects change of momentum is equal and opposite to one another

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## CHANGING MOMENTUM

in a collision the persons momentum is reduced to 0kg m/s which creates a large force to be felt, increasing the time, reduces the force felt.

CRUMPLE ZONES: chnage the momentum safely by reducing the force of a collision by having a longer time for the car to fully stop

SEAT BELTS: change the momentum safely by exerting a force on the person wearing the seat belt to increase the time to decrease the momentum

AIR BAGS: change the momentum safely by cushioning thr person in a collision to decrease the force of the crash on the passenger

if we had no friction the object would keep on moving with the same momentum UNTIL something else stops it.

counter force > | <driving force

If the counter force and driving force are equal we wull niove ar a constant speed (or stationery)

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## DOING WORK

doing work = when you push or pull something and make it work, you do work

work (done in joules) = force(N) x distance(m)

the amount of work you do = the amount of energy transferred

doing work against gravity:

• if you lift something up, you must exert enough force to balance the force of gravity
• the upward dorce is equal to thre weight of the object
• you transfer some energy from your body and this increases the graitational energy ofn for the thing you lift

change in gravitational potential energy = weight x vertical height gain

Falling objects convert into kinetic energy

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## KINETIC ENERGY

kinetic energy is a energy of movement, anything thats moving has kinetic energy (K.E)

the fast something is moving the more kinetic energy is has

KE = 1/2 x mass x (velocity²)   velocity - KE x 2/mass

(J) = 1/2 x (kg) x ([m/s²])  Some KE is wasted as heat or light or sound energy

Falling objects convert into kinetic energy

kinetic energy gained = gravitatonal energy lost

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