# Forces

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• Created by: S_a_c
• Created on: 02-06-13 11:24

## Movement and Position

Speed = distance / time

Distance-time graphs : gradient shows speed

Velocity = speed in a particular direction (vector quantity)

Displacement = distance moved in a particular direction (also a vector)

Velocity-time graphs : gradient shows acceleration ....... area underneath shows distance travelled

Acceleration = change in velocity / time

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## Forces

A force causes an object to speed up, slow down or change direction

Vector quantity - has both magnitude and direction

Scalar quantity - has only magnitude

A force is a vector

Force = mass x acceleration

Unbalanced forces - two forces acting oppositely but one  is  greater than the other

Object will move in direction of greater force

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## Friction

Friction is the force opposing motion

It can affect the stopping distance of a car

(Stopping = thinking + braking)

• Thinking can be affected by alcohol, drugs, tiredness etc
• Braking can be affected by road surface, weather, mass of car, speed

Drag force acts on falling objects :

As  an object falls through the air, its speed increases. As this is increasing, the drag force acting on it also increases. This happens until the two opposite forces are balanced (both the same size). The object remains at a constant velocity but is still falling

= terminal velocity (this can be shown using sycamore seeds or parachutes)

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## Momentum

Momentum is:

• measure of how difficult it is to stop something that is moving
• mass x velocity
• conserved - momentum before = momentum after

Collisions:

• elastic = no energy is lost
• partially elastic = some of the movement energy is lost (converted to heat or sound)
• inelastic = objects stick together

Force = change in momentum / time taken   (so think about crumple zones / air bags)

Newtons laws of motion:

• unless a resultant force acts on an object it will continue to remain at rest/constant speed
• the rate of change of momentum is proportional to the force acting on it
• for every action there is an equal and opposite reaction)
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## Moments

Moment = turning effect of a force

= force x perpendicular distance from pivot

An object is in balance if : sum of anticlockwise = sum of clockwise movement

The weight of a body acts through its centre of gravity

The upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on a beam

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## Springs

Hookes Law :

the extension of a spring is proportional to the weight applied

BUT springs/wires reach their elastic limit - this is the weight after which they will not return to their original length/shape

Materials can be :

• Elastic - object returns to original shape once force applied is removed
• Plastic - permanently changed
• Brittle - breaks when any force is applied
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## Astronomy

Gravitational field strength = force acting per kilogram mass (10 on earth)

It depends on :

• the masses of the objects
• the distance between them

orbital speed = 2 x pi x orbital radius / time

the universe > galaxy > solar system

• universe = collection of galaxys
• galaxy =  collection of stars
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