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PHYSICS UNIT 3 - FORCES (Force & Extension (When the line of best fit…
PHYSICS UNIT 3 - FORCES 
Force
Forces
contact - when 2 objects have to be touching for force to act e.g.. friction, air resistance, tension in ropes, normal contact force .
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Contact Friction - friction between our shoes and the floor stop us from slipping - car has friction & thrust
non-contact - when objects don't need to touch for force to act e.g.. magnetic force, gravitational force, electrostatic force.
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Vector notation
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Vector quantities = force, velocity, displacement, acceleration, momentum etc.
Vectors are usually represented by an arrow - length of arrow shows magnitude & the direction of arrow shows the direction of the quantity.
Scalar quantities
Scalar quantities have magnitude & no direction = speed, distance, mass, temperature, time etc.
Force & Extension
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If a spring is supported at the top & weight attached to bottom, it stretches
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Spring constant measured in newtons per metre & extension in cm or mm so convert extension into metres
When the line of best fit is straight before it exceeds the limit of proportionality there is a linear relationship between force & extension
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Gravitational force
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Weight = force acting on an object due to gravity. Will change with its location e.g.. Earth or Moon.
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Mass = amount of stuff in an object & will have same value anywhere in universe. Not a force, measured in kilograms using balancing scales.
Gravity attracts all masses e.g.. on planet surface makes all things fall towards ground, which gives everything a weight.
Forces & Elasticity
When apply force to object it may stretch, compress or bend
Need more than one force acting on object or the object would move in direction of applied force instead of changing shape
Work is done when a force stretches or compresses an object & causes energy to be transferred to elastic potential energy store of object.
Elastically deformed
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= when all the energy is transferred to object's elastic potential energy store & can go back to original shape & length when force removed e.g.. spring
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Practical - Investigation of the force-extension graph for a spring
Measure length of spring when no load applied with a mm ruler attached to stand. Add a thin strip of tape to bottom of spring to make reading more accurate
Add a mass to the spring & allow spring to come to rest. Record mass & measure new length of spring. Extension = change in length.
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