# Physics - P2.2 - Forces

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## P2.2.1 - Forces Between Objects

• Forces measured in newtons (N)
• Objects always exert equal and opposite forces on each other - action and reaction forces
• Car hits a barrier - exerts force on barrier - barrier exerts force on car equal in size and opposite in direction
• Car is being driven - force from tyre on ground pushing backwards - equal and opposite force from ground on tyre pushing car forwards
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## P2.2.2 - Resultant Force

• Many objects have more than one force acting on them
• Resultant force: single force with same effect on object as all original forces acting together
• Resultant force = 0: object at rest - stay at rest. object moving - continue to move at same speed and in same direction
• Force ≠ 0: acceleration in direction of force - object at rest - move in direction of force, object moving same direction as resultant force - accelerate in same direction, object moving different direction of force - decelerate
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## P2.2.3 - Force and Acceleration

• Resultant force always causes acceleration - no acceleration: resultant force = 0
• Acceleration: change in velocity - object accelerates if it changes direction but goes at the same speed - resultant force is needed to change objects direction
• F = m x a
• F - resultant force - N
• m - mass - kg
• a - acceleration - m/s²
• Bigger resultant force = bigger acceleration
• Larger mass of object = bigger force needed for it accelerate
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## P2.2.4 - On the Road

• Vehicle travelling at steady speed: resultant force = 0 - driving forces equal and opposite to frictional forces
• Faster speed of vehicle = bigger deceleration needed to stop it in particular distance - bigger braking force needed
• Stopping distance - distance travelled during reaction time (thinking distance) and under braking force (braking distance)
• Thinking distance - increased if driver is tired or under influence of drugs ir alcohol
• Braking distance - increased if road is poorly maintained, bad weather conditions or car condition (eg. worn tyres or brakes)
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## P2.2.5 - Falling Objects

• Freely falling object - resultant force is gravity - make objects close to Earth's surface and accelerate at 10m/s²
• Force of gravity: weight, acceleration: acceleration due to gravity
• F - resultant force (N) = m - mass (kg) X a - acceleration (m/s²)
• W - weight (N) = m - mass (kg) X g - acceleration due to gravity (m/s²)
• If object is on Earth (not falling) - g: gravitational field strength (N/kg)
• Object falls trough liquid - fluid exerts drag force on object, resisting it's motion - falls faster = bigger drag force - eventually equal to objects weight: resultant force = 0: stops accelerating - moves at constant velocity: terminal velocity
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## P2.2.6 - Stretching and Squashing

• Hang small weights from spring: stretches - increased legth from original = extension - remove weights -> spring returns to original length
• Things that behave this way: elastic - regains original shape when deforming forces are removed
• Plot a graph of extension against force applied: straight line through origin - extension ∝ force applied - apply a too big force: begins to curve - exceeded limit of proportionality
• Hooke's Law - elastic objects obey Hooke's law: extension ∝ force applied
• F - force applied (N) = k - spring constant (N/m) x e - extension (m)
• Stiffer spring = bigger spring constant - elastic object stretched: work done - stored as elastic potential energy in object - stretching force removed: energy released
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## P2.2.7 - Force and Speed Issues

• Reducing speed reduces amount of fuel used to travel a particular distance and reducing air resistance of vehicle (making it more streamlined) reduces fuel: fuel economy
• Speed cameras used to discourage motorists from speeding - determine speed of motorists at particular point - used in pairs to calculate average - motorists travelling above speed limit face fines and other consequences
• Skidding occurs when brakes are applied too harshly - wheels lock and slide along road surface, increasing stopping distance
• Anti-skid road surfaces - reduce or prevent skidding - rougher then normal: increased friction between tyres and road - used where drivers likely to brake, eg. near traffic lights or road junctions
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