Physics - P2.2 - Forces
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- Created by: Katatatitee
- Created on: 26-04-14 08:50
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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