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Forces (Contact and non-contact forces (Vectors- magnitude and direction;…
Forces
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Forces and elasticity
When force applied, object may stretch, compress or bend- more than 1 force must act on object.
Elastically deformed- can stretch back into original shape.
Inelastically deformed- unable to return to original shape and length after force removed.
Work done when force stretches or compresses an object and causes energy to be transferred to elastic potential energy store of object. If elastically deformed all energy is transferred to objects elastic potential energy store.
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Limit of proportionality- when extension is no longer proportional to force (curves off on graph). As long as spring isn't stretched past limit of proportionality, work done is found in:
Weight, mass and gravity
Gravitational force- force of attraction between masses. Only noticed when mass is really big (planet). Has 2 important effects:
- On surface of planet, makes all things fall towards the ground.
- Gives everything a weight.
Weight- force acting on an object due to gravity close to Earth. Force caused by gravitational field around Earth.
Gravitational field strength varies with location. The stronger, the closer to the mass causing the field and stronger for larger masses.
Weight of object depends on strength of gravitational field at the location of object. Weight of object changes with location.
Weight (newtons) measured using calibrated spring balance or newton meter.
Mass (kg) measured with mass balance.
Earth- 9.8 N/Kg--------Moon- 1.6 N/Kg Increasing mass of object increases weight. Weight is directly proportional to mass.
Stopping distances
Stopping distance-distance it takes to stop a car in an emergency.
Thinking distance- the time between the driver seeing the hazard and braking.
Braking distance- distance taken to stop under the braking force (once brakes applied).
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Thinking distance affected by: speed, reaction time.
Braking distance: speed, weather/ road surface, condition of tyres, quality of brakes.
When brake pedal pushed, causes brake pads to be pressed onto wheel- friction; work done. Work done between brakes and wheels transfers energy from kinetic energy stores of wheels to thermal energy stores of brakes- increases temperature.
The faster a vehicle is moving- more energy to kinetic energy stores so more work needed to be done to stop it; greater braking force needed to make it stop within certain distances.
Larger braking force= larger deceleration. Can be dangerous; cause brakes to overheat or vehicle to skid.
Distance, displacement, speed and velocity
Distance = scalar------Displacement = vector. Measures distance and direction from start to finish.------Speed-how fast you're going.
Velocity is speed in given direction. So object can change direction but have same speed- changing velocity.
Average speeds of everyday objects:
Walking person- 1.5 m/s------Running person- 3 m/s------Cycling person- 6 m/s
Car- 25 m/s------Train- 30 m/s------Plane- 250 m/s
Things that affects speed- fitness, age, distance travelled and terrain (surface of ground).
Speed of sound (330 m/s) changes depending on what sound waves are travelling through and the speed of wind.
Things that affect speed of wind- temperature, atmospheric pressure, if there are large objects nearby.
Acceleration- change in velocity in a certain amount of time.
Constant acceleration=uniform acceleration.
Acceleraion due to gravity is uniform for objects in free fall- 9.8 m/s2 near Earth's surface and same value as gravitational field strength.
Terminal velocity
If object has no force propelling it, it will slow down and stop due to friction.
Friction acts in opposite direction to movement. To drive at steady speed, driving force must balance frictional forces.
Get friction between 2 surfaces in contact or when object passes through a fluid (drag). Reduce this by using lubricant.
Drag- resistance in fluid e.g. air resistance. Reduce this by having streamlined object.
Parachutes work in opposite way- need as much drag as possible. Frictional forces from fluids increase with speed. So car has much more friction to work against at higher speed.
When falling object sets off, force of gravity is higher than frictional force slowing it down so accelerates.Speed increases; friction builds up- reduces acceleration until frictional force= accelerating force.This means it has reached it's terminal velocity. Fall at steady speed.
The less streamlined object is, the lower the terminal velocity. Large surface areas have lower terminal velocity.
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Newtons 1st and 2nd law
1st law- if resultant force on stationary object is 0, object will remain stationary. If resultant force on moving object is 0; carry on moving at same velocity.
A non-zero resultant force will always produce acceleration in direction of force. Can take 5 forms; starting, stopping, speeding up, slowing down and changing direction. On free body diagram, arrows will be unequal.
2nd law- Force is directly proportional to acceleration.------Acceleration is indirectly proportional to mass.
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Calculating forces
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If all forces combined to give resultant force of 0, object is in equilibrium. If tip and tail of both forces meet, resultant force is 0. For 3 forces, scale diagram will form triangle.
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Momentum
Momentum- vector quantity. In a closed system, total momentum before event= after event- conservation of momentum. Collision-event where momentum of each ball changes but overall momentum stays the same. If momentum before event is 0, momentum after is 0.