P2.3.1 Energy and Work
- When an object moves, a force must have been applied to it. When this happens, work has been done, causing energy transfer.
- When an object is moved by a force, work has been done on the object by the force.
- Energy transferred=work done.
- If the work you do on an object to lift it up is 30J, the energy transferred to it must be 30J.
- Work done =force applied distance moved in × direction of force.
- Work done-joules, J. Force newtons, N. Distance-metres, m.
- The word equation in symbols: W=F × d. If d=0, then W=0.
- Work done to overcome firction is mainly energy transfer into heat.
- Rubbing your hands together warms them, and your muscles do work to overcome the friction that is created. The work those muscles do is transferred into energy.
- Brake pads become hot if brakes are applied for too long; friction between the break pads and wheel discs opposes the motion of the wheel, and the kinetic energy is transferred into heat energy. Some energy is transferred to sound; the wheels make a screeching noise.
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P2.3.2 Gravitational Potential Energy
- This is energy stored in an object due to its position in the Earth's gravitational field.
- Each time an object is lifted upwards, its graviational potential energy increases by an amount equal to the work done on it. The same applies when the object moves down.
- Change in gravitational potential energy (J) = weight (N) × change in height (m)
- Weight in Newtons = mass × gravitational field strength (9.8 N/kg on earth).
- Putting the above equations together would make: Ep= m × g × h
- Ep=Change in gravitational potential energy, J
- m=mass in kg.
- g=gravitational field strength in N/kg.
- h=change in height in m.
- Power is the rate of energy transfer, calculated using the equation: P=E/t
- P=power in watts, E=energy in J, t=time in s.
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P2.3.3 Kinetic Energy
- Every moving object has kinteic energy. The more mass and the higher the speed of the object, the more kinetic energy there is.
- It can be calculated using the equation: Ek = 1/2 x m x v2
- Kinetic energy recovery systems (KERS) in vehicles store energy when vehicles brake and use it later. Some vehicles, including hybrid cars, use an electric generator to transfer kinetic energy into electrical energy, which is then stored in a battery.
- Elastic Potential Energy
- When a rubber band is stretched, the work you do is stored as elastic potential energy. Archery is an example of this type of energy being transferred into kinetic energy.
- An object that is elastic regains its shape after being stretched or squashed. A rubber band is an example.
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- All moving objects have momentum. It is important to anyone who plays a contact sport-in rugby, if a player has high momentum, then he/she is difficult to stop. It has a size and direction.
- Equation: p=mxv
- p: momentum in kilograms metres/second-kgm/s
- m: mass, kg.
- v: velocity, m/s.
- The law of conservation of momentum says that in a closed system, the total momentum before and event is equal to the total momentum after the event.
- This law can be used to predict what might happen when objects collide in a collision and when they move apart in an explosion.
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- Momemtum has size and direction. Direction-wise, this is when the two objects move away/recoil from each other, in opposite directions.
- When object A and B don't move before collision:
- Momentum of object A after explosion=mass of A x velocity of A
- Momentum of object B after explosion=mass of B x velocity of B
- Total momentum before explosion=0 as A and B weren't moving.
- Therefore (mass of Axvelocity of A) + (mass of B x velocity of B)= 0
- And therefore, (mass of A x velocity of A) = - (mass of B x velocity of B)
- The negative sign shows that B is in the opposite direction of A-and that A and B move apart with equal and opposit amounts of momentum.
- In action-when a shell is fired from and artillery gun, the gun barrel recoils backwards, but it is slowed down by a spring to lessen its backward motion.
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P2.3.6 Impact Forces
- Crumple zones at the front and back end of a car are designed to reduce impact force. In front-end impact, the momentum of the car is reduced. In rear-end impact, the momentum is increased.
- If object A collides with object B, the impact force would act for a certain amount of time and cause it to stop. A soft pad would increase the impact time and the momentum would therefore be lost over a longer time and its kinetic energy would be transferred over a greater distance.
- The longer the impact time, the more the impact force is reduced.
- In a collision, the momentum of an object often becomes 0 as it eventually stills.
- If the impact time is short, the impact force is large.
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P2.3.7 Car Safety
- Seat belts:
- It stops a person continuing forwards when the car stops suddenly, so they don't accidentally hit the windscreen.
- The seat belt acts across the chest, spreading the force out and increasing the impact time.
- Air bags:
- These are put in the front and sometimes in the sides of a car, to protect people from an impact on the side of the car or windscreen. The inflated air bag spreads the force of impact across the upper body and increases impact time.
- The impact force is therefore lessened and it is more effective than a seat belt.
- Child car seats:
- It is the law that any baby or child has to be strapped in a car seat, applying to children up to twelve years old or up to 1.35 metres in height. Different types of car seats have to be used for different age ranges.
- Baby seats have to face backwards, and children under four should usually be in a car seat fitted onto a back seat.
- After a car crash, the police use measurements from the scene and the conservation of momentum to calculate the speed of the vehicles before the collision.
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