Forces And Energy
Visão Geral
Lift a bag of rice onto a shelf and you have just paid a bill. The bag was low down and now it is high up, and the difference between those two situations cost you something real: a force from your arms, moved through a distance. Physics has a single word for what you paid, and it is the same word whether the payer is a muscle, an engine, a stretched spring or a waterfall. That word is work, and the receipt it prints is measured in joules.
This lesson turns that idea into the relationships you can put numbers into. You will learn what work really means and why holding something heavy above your head does none of it, how a raised mass and a stretched spring both hold energy in waiting, why a car travelling at twice the speed carries four times the kinetic energy and needs four times the distance to stop, and how power separates a machine that does a job quickly from one that does the same job slowly. This topic hands more equations to the Physics Equation sheet than any other on the course, and the sheet comes into the exam with you, so the marks are never for reciting them. They are for knowing which one the question wants.
Objetivos
- Work is done when a force causes an object to move through a distance. The relationship between work done, W, force, F, and distance, d, moved in the direction of the force is:
- Energy is transferred when work is done. Work done against frictional forces causes energy transfer by heating. Students should be able to discuss the transfer of kinetic energy in particular situations, for example shuttle re-entry into the atmosphere or meteorites burning up in the atmosphere and braking systems on vehicles.
- The amount of elastic potential energy stored in a stretched spring (assuming the limit of proportionality has not been exceeded) can be calculated using the equation:
- An object gains gravitational potential energy when it is raised vertically because work is done against the gravitational force. The relationship between gravitational potential energy, Ep , mass, m, gravitational field strength, g, and height, h, is:
- The kinetic energy of a moving object depends on its mass and its velocity. The relationship between kinetic energy, Ek , mass, m and velocity, v, is: Students should understand that when the mass of an object is doubled, if it is travelling at the same speed it will have twice the kinetic energy. They should understand that an object travelling at twice the speed of another object with the same mass will have four times the kinetic energy and should be able to apply this idea in the context of road safety.
- Power is the rate at which energy is transferred or the rate at which work is done. The relationship between power, P, work done, W, or energy transferred, E, and time, t, is: and
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- A horizontal force of 20 N pushes a box 3.0 m across a floor in the direction of the force. How much work is done on the box? A. 6.7 J B. 17 J C. 23 J D. 60 J Answer: D
- A machine transfers 3000 J of energy in 15 s. What is its output power? A. 0.005 W B. 200 W C. 2000 W D. 45 000 W Answer: B
- A car has kinetic energy Ek when it travels at 10 m/s. What is its kinetic energy when it travels at 30 m/s? A. 3 x Ek B. 6 x Ek C. 9 x Ek D. 30 x Ek Answer: C
- A crane raises a 200 kg load through a vertical height of 5.0 m. The gravitational field strength is 9.8 N/kg. How much gravitational potential energy does the load gain? A. 40 J B. 1000 J C. 9800 J D. 98 000 J Answer: C
- A sledge slides across level ground and gradually comes to rest. What happens to its kinetic energy? A. It is destroyed by the friction B. It is transferred to the surroundings by heating C. It is stored as gravitational potential energy D. It is stored as elastic potential energy in the ground Answer: B
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