1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can this method turn potential energy into kinetic energy?

  1. Nov 23, 2013 #1
    I am wondering if this method is in use already somewhere, or if its possible to be used. For example, using the equation of gravitational potential energy a 100kg weight that freefalls 100 meters has 98,000 J of energy, which equals 24,000 watts after approx 4 seconds of freefall. Assuming I used the equation correctly, I'd like to know if any or all of those watts can be 'harvested' into usable electricity? I'm thinking that maybe a device, like the brakes on an electric car, could bring the weight to a controlled stop after the 4 seconds of freefall. If thats possible would the amount of time for the controlled stop effect the electricity produced?
    My guess is that even if all 24,000 watts of energy could theoretically be 'harvested' the rate of electricity made available would decrease as the time for the controlled stop increased. If the stop took 1 second, for example, then the rate would be 24,000 watts/second, but if the stop took 60 seconds the rate would be 400 watts/second. Is that right, or anywhere in the vicinity of being right?
  2. jcsd
  3. Nov 23, 2013 #2


    User Avatar
    Gold Member

    Watt is not the unit of Energy.(It is joules)Watt is the unit of power.

    Anyway,what the use of this?We already generate electricity by using the GPE of water.Water falling from a water fall,fall over a turbine,as the turbine rotates,electricity is produced.
  4. Nov 23, 2013 #3
    Thanks for the tip about the difference between power and energy. I'm wondering if its correct, mathematically and the correct terminology, to say that the scenario I described constitutes a 24.5KW power plant, if only for 60 seconds?
  5. Nov 23, 2013 #4


    User Avatar
    Science Advisor

    What you actually have is an “engine” not a power plant.
    An engine converts energy from one form to another.
    More powerful engines can convert more energy per unit of time.

    It is important to realise that power is the rate of flow of energy.
    One watt is equal to one joule per second.
    More “powerful” equipment can handle higher energy flows.

    When you buy electricity for your house you are buying energy.
    It is often wrongly and misleadingly called the “power” supply.
    At the end of a billing period you receive a bill for total energy used over that period.
    Energy is traditionally sold in units of “kilowatt hours” which sounds like power.
    But the units of power are (energy / time), so (power * time) is energy because the time dimensions cancel.

    Energy and money are usually directly related.
    Your accumulated pay at the end of a period of time is like accumulated energy.
    Your rate of pay, (pay per unit of time), is equivalent to your financial power.
  6. Nov 23, 2013 #5
    A 24.5kW power plant produces 1470000 J in 60 seconds. You only have 98000 J available. You must have made a mistake somewhere.
  7. Nov 23, 2013 #6


    User Avatar

    Staff: Mentor

    You said 4 seconds in your first post. Where did you get 60?
  8. Nov 24, 2013 #7
    Firstly, the very wires in the engine to "harvest" energy have resistance in the real world. As a general rule of thumb, energy is usually lost in process by one form or another, such as friction, resistance, sound, etc. So, in the real world, not all of it will be collected to begin with.

    Yes and no the the second bold portion. Theoretically, it is moving with a certain kinetic energy, and in order to make it come to a halt, you would have to match it. In the real world, however, there are other factors that will play into it. While your engine creates electricity from the moving object, will the heat of the engine change the resistance in the wires, or the friction of the mechanical parts that will inevitably lower efficiency? That is the problem with doing things on paper.
  9. Nov 24, 2013 #8


    User Avatar
    Science Advisor

    The power rating of an engine is the maximum rate it can convert energy.

    If an electric car weighs 1 ton = 1000 kg and is travelling at 56mph = 25 m/sec
    then the KE = 0.5 * 1000 * 25^2 = 312.5 kJ
    To regeneratively brake from 56 mph = 25 m/s to a stop in;
    20 seconds requires a generator rated at 15.625 kW = 20.95HP
    10 seconds requires a generator rated at 31.25 kW = 41.9 HP
    6.25 seconds requires a generator rated at 50.0 kW = 67.0 HP
    5 seconds requires a generator rated at 62.5 kW = 83.8 HP
    3 seconds requires a generator rated at 104.17 kW = 139.7

    When regenerative braking is available that uses the drive motors as generators, the acceleration time and braking time will be very similar. Because braking efficiently and quickly is important the motor would need to be rated at about 67 HP. But if you could settle for less acceleration and if friction brakes were provided for emergency braking the engine power could be reduced maybe to 30 HP. There is no requirement that the full power of the motor / generator provided ever be fully utilised, but it needs to be there in case it is needed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook