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!

Newton's third law and energy transfer

  1. Apr 30, 2013 #1
    If newtons third law says there is an equal and opposite reaction how can there be energy transfer.
    For example if I push a box and give it 1000 joules of energy wouldn't it "give back" the energy. Also does an object have to be moved for it to gain energy, if I were to punch a book many times would it gain energy or is it just giving back the energy becausE of newtons third law. And if I drop a ball from a height of 10 feet wouldn't it gain energy after it hits the ground because I am supplying it with potential energy than it is converted into kinetic as it falls once it hits the ground the energy is transferred into the ground and then back into the ball therefore giving the ball more energy than when it started. Please correct me if I am wrong
    Last edited: Apr 30, 2013
  2. jcsd
  3. Apr 30, 2013 #2
    Recall the definition of work, which is force acting over a distance; the direction of the force matters. If I push a box 1m along the X axis, with a force of 1N, I have expended 1J of energy, because the displacement is in the direction of the force. The box on the other hand absorbed 1J or energy(or rather friction did), as the force it applied was opposite the displacement.
  4. Apr 30, 2013 #3


    User Avatar

    Staff: Mentor

    Welcome to PF!
    Newton's 3rd law is about force not energy. If you apply a force to a box, it applies a force back at you.
    There are different kinds of energy. If you punch a book a bunch of times and it doesn't move, it will absorb energy due to deformation and heat up.
    It loses potential energy and gains kinetic energy until it hits the ground. Then it loses the (linear) kinetic energy too, converting it to heat and sound.
  5. Apr 30, 2013 #4
    In my first question you said that the box applies a force back to you, but doesn't it take energy to supply a force. And in the second question about punching the book, would the same outcome happen to any object punched repeteadly.
  6. Apr 30, 2013 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    No. It does not take energy to apply a force.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook