Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Internal/External Forces

  1. Feb 13, 2004 #1

    I need to know the difference between internal and external forces. From what I can understand, Internal forces will not change the total mechanical energy of the object, while external forces will. What are some examples of internal of external forces?

    Thanks for any help.
  2. jcsd
  3. Feb 13, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    If two parts of a system exert forces on each other, those forces are said to be internal forces for the system.

    An external force is a force between a part of a system and something outside the system.

    So whether a force is internal or external depends on what you define as your system.


    1) a baseball. The forces holding the particles of the baseball together are internal forces; the force of a bat hitting the ball is an external force on the ball; so is gravity.

    2) a cat and dog fighting in a sack. The two are killing each other, but considered as one system the forces they exert on each other are all internal.

    2') the cat considered separately. Considering the cat as a separate system, forces between the cat and the dog are now external forces.

    3) A car. The parts of the car exert forces on each other: the pistons push the rods, the axle pushes the wheels, etc. All these forces are internal to the car as a whole. But the friction that the ground exerts on the tires is an external force. So is the weight of the car, and the force the ground exerts to hold up the car.

    I hope this helps a little.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook