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Old May 2nd 2018, 02:38 AM   #11
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Originally Posted by avito009 View Post
Well the answer is really simple if you see. When a person applies a force on the dashboard and the car moves forward he is applying an external force.
Internal means inside and external means outside. The man is inside the car, so any force he exerts on the car is internal. Simple!

Take for example a baseball bat hitting a ball. Here there is an external force on the ball by the baseball bat. So in our example we can say that the hand of the person inside the car is the baseball bat and the ball is the dashboard.
But the ball isn't a box that contains the baseball bat. It's a totally different situation.

So answer is simple it is an external force applied on the car which moves it forward. So the person who asked me the question stating that it was an internal force since the driver was in the car was wrong.

Internal forces are: The gravity forces, magnetic force, electrical force, and spring force. Since none of these are involved in our question it proves that the force applied is external.
No... internal forces cannot be conveniently separated by types like that. Consider gravity and the planet Earth... all the forces of attraction between all those masses within the planet are internal forces (e.g. force between Earth's crust and mantle). Those internal forces do not have an impact on the net motion of the Earth throughout the solar system. However, if you consider the Sun, the gravitational attraction between the Earth and the Sun is external to the Earth. It has a massive impact on net motion of Earth. We're lucky that the Sun is there to keep the Earth from floating off into space!

By the way... it is possible for internal forces to move the control volume that contains them. However, for this situation to occur, the object that creates the internal force has to be free to move inside the volume (e.g. the man is not sitting in a seat or standing on the floor of the car) and upon making an initial force, the object is going to experience recoil, move within the space inside of the control volume and then interact in a similar way with the other side of the volume, which will be in the opposite direction to the initial force (e.g. if the man dropkicks the dashboard, the man will move towards the rear of the car due to recoil and then slam against the back of the car, exerting another force with similar magnitude). Therefore, internal forces can only really wobble or jiggle an object... they can't really result in any sort of "useful" net motion... much to the dismay of people trying to engineer perpetual motion machines.

In typical situations, like gas particles colliding with container walls, you consider the sum of these internal forces exerted on the container walls to be an internal pressure which is uniform inside the container and any differences or perturbations in pressure tend to either even themselves out over time or result in convection or other internal motions... no significant external motion results.

Final note: you can actually do an experiment with this. Get a smooth, light and transparent box (e.g. plastic) and put a small ball inside it that has a similar (or perhaps slightly lower) mass . Then shake the box such that the ball starts moving inside it and put the box down. The ball will hit one wall, move the box a little bit, then recoil and go the other direction. It should keep going for a while, but friction forces will result in a short experiment. For best results, put the box on a big block of ice
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Last edited by benit13; May 2nd 2018 at 03:07 AM.
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