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Our physics teacher made us define a force as

“A push or pull, which causes at least one of the following- Change in velocity, change in shape, change in size”.

If I push a wall really hard such that I cant break it- ie. I cannot change its velocity and cannot deform it. Then would the push not be considered a force?

Qmechanic
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Saanvi Kalra
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2 Answers2

1

Your teacher's definition is close.
Force can cause the stated effects, but it doesn't have to. In your wall example, you are exerting a force, even though the wall doesn't move or deform. There's a counter force (from the wall) opposing your push that keeps it still according to Newton's Third Law and maintains it's initial shape and position.

So, a more precise definition of force would be:

Force is an interaction that can cause a change in an object's state of motion (including staying still) or its shape/size.

CompassBearer
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Joshua
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0

I prefer the definition from Wikipedia:

A force is an influence that can cause an object to change its velocity ... unless counterbalanced by other forces.

I see two problems with your teacher's definition:

  1. There might easily be counterexamples e.g. what about a couple - a pair of equal and opposite forces that do not change the linear velocity of an object, but do change its angular velocity. The definition could be expanded to include such counterexamples, but then it becomes more and more ad-hoc.
  2. Although the definition might apply to real world objects, it does not apply to the ideal objects that appear in elementary mechanics and dynamics e.g. inextensible strings and perfectly rigid bodies. So it could easily cause confusion for beginners. For example, a brick sits on the ground and is subject to two equal and opposite forces - its weight and the normal force from the ground. These forces do not change the brick's velocity because the net force on the brick is zero. And although we know that a real brick will be very slightly deformed by these forces, you would be hard pressed (pun intended) to measure this deformation, and for almost all purposes we can treat the brick as a perfectly rigid object.
gandalf61
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