Uniform motion?Do objects really move Uniformly?

Question

Newton's first law

Answer 1

Because there is no absolute reference frame for motion, you can only define uniform motion with respect to a specified coordinate frame.

For example, a ball falling in a vacuum is not in uniform motion with respect to an earth-fixed reference frame; the ball accelerates.

When the forces on an obejct sum to zero, that object is in uniform motion with respect to a reference frame that is non-accelerated. That means that if you compare gravity acceleration between the a frame fixed to the object, and the frame you are wondering about, if the gravity is the same, the frames are not accelerated with respect to each other.

Others have suggested that uniform motion requires absence of forces. This is not quite true - it requires absence of *net* forces. For example, a car traveling a constant velocity on a highway is not in a force-free environment: you can identify forces of air friction, propulsion from the wheels, weight, and support from the pavement. However it is considered to be in uniform motion with respect to a reference frame fixed to the earth, because all of those forces sum to zero, and because the apparent gravity in a frame fixed to the car is the same as in a frame fixed to the earth.

Answer 2

Only where there is no friction - so a ball falling in a perfect vacuum does move uniformly, but inroduce any energy loss (friction due to air resistance, or contact with a non perfectly smooth surface)and motion is non-uniform. In that case, "fudge factors" such as coefficients of friction have to be introduced to predict what will happen. Therefore, Newtons law is a very general case to be adapted to the situation as needed.

Answer 3

yes objects do move uniformly in the absence of any externel force such as friction or any other force

Answer 4

Use F=dp/dt, where p is the linear momentum. Thus dp is the change in momentum, from its initial value to zero, for example, and if the impact is done in a time dt you have your final result.