## constant velocity at rest

why do forces considered constant velocity and rest to be the same ??

note :in the diagrams u draw forces of equal magnitudes representing by this a moving abject (with constant velocity ) i don't understand why ?? they are balanced so they have to cancel each other .(resulting an object at rest !!).
thanks

fatma
1964

accept rate: 25%

Tim Gibson
3.9k51927

Good question. You have hit upon one of the important ideas in physics. A net force on an object is related to a change in the velocity of that object. If the the net force (or sum of the forces) equals zero, then the velocity of the object does not change. An object at rest is really just a special case of an object moving with a constant velocity.

The reason this is hard to understand is because most of our everyday life involves friction. When an object is moving at constant velocity, the zero net force is often the sum of a frictional force and some other applied force.

An air hockey disk is an example of an object with a zero net force when friction is absent.

Here is something else to think about. Whether an object appears to be still or move at constant velocity depends on how the frame of reference is defined. Think of a passenger riding on a train that is moving at constant velocity relative to the ground. Relative to the train, the passenger has a zero velocity. Relative to the ground, the passenger has a non-zero velocity. This one situation is described in two different ways (or frames of reference). It makes sense that the forces could be described in the same way for both frames of reference.

Pat Hallquist
9885

alot of thanks

(29 Jul '12, 18:00)

I think it is because to be a force you need a mass and an acceleration. F=ma........so if there is no acceleration then there is no force. You can still have other things going on with distance, velocity and time.......but you dont have a force. If you are in the Y plane you have the acceleration of gravity but if you are just in the X plane then you can have situations where you do not have acceleration and therefore you dont have Force.

david
3.8k19

but how?? if there is no force acting on an object how can it move with a constant velocity? u need a force to make any object move !!!!!

(29 Jul '12, 17:28)

1

You need a force to GET AN OBJECT MOVING (acceleration). It turns out you don't need a force to KEEP an object moving. If you ever see videos of astronauts in space, you can see this. They float around at constant speed throughout the spaceship.

(30 Jul '12, 20:49)

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