# How can a body have constant velocity when net force on it is zero?

Considering Friction is here.. How can a body move with uniform velocity, to move with uniform velocity it should have acceleration equal to zero, for this suppose a body on which a force F is acting on right then the F(k) {Kinetic friction} will be on left (all this happening in x-axis). To have zero acceleration F=F(k), then how can a body even move having zero Force on it along with uniform velocity... I didn't get this theoretical concept.

• You confuse force and net force. Net force is zero, counteracting moving and friction forces are not zero. When the former ceases, the motion (progressively) stops. – Poutnik Jun 22 '19 at 11:08
• Do you understand what the term "net force" means? – Aaron Stevens Jun 22 '19 at 12:03
• I'm not sure what you are asking. Are you asking how a body can move at all if there is zero net force? or are you asking how a body starts to move? Does your question specifically involve friction, or are you just using friction as an example of a force? – garyp Jun 22 '19 at 13:34
• I am asking is that, if a body has a constant speed then net Force on the body is zero and force is directly associated with acceleration, so how a body even move with constant velocity.. Or what forces will be acting on a body moving with constant speed. – Kenji Jun 22 '19 at 15:12
• This thread shows why what many of take for granted took until Galileo to be discovered. – my2cts Jun 22 '19 at 17:22

Frictional forces works in two different regimes; static and dynamic.

Immagine a solid resting on a table. When you pull from it with a force (we call that force in this context the acting force) the frictional force apears. It was not there before. This is the static friction force. It increases in magnitude at the same pace that you increase the acting force, thus is always perfectly counteracting your pull and therefore the object doesn't move. Or more precisely the object doesn't change its state of motion (which is a specific case of uniform rectilinear motion where velocity is zero).

If you keep increasing the pull, then the static friction would keep increasing to counterbalance it, until it reaches a critical threshold (a maximum force) where the frictional regime transforms to the so called dynamical. Above this force (this pull / acting force) the table can no longer resist your attempts to move the object and friction stops increasing as a response to your acting force. Then your acting force is greater than friction (since it can't keep increasing the bet) so the net force (the total sum of forces) is non zero, A.k.a. you start to accellerate the object.

You can see the response of the frictional force with respect to the acting force you are pulling the object with in this diagram:

THEORETICAL INTENSITY OF THE FRICTIONAL FORCE VS THE ACTING FORCE

EXPERIMENTAL DATA SHOWING INTENSITY OF THE FRICTIONAL FORCE VS THE ACTING FORCE

So, to finally answer your question. Can a body have constant velocity when net force on it is zero? Yes, and not only that. It will ALWAYS have constant velocity when the net force (the sum of all the forces acting on an object: in this case pulling and frictional forces response) is zero. This is the statement of Newton's 1st law of motion. It doesn't matter if there is friction or not, the statement is always valid. In the static regime, when you are not pulling very hard from the object and the frictional response is counteracting your pull the net force is zero and in fact the object mantains a constant velocity (zero velocity in this particular case). When you pull harder than a certain threshold force the friction is not completely counteracting your pull and thus there is a net force in the direction of the pull slowly accellerating the object (Newton's 2nd Law). This means that the object would change its state of motion from been at rest to start to move.

It would be difficult to find a location in space where there is absolutely no force whatsoever acting on a body, but there are locations where the force of gravity is so negligible that there is virtually no force acting on it. In such a location the object is free to follow Newton's 1st Law of Motion: a body in uniform motion will continue to travel in a straight line until acted on by a force.

The simple answer an external force was initially required to overcome static friction. Once the object starts moving we switch to kinetic friction which, as shown in @Swike graph is less than static friction. If you maintain the force you were applying to overcome static friction your force will exceed kinetic friction and the object will accelerate. However, if you reduce your force to exactly equal the kinetic friction force, then there is no net force and the object will continue to move at the velocity it had before your reduced your force. It now moves with constant velocity with no net external force.

Hope this helps.