# How can a body even move if no net force is acting on it?

Studying NLM currently...I came across various questions where a body was moving with constant velocity and and I had to consider net force on it as zero. I get it theoretically...

$$\mathbf F=m\mathbf a$$ and for constant velocity acceleration is zero so force should be zero...

But how's that even possible? Cause of motion is force and how can body be moving if no net force is acting on it?

• "Cause of motion is force" Only if you believe Aristotle knew everything worth knowing about physics. The three-word summary of Newtonian mechanics is "Aristotle was wrong". – alephzero Jul 21 '19 at 15:11
• How can a body even stay still if no net force is acting on it? – Andrew Steane Jul 21 '19 at 15:21
• Given no net force on an object, that object's inertia preserves its current state of motion, per Newton's 1st law. Once an object is moving at constant velocity, it DOES NOT take a continuing force to keep it moving at that velocity. This is a common and intuitive misconception, and it is definitely wrong. – David White Jul 21 '19 at 15:38
• Force causes acceleration not velocity. At some time in the past there was a force but you don't know the magnitude or the time it lasted .... just look at the problem in the present. – PhysicsDave Jul 21 '19 at 15:48

Newton's 1st Law of Motion:

A body in uniform motion will continue to travel in a straight line until it is acted on by a force.

It only needs a force to accelerate or change direction, not to keep moving. Also, motion is relative, so whether the body under consideration is moving or not depends on your frame of reference. Your chimney pot will seem to you to be at rest. but from the frame of reference of an observer on Mars, it is whizzing around at 1,000 mph with the rotation of the Earth (assuming you live near the equator).

• "It only needs a force to accelerate or change direction, not to keep moving". That would not be the case if you accelerated it on a surface with friction. You need to continue to apply a force at least equal to the kinetic friction force to keep it moving. – Bob D Jul 21 '19 at 16:15
• @BobD He means net force, I assume. Who applies it does not matter IMHO. – Apoorv Khurasia Jul 21 '19 at 19:42
• @ApoorvKhurasia Perhaps. Then, IMHO, he should so indicate. – Bob D Jul 21 '19 at 20:26

F=ma and for constant velocity acceleration is zero so force should be zero...

But how's that even possible? Cause of motion is force and how can body be moving if no net force is acting on it?

Example 1: a body moving on a frictionless surface at constant velocity has no net force acting upon it. A force would be required to start it moving, but no force is necessary to keep it moving.

Example 2: A body moving at constant velocity on a surface with friction where the force pushing the body exactly equals the opposing kinetic friction force. There is no net force, yet the body is moving. Once again a net force would be required initially to overcome static friction, but once the body starts moving all that is needed is a force equal to the opposing kinetic friction force to keep it moving.

Hope this helps.

An object in motion had some force act upon it in the past. If the force were STILL acting upon it, it would be accelerating, not just moving at a constant velocity. Once energy is spent to start an object moving, the energy must be conserved as kinetic energy, it cannot just disappear and the object stop. It will continue moving at the same speed and direction until another force acts upon it. Hope this will help you understand Newton's first law.

• Once the object starts moving its energy need not be conserved as kinetic energy. An example is starting an object moving on a surface having friction. Once the force is removed the object will eventually come to a stop and its kinetic energy will have been dissipated as heat at the contact surfaces. – Bob D Jul 21 '19 at 16:12
• @Bob D: yes but the friction acting upon it is another force – Adrian Howard Jul 21 '19 at 18:11
• I don't wish to belabor the point, but your statement only referred to energy spent starting the object being conserved as kinetic energy. I suggest you consider editing it along the lines of "Once energy is spent to start an object moving, the energy must be conserved as kinetic energy ....unless another force subsequently acts on it". But it's just a suggestion. – Bob D Jul 21 '19 at 18:19
• @Bob D: Yes I did edit it, I originally left it out as he said he was studying Newton's laws so I assumed he had read that part. but it should be added for future readers. thank you (if this comment is posted twice forgive me my computer seems to have lost the first attempt) – Adrian Howard Jul 21 '19 at 18:44
• Looks good to me now. Regards. – Bob D Jul 21 '19 at 18:56

Momentum = mass x velocity - The moving object has momentum that is keeping it moving with zero net force acting on it.