Why F=MA and not F=MV?

[parliamentarian]

I was just thinking, wouldn't it make more since for force to equal mass times velocity.

Lets say something is moving at a constant velocity of 30m/s. Thus its acceleration is essentially 0. Which would mean its force is 0. Yet its obvious that the object has a force.

Another scenario, what if something is moving with an acceleration of 100m/s2. Its force should be higher at 10 seconds, when its v=1000m/s than at 2 seconds when its 200m/s. But according to the formula it will always have the same force.

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[WashMe]

because p (momentum)=mv, and momentum does not = force.

lol

actually, it's just an observation that fits an equation, that's all. like every other equation in physics, it is empirical. if you actually want to know WHY F=ma, you'd have to ask God or something.

In your first example, remember that you assume no friction or resistance in constant velocity problems. based on the empirical equation, there is no acceleration and no net force.

In your second example, remember that the change in velocity at each instant in time is constant, even thought the velocity is increasing linearly.

Edit: think of it like this- the object has momentum, not force; constant force is applied to the object but the momentum increases linearly. If the momentum (m* velocity, which is a vector) is not changing, then there is no force.

 

[werd]

I was just thinking, wouldn't it make more since for force to equal mass times velocity.

Lets say something is moving at a constant velocity of 30m/s. Thus its acceleration is essentially 0. Which would mean its force is 0. Yet its obvious that the object has a force.

Another scenario, what if something is moving with an acceleration of 100m/s2. Its force should be higher at 10 seconds, when its v=1000m/s than at 2 seconds when its 200m/s. But according to the formula it will always have the same force.

the moving object only exerts force if it impacts something else. the impact causes a velocity change (acceleration) and the force imparted is proportional to the velocity change. something moving at a high velocity relative to the object it impacts can exert more force on that object because there will be a greater change in velocity upon impact. to this extent, it's argued that rubber bullets actually impart more force on impact because they bouce off and head the other direction, hence a greater change in velocity, therefore greater acceleration at impact and greater force.

the other reason that F = M*a is because it's how the units work out. if you multiply mass and velocity, you don't come out with the units in which force is measured.

 

[BloodySurgeon]

I was just thinking, wouldn't it make more since for force to equal mass times velocity.

Lets say something is moving at a constant velocity of 30m/s. Thus its acceleration is essentially 0. Which would mean its force is 0. Yet its obvious that the object has a force.

Another scenario, what if something is moving with an acceleration of 100m/s2. Its force should be higher at 10 seconds, when its v=1000m/s than at 2 seconds when its 200m/s. But according to the formula it will always have the same force.

Well when the object has constant velocity, the net force equals zero as well as the acelleration. In addition, when we are dealing with circular paths, there is centripetal acceleration due to the force of gravity pulling towards the center and we use the formula F=mv^2/r.

We need a Force to increase an objects speed over time (which is the same as giving an object acceleration). With no net force, the object will either be at rest or at constant velocity. If it has been in motion and the forces cancel each other out, then the object will stay in a straight motion unless acted upon by an external force.

Also, in your second senario, think of it this way. If you are pushing a box on the ice which has negliable friction. You give a constant force which equals your push. The object starts to speed up. At first it was at rest, but each time you push it with the same force it goes faster and faster. This is Newton's second law at hand. The net force and acceleration is the same throughout an interval of time, but the velocity is increasing or decrease each second. F=ma and if the object was at rest initially... a = Vf/t