Newton's 2nd Law: Force, Mass and Acceleration

[ttesss]

I´m currently studying Newtons 2nd law and I don't get how can force only depend on mass and acceleration and not on velocity..
I mean, if I punch something with my fist going at constant velocity ( acceleration=0) it stills has a force right?

 

[A.T.]

...going at constant velocity ( acceleration=0) it stills has a force right?

No.

 

[russ_watters]

I mean, if I punch something with my fist going at constant velocity ( acceleration=0) it stills has a force right?

You can't punch something with constant velocity. The instant you make contact, it will accelerate and your fist will decelerate (and both will deform).

Have you studied momentum yet? That's the concept you are looking for.

 

[russ_watters]

You can't punch something with constant velocity.

[sigh] Before I get a "but what if...?" Ok, sure, it is theoretically possible, but practically speaking it is really not.

 

[Mister T]

Summary:: F=m x a

I mean, if I punch something with my fist going at constant velocity ( acceleration=0) it stills has a force right?

When you say it still has a force what do you mean by "it"? If you're talking about your fist then you need to look at the net force on your fist. The thing that your fist hits will exert a force on your fist. But the rest of your body also exerts a force on your fist. If the two forces exerted on your fist are equal but opposite, then the net force on your fist is zero and the acceleration of your fist is also zero.

 

[sophiecentaur]

Summary:: F=m x a

it stills has a force right?

Whilst it's on its way, at a constant velocity, your fist 'has' no force. The force comes into the situation during actual contact when the fist or the object or both, changes velocity - i.e. accelerates. Then the force is its mass times the acceleration. If there is something resilient in the way (a cushion or spring) then the acceleration may be less so the force will also be less (but spread over a longer distance).

 

[DrStupid]

Summary:: F=m x a

I´m currently studying Newtons 2nd law and I don't get how can force only depend on mass and acceleration and not on velocity..

Newton 2 just tells you what forces do (changing the momentum of a body with the rate m·a) but not what they depend or not depend on. That is subject of force laws. Of course there can be forces that depend on velocity (e.g. drag). Than you have F(v) = m·a.

 

[vanhees71]

I'm a bit puzzled by this question since forces usually depend on position and velocity but not on acceleration. The paradigmatic examples are Newton's gravitational interaction
$$\vec{F}_{12}=-\frac{G m_1 m_2 (\vec{r}_1-\vec{r}_2)}{|\vec{r}_1-\vec{r}_2|}$$
and Lorentz's force on a charge in an electromagnetic field (in SI units)
$$\vec{F}=q [\vec{E}(t,\vec{x}) + \vec{v} \times \vec{B}(t,\vec{x})],$$
where $\vec{x}$ is the position of the particle and $\vec{v}$ its velocity.

Except for gravitational interactions forces usualy don't depend on mass either.

 

[wolf1728]

This is a reply to ttess (posting #1)
Weight is a force and as we know force = mass * acceleration, (where acceleration = 9.80665 meters / sec²).
A mass of 1 kilogram on the Earth's surface has a force of 9.80665 Newtons.
Basically, we know that objects don't need velocity to have weight (or force), just acceleration.