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Joined: 23 Apr 2009
Posts: 100

Posted: 04 Jun 2011 04:20:14 am    Post subject:

Hey Unitedti, it's been a while. I have question about Forces. Lets say, in a real life situation, you apply a Force to an object on a horizontal surface. Now assuming you don't get tired, if you keep applying some Force, at some point in time I would assume that this object would reach a max velocity. After thinking about it, I came to the conclusion that the Force on the object in question cannot be constant, because if it were then acceleration would also be constant, which means velocity would keep rising, never reaching a max. I also considered Friction as the reason why the total force was was not staying constant, but I figured unless the Normal Force changes or the surface changes, (which I'm assuming stays the same) then Friction remains constant.

I guess what I'm asking is how do you represent mathematically this situation or better yet how do you
figure out/represent the Force in this situation and therefore the acceleration and velocity?

Also I like to add that I am well aware that When the net force in a certain direction is zero, then the velocity is constant in that direction, or in this case a max velocity. What I am asking what Forces end up equaling zero.

I would appreciate any help.

Last edited by Guest on 09 Jun 2011 12:26:20 am; edited 1 time in total
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Active Member

Joined: 24 May 2003
Posts: 747

Posted: 10 Jun 2011 08:08:56 pm    Post subject:

The force required to accelerate a given mass at a certain rate is constant. This is represented by F = ma .. or a = F/m. As long as force and mass stay constant, it will keep accelerating at the same rate forever. At least until you get to incredibly fast speeds where relativistic effects increase the mass of the object and more force is needed to keep accelerating it, with a limit of infinite force needed when the object is at the speed of light.

When you're pushing something along the table and it isn't going any faster or slower, then velocity is constant, which means there is no acceleration in any direction. Net force has to be zero then, so that acceleration is also zero, which means you are giving it just enough force to counteract friction and no more. That force needed to counteract friction should theoretically probably stay constant no matter the velocity of the object. It probably slightly increases because of heat issues and deformation of the object.


Only two forces in play are:

Force of friction = uFn
where u = coefficient of friction
and Fn = normal force, which equals mg


Force from your hand pushing the object

Net force = ma,

Ffriction + Fhand = ma,

if acceleration is zero then:

Ffriction + Fhand = 0,

Ffriction = -Fhand

uFn = -Fhand

umg = -Fhand

So your hand perfectly counterbalances the force of friction, and you're pushing with umg newtons

Last edited by Guest on 10 Jun 2011 08:15:35 pm; edited 1 time in total
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Joined: 23 Apr 2009
Posts: 100

Posted: 11 Jun 2011 06:56:42 am    Post subject:

Thanks Joe that make a little more sense. At least I know which forces balance out.

I know the applied Force is more when you first start pushing it, first to overcome static friction then to overcome kinetic friction, which is usually less than static friction.

And like you said, when it reaches a max velocity, the frictional force and the applied force are equal, which means that the applied force is less than it was at the beginning.

So what makes the applied force go down? Does it have something to do with the mass?
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Joined: 17 Aug 2011
Posts: 3

Posted: 19 Aug 2011 12:06:58 am    Post subject:

Hello I'm anandjo!

I like to help you also...

Use the Force Body Diagram(FBD) so that you can easily identity the forces, frictions, gravitational forces or it weight=mg.

Hope I can help..
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