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basic physics - force needed to move an object

Hi

If you had a 100kg object moving at a constant speed of 5m/s, how would you calculate how much force was needed to get this object moving in the first place? I don't really know how to do this because i only really know force = mass * acceleration and it don't know how you cater for the fact the object is travelling at a constant speed. I didn't think you could assume the object went from not moving at all to travelling at a constant speed. Perhaps you can make this simplification at a basic level.

Thanks
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andieje
Asked:
andieje
1 Solution
 
stranger9002Commented:
I think there is somethink wrong with the question. Considering that you can accelate the object almost infinately long time the force needed would be almost infinately small. But maybe the question should be how much mechanical work is needed, beacase that will be in that case constant.
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SuperdaveCommented:
It would depend entirely on friction.  With no friction it could be an arbitrarily small amount of force over a long enough time.  If it went from not moving at all to travelling at a constant speed it would require an infinite force.
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stranger9002Commented:
I just went through it again. Constant force inputed will result to constant acceleration instead of speed. Maybe this will help http://en.wikipedia.org/wiki/Impulse_%28physics%29
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aburrCommented:
"If you had a 100kg object moving at a constant speed of 5m/s, how would you calculate how much force was needed to get this object moving in the first place?"
From the information given, you can not.   You need the time that the force is applied. short time, big force, long time, little force.
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" it don't know how you cater for the fact the object is travelling at a constant speed."
If the object is traveling at a constant speed, there is NO force.
-
(a problem of this type is often better solved from an energy standpoint)
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the_billCommented:
Hi, you'd need to know how long the force was applied for, before you can calculate how large the force was. assuming unit time (ie 1s) then
f = ma
v = u + at ==> a = (v-u)/t
f= m(v-u)/t
u = 0 and t = 1s
f = mv = 100 * 5 = 500N

Assuming there is no friction, drag, or any other resistive force, as soon as one second elapsed & you removed the 500N force, the object would just keep moving at 5m/s and not go faster or slower.

things only go slower in real life because of friction & wind resistance. To keep going at that speed in life, your calculations would become more complex, as it would depend on where on earth you are, temperature, humidity etc etc, but thats out of scope!
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ozoCommented:
> If you had a 100kg object moving at a constant speed of 5m/s, how would you calculate how much force was needed to get this object moving in the first place?

It depends on the distance that the force is allowed to act through.

To accelerate 100kg  up to 5m/s within a mm would require 1250000 N
but 1.25 N acting over a kilometer would also do it
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andiejeAuthor Commented:
Aburr, i know that if the object is travelling at a constant speed there is no force on it but didn't it need a force on it to get it moving in the first place?

I can see you need the amount of time the force was applied to get the force.

I understand bill's answer perfectly (until tomorrow when i do something new and forget this again).
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andiejeAuthor Commented:
thanks
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