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Originally Posted by Shan2nu If mass in the absence of friction affects rate of acceleration, it should also work the other way around. So youre saying that an object with more mass will also come to a halt slower than a light object in the absence of friction? |
Without friction, an object will never slow down. Slowing down is an acceleration so it needs a force. If you did introduce friction to slow the body down, then it is slightly different.
The friction acting on a body is given by the product of a friction coefficient (depends on the surface) and the normal force exerted by the surface on the body. If we consider a box moving on a horizontal surface, then the normal force is equal to the weight. So the frictional force depends on the weight. Now, the force of friction may be higher but a heavier object means that the mass is also higher. If, therefore, you push a big 1000 kg box on a flat surface with friction till it reaches velocity v and let it go, it will have a magnitude of deceleration a m/s/s, say. Now if you push a 1 kg box to the same velocity on the same surface, it will also decelerate at a m/s/s after you let it go. They will both stop in exactly the same distance. (Note that air resistance is not considered here)
The math is simple.
f=uN (f is the force of friction, u is the friction coefficient and N is the normal force)
So, f=uW (In this case, W, the weight is equal to N.)
So, f=umg (W is mg)
The total force acting on the body is F, say. Since friction is the only force we have,
F=f
But we know by Newton's second law,
F=ma
Thus, we have
umg=ma
So,
a=ug which is clearly independent of m.
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Originally Posted by Shan2nu But even in a world with no friction, as long as the force acting on the object is the same the rate of acc should also be same, irrespective of its weight. |
No, this is not so. The acceleration will be inversely proportional to the mass. So a heavier object will have lower acceleration for the same force.
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Originally Posted by Shan2nu I think you're trying to say that even without friction, gravitaional pull creates some sort of resistance (in other words mass of the object). Is that right? |
Not quite. If it helps, here's the math
F=GMm/r^2, where F is the force on a falling body, M is the mass of the earth (constant), r is the distance of the body from the earth's centre (virtually constant at low heights) and m is the mass of the object that's falling.
F=ma, by Newton's second law
So ma=GMm/r^2
So a=GM/r^2 which is independent of m. GM/r^2 is a constant and is given the notation g. The same g that is 9.8m/s/s
So, a=g for all bodies. No m in the equation which simply means that irrespective of the mass, acceleration of the falling object is 9.8m/s/s
This explains the falling body idea.
If you find that math unhelpful, think of it this way. A strong powerful man can throw a heavy metal ball ten feet. A weak sick boy can throw a tennis ball ten feet. In the first case, the heavy object had a large force acting on it and the light object had a small force. Yet both had the same acceleration (and consquently, same distance travelled). This is what happens to a falling body. Heavy objects are pulled with great force but their mass is also high. So the acceleration ends up being the same.
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Originally Posted by Shan2nu But dude, if the 1600kg car is, lets say levitating (since you mentioned its not using any wheels), It means that the upward force acting on it is the same as the gravitational pull acting on it.
So in such a situation, wont the 2 forces working against each other be like pushing an object in the absence of gravity (during its state of levitation)? Would the mass still affect acceleration?
Shan2nu |
In levitation, yes, the forces of gravity and the applied force balance each other. Hence the body stays where it is and has no acceleration. The correct way to think of it is that the total force on the body is zero. Thus, there is zero acceleration. If, after adding all the forces and cancelling the opposing forces, there remains a net force, then the body will accelerate in that direction and the
magnitude of its acceleration will depend on the mass and the net force.