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Tue 10 Jan, 2006 04:59 am
Which of the following phenomena can be explained by Newton's Second Law of motion?
When a body is moving with a constant velocity, there is no unbalanced force acting on the body.
A moving body keeps its velocity by its own inertia if there is no unbalanced force on it.
When the magnitude of the unbalanced force acting on a body is doubled, the acceleration is also doubled.
A. (1) only
B. (3) only
C (1) and (2) only
D (2) and (3) only
The solution is B and it states that (1) and (2) are explained by Newton's first Law.
I understand that (1) is exexplained by Newton's first Law, but can it be explained by Newton's second law as well?
Newton's 2nd law is:
Quote:An applied force is equal to the rate of change of momentum.
Your statement (1) is:
Quote:When a body is moving with a constant velocity, there is no unbalanced force acting on the body.
From Newton's 2nd law, one may deduce that if a force of zero is acting on an object (no force), it's momentum is constant. Since momentum = mv, this means that it's velocity is constant. This gives statement (1).
I think all three of these could be explained by the second law, but the first law is a special case of the second law for cases where the applied force is zero. I could simplify your cases as follows:
1) Constant velocity implies no external force
2) No external force implies no change in velocity
3) 2x force implies 2x acceleration (change in momentum)
As Brandon posted, "An applied force is equal to the rate of change of momentum." For the first two, you could say "zero force = zero change in momentum" and say that the second law proves the statement and I would agree from a practical point of view. From a classroom point of view, the first law specifically deals with zero force, so those are from the first law. Number 3 is a direct application of the law. If you double the force, you double the acceleration.
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