A girl holds a stone in her hand and can move it up or down or keep it still. Tr

Question

A girl holds a stone in her hand and can move it up or down or keep it still. True or false & why:
A) The force exerted by her hand on the rock is always the same magnitude as the force of gravity on the stone.
B) The force exerted by her hand on the rock is the reaction force to the force of gravity on the stone.
C) The force exerted by her hand on the stone, is always the same magnitude as the force on her hand by the stone but in the opposite direction.
D) If the girl moves her hand down at a constant speed, then her upward force on the stone is less than the force of gravity on the stone.
E) If the girl moves her hand downward but slows the stone to rest, then the force of the stone on the girls hand is the same magnitude as the force of gravity on the stone.

Explanation / Answer

Refer to the basic equation that is a result of Newton's 2nd law. The sum of the forces on an object is equal to its mass times its acceleration. Let W be the weight of the rock and F be the force she applies to it, then: F - W = m a Anytime acceleration (a) is non-zero, then F can't equal W. A. False==>Newton's 2nd law. If her hand is accelerating the rock (up or down), then the force applied by her is not the same as the force of gravity. B. False==>Newton's 2nd law. Only true if there is no acceleration. C. True==>Newton's 3rd law. Every action has equal and opposite reaction. D. False==>Newton's 2nd law. If her hand is not accelerating downward then she is applying a force exactly equal to the weight of the stone onto the stone (note that she had to accelerate at some point to achieve this constant velocity, but once achieved the situation is as I just described) E. False==>Newton's 2nd law again. There is acceleration thus the weight of the rock and the force she applies to it can't be the same.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.