The potential energy of a system can be negative. You drop a ball to the ground

Question

The potential energy of a system can be negative. You drop a ball to the ground from rest, and, from the same height, throw a second identical ball down to the ground. Their change in gravitational potential energies are different. If the force on an object is zero, the potential energy of the system Is zero. When air resistance is taken Into account, a ball thrown directly upward spends more time on the down than on the way up. If the potential energy of the system is zero, the force on the object is zero.

Explanation / Answer

1.true, because all potential energy is relative to a certain point. If the gravitational force is pulling away from the point called 0, then the gravitational potential energy can be negative. For example, call your current distance from the center of the planet the point of reference. If you went up a mountain, the gravitation potential energy would be greater. Similarly, if you went into a chasm, the potential energy would be negative. There is no definite zero for potential energy, it's all relative.

2. False. The potential energy of an object does not depend on its velocity, it only depends on iis position. Since the two balls begin and end in the same place, their changes in gravitalional potential energy are identical.

3. False. If there is no net force acting on an object, one can say that its potentia'l energy is constant. But this does not mean that constant potentia.l energy is zero The potential energy depends on selected reference height. Let's take a book rest on a table surface as an example' The net force on the book (the sum of weight and normal forcas) is zero lf the rcference height is selected orl the table surface, th€ potantial energy is zero. Otherwise, the potential energy will be non-zero

4. True.

This question can be answeled using either work-gnergy relalion or forces acting on i6e ball. Using tt!" *ork-energy relation it should be realized that because air resistance does non-conservaiive work on the ball, the ball's kinetic energy will be reduoed when it rcaches you again, resulting in a less final speed This means that the ball had a higher average speed on the way up than on the way down, Since the ball traveled the same distance up and don, it must have taken longer on the waY down. Using the forces acted on the ball: on the way up, gravity and airresistance are both acting downward on the ball, On the way down, however, gravity and air resistance have opposite direction, since the ball have a greater net force acting on it on the way on, it will slow down more quickly.

5. False.

The explanation in (3) also applies to this question. The potential energy depends on the reference height, for example, ifl drop a ball and take the reference height to be the initial h€ight ofthe ball, then the initial potential energy is zero even thought there is a force (gavit, on lt

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