A closed system consists of a pendulum that is swinging back and forth. If the pendulum's kinetic energy decreases, what else must happen to the
energy of the system?
a
A. Its gravitational potential energy must decrease.
B. Its total mechanical energy must increase.
O C. Its total mechanical energy must decrease.
O D. Its gravitational potential energy must increase.

Taking into account the definition of kinetic, potencial and mechanical energy, the correct answer is option D:  If  the pendulum's kinetic energy decreases Its gravitational potential energy must increase.

Kinetic energy is a form of energy. It is defined as the energy associated with bodies that are in motion and this energy depends on the mass and speed of the body.

Kinetic energy is defined as the amount of work necessary to accelerate a body of a given mass and at rest, until it reaches a given speed. Once this point is reached, the amount of accumulated kinetic energy will remain the same unless there is a change in speed or the body returns to its state of rest by applying a force.

On the other hand, potential energy is the energy that measures the ability of a system to perform work based on its position. In other words, this is the energy that a body has at a certain height above the ground.

Gravitational potential energy is the energy associated with the gravitational force. This will depend on the relative height of an object to some reference point, the mass, and the force of gravity.

Finally, mechanical energy is that which a body or a system obtains as a result of the speed of its movement or its specific position, and which is capable of producing mechanical work. Then:

Potential energy + kinetic energy = total mechanical energy

The principle of conservation of mechanical energy indicates that the mechanical energy of a body remains constant when all the forces acting on it are conservative (a force is conservative when the work it does on a body depends only on the initial and final points and not the path taken to get from one to the other.)

Therefore, if the potential energy decreases, the kinetic energy will increase. In the same way, if the kinetics decreases, the potential energy will increase.

This principle can be applied in this case. The mechanical energy of the pendulum remains constant when all the forces acting on it are conservative.

So, the correct answer is option D:  If  the pendulum's kinetic energy decreases Its gravitational potential energy must increase.

Learn more about mechanical energy:

The correct choice is D. Its gravitational potential energy must increase

Explanation:

Conservation of Mechanical Energy

The total amount of mechanical energy, in a closed system in the absence of dissipative forces like friction or air resistance, remains constant.

This means that energy cannot disappear or appear and that potential energy can become kinetic energy or vice versa.

In a closed system like a pendulum, two types of energies are considered: Gravitational potential (U) and kinetic (K). Thus, the sum of both energies must remain constant in time.

Suppose the pendulum is at a state where U=150 J, and K=350 J. The total mechanical energy is:

M = 150 J + 350 J = 500 J

If the kinetic energy decreases to a new value, say K = 200 J, then the gravitational potential must increase to compensate for this new condition, that is: U = 300 J

The correct choice is D. Its gravitational potential energy must increase

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s