Video: Definition of Kinetic Energy

Which of the following is the term used to refer to the energy that a moving object has due to its mass and its velocity? [A] Internal energy [B] Kinetic energy [C] Mechanical energy [D] Potential energy


Video Transcript

Which of the following is the term used to refer to the energy that a moving object has due to its mass and its velocity? A) Internal energy, B) Kinetic energy, C) Mechanical energy, D) Potential energy.

Okay, so in this question, we are considering an object that is moving and we’re trying to figure out which one of these four is the term that refers to its energy due to its mass and velocity. Now, the first option is internal energy. Well, when we say internal energy, we should immediately think of a system of particles. So, for example, if we have a box with lots of particles of gas inside this box, then the internal energy of this system of particles is the total energy contained within this system.

In other words, the kinetic energy, the motion energy due to these particles moving around inside the box, all at different velocities and in different directions. And additionally, the potential energy between these particles, as a result of their positions in space relative to each other. But in simpler terms, it’s the total energy contained within this system. And so, it’s not simply the energy due to the mass and velocity of a moving object. Hence, this is not the answer we’re looking for.

Option B, then, is kinetic energy. Well, kinetic energy is indeed movement energy. It is the energy that an object has because of its mass and its velocity. And, we can see this by recalling that the kinetic energy of an object, KE, can be found by multiplying half by the mass, 𝑚, of the object by the velocity, 𝑣, of the object squared. And hence, the kinetic energy of an object is dependent on the mass and the velocity of the object. So, it looks like option B is the answer that we’re looking for.

Quickly looking at the remaining options though, we can see that option C is mechanical energy, which is actually defined as the sum of a system’s kinetic energy and potential energy. Well this time, the system that we’re talking about doesn’t have to be a system of particles. It could be a macroscopic or large-scale system. But the point is that this is not the energy that our moving object has by virtue of its mass and its velocity. And hence, option C can be crossed out as well.

And finally, option D is potential energy. Now, potential energy is the energy between objects due to their relative positions to each other. One good example of this is if we have the Earth and if we’ve got an object, for example, a tennis ball suspended above the Earth. Then, this tennis ball has some gravitational potential energy at this distance from the Earth’s surface, by virtue of the fact that the Earth has a gravitational field pulling all objects with mass towards itself.

And so, because this tennis ball is in the Earth’s gravitational field, it has some gravitational potential energy. But once again, this is potential energy due to the tennis ball’s position relative to the Earth, more specifically the height above the surface of the Earth, and therefore not due to its velocity. Once again, the position is important, not the speed at which the object is moving. And hence, we can cross out option D as well. At which point, we’ve found the answer to our question. The term used to refer to the energy that a moving object has due to its mass and its velocity is kinetic energy.

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