Concise Physics - Middle School 8: Energy

Question and answer flashcards covering Mechanical Energy, Potential Energy, Kinetic Energy, and Energy Conversions from Middle School Physics Class 8.

What is the S.I. unit of energy and its symbol?

The S.I. unit of energy is the joule, represented by the symbol $J$.

What does it mean for a body to possess an energy of one joule?

A body is said to possess an energy of one joule ($1\,J$) if it can do one joule of work or if one joule of work is done on it.

What is mechanical energy and what are its two forms?

Mechanical energy is the energy possessed by a body due to its state of rest or state of motion. Its two forms are (1) Potential energy and (2) Kinetic energy.

What are examples of bodies that possess mechanical energy as shown in Fig. 4.10?

A car in motion, an arrow on a stretched bow, moving arms of a clock, a rock on a high hill, and a swinging pendulum.

What is potential energy and what is the origin of the word 'potential'?

Potential energy is the energy possessed by a body due to its state of rest or position. The word 'potential' comes from a Latin word meaning 'to be able.'

What are the two forms of potential energy?

The two forms are: (1) Elastic potential energy (when work done changes the size or shape of a body) and (2) Gravitational potential energy (when work is done to move a body to a height above the earth's surface).

What is the formula for calculating gravitational potential energy ($P.E.$ or $U$)?

$P.E. = mgh$, where $m$ is the mass of the body, $g$ is the force of gravity on a mass of $1\,kg$, and $h$ is the height above the ground.

What two factors affect the potential energy of a raised body?

(1) The mass of the body (greater mass means greater potential energy) and (2) Its height above the ground (greater height means greater potential energy).

How does mass affect potential energy, according to the textbook example?

The potential energy of a bucket of water of mass $20\,kg$ at the first floor of a house is double the potential energy of a bucket of water of mass $10\,kg$ at the same floor.

How does height affect potential energy, according to the textbook example?

The potential energy of a bucket of water of mass $10\,kg$ at the second floor at height $6\,m$ is double the potential energy of the same bucket of water of mass $10\,kg$ at the first floor at height $3\,m$.

How does a falling stone drive a nail into wood using energy conversion?

The potential energy stored in the stone at its raised position changes into kinetic energy as it falls. This kinetic energy does work on the nail when the stone strikes it, driving it into the wood.

How is work done by a falling stone using a pulley in Fig. 4.15?

When a falling stone reaches a pan attached to one end of a rope over a fixed pulley, its potential energy allows it to do work by lifting up a weight attached to the other end.

What is kinetic energy?

Kinetic energy is the energy possessed by a body due to its state of motion. It is equal to the work done on the body to set it into motion from rest.

How does a fast-moving stone break a window pane as shown in Fig. 4.16?

The fast-moving stone possesses kinetic energy, which enables it to do work on the window pane upon impact, breaking it into glass pieces.

What factors affect the kinetic energy of a moving body?

The kinetic energy of a moving body depends on two factors: (1) The mass of the body and (2) The speed (or velocity) of the body.

How is potential energy converted to kinetic energy to produce hydroelectricity?

Water stored in a dam possesses potential energy. When it falls onto a turbine, its potential energy changes into kinetic energy to rotate the turbine, which connects to a dynamo to generate electricity.