# PHY101 Physics Assignment 1 Solution Spring 2013

Question # 1
The energy in a light wave is transmitted through space by
a. the Successive collisions of series of electrons
b. the successive collision of series of atoms
c. electric field variations
d. electric field and magnetic field variations
e. the successive collision of series of air molecules Also write the reason of your above selected choice as well. Marks = 3+4

Idea Solution:

unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.

Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 m/s (a speed value commonly represented by the symbol c). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 108 m/s. This is depicted in the animation below.

The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorption of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 108 m/s) through the vacuum of interatomic space, the absorption and reemission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below.

The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorption and reemission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.

Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects. When light waves of these frequencies strike an object, the electrons in the atoms of the object begin vibrating. But instead of vibrating in resonance at a large amplitude, the electrons vibrate for brief periods of time with small amplitudes of vibration; then the energy is reemitted as a light wave. If the object is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted. If the object is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material. Rather the electrons of atoms on the material’s surface vibrate for short periods of time and then reemit the energy as a reflected light wave. Such frequencies of light are said to be reflected.
Question # 2
You start on a beach. You go to the moon. You come back. You go to Hollywood. You then go to the summit of Mount Everest. Have you increased your gravitational potential energy more than if you had climbed to this summit without all the other side trips? Marks 5

Idea Solution:

# Gravitational Potential Energy

From the work done against the gravity force in bringing a mass in from infinity where the potential energy is assigned the value zero, the expression for gravitational potential energy is

This expression is useful for the calculation of escape velocity, energy to remove from orbit, etc. However, for objects near the earth the acceleration of gravity g can be considered to be approximately constant and the expression for potential energy relative to the Earth’s surface becomes

where h is the height above the surface and g is the surface value of the acceleration of gravity.

Gravitational potential energy is acquired by an object when it has been moved against a gravitational field. For example, an object raised above the surface of the Earth will gain energy, which is released if the object is allowed to fall back to the ground. In order for an object to be lifted vertically upwards, work must be done against the downward pull of gravity. This work is then stored asgravitational potential energy. When the object is released and falls towards the Earth, the potential is converted into kinetic energy, or movement.

A pendulum is a good example of the relationship between gravitational potential and kinetic energy. At its highest point, the pendulum has only potential energy. As it descends, this is converted into kinetic energy, reaching a maximum at its lowest point, where it has no potential energy. As it swings up again, the kinetic is converted to potential energy.

Question # 3
A baseball travelling with horizontal velocity V is caught by a glove. Which of the following statement is true?
a. The ball applies a greater force to the glove than the glove applies to the ball.
b. The ball applies a greater impulse to the ball than the ball applies to the glove
c. This is an example of an elastic collision
d. The padding of the glove reduces the force of the ball on the glove
e. The padding of the glove reduces the impulse of the ball on the glove
Also write the reason of your above selected choice as well. Marks = 3+4
Question # 4
Inertia is the tendency of an object
I. Initially at rest to stay at rest
II. Initially in motion to continue moving with constant speed
III. Initially in motion to continue moving in a straight line
Which of the following is true?
a. I only
b. I & II Only
c. I & III Only
d. II & III Only
e. I ,II & III
Also write the reason of your above selected choice as well. Marks = 2+3

Idea Solution:

Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. The principle of inertia is one of the fundamental principles of classical physics which are used to describe the motion of matter and how it is affected by applied forces. Inertia comes from the Latin word, iners, meaning idle, or lazy. Isaac Newtondefined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states:[1]

The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.

In common usage the term “inertia” may refer to an object’s “amount of resistance to change in velocity” (which is quantified by its mass), or sometimes to its momentum, depending on the context. The term “inertia” is more properly understood as shorthand for “the principle of inertia” as described by Newton in his First Law of Motion; that an object not subject to any net external force moves at a constant velocity. Thus an object will continue moving at its current velocity until some force causes its speed or direction to change.

Question # 5
You are asked to push two wheelbarrows up a hill. One wheelbarrow is empty, and you are able to push it up the hill in one minute. The other is filled with huge rocks, and even after you push it for an hour, you cannot budge it. In which case do you do more work on the wheelbarrow, elaborate your answer? Marks 6