Exam 2.3

The difference between speed and velocity is that _________.

velocity also includes a direction. Velocity describes speed in a particular direction

The acceleration of gravity on Earth is approximately 10 m/s2 (more precisely, 9.8 m/s2). If you drop a rock from a tall building, about how fast will it be falling after 3 seconds?

30 m/s. To find this answer, remember that the acceleration of gravity means that each second, an object’s downward speed increases by 10 m/s. Therefore, if it starts from rest (speed = 0), after 1 second it is falling at 10 m/s, after 2 seconds it is falling at 20 m/s, and after 3 seconds it is falling at 30 m/s.

Momentum is defined as _________.

mass multiplied by velocity. Notice that because velocity includes direction, momentum also includes direction.

Suppose you lived on the Moon. Which of the following would be true?

Both your weight and your mass would be the same as they are on Earth.

Both your weight and your mass would be less than they are on Earth.

Your weight would be less than your weight on Earth, but your mass would be the same as it is on Earth.

Your mass would be less than your mass on Earth, but your weight would be the same as it is on Earth.

Your weight would be less than your weight on Earth, but your mass would be the same as it is on Earth. You would weigh only about 1/6 as much on the Moon as you do on Earth, but your mass would still be the same.

In which of the following cases would you feel weightless?

While falling from a roof

While accelerating downward in an elevator

While parachuting from an airplane

While walking on the Moon

While falling from a roof. You are in free-fall when falling off a roof, so you are weightless (until you hit the ground)

Which of the following statements is not one of Newton’s Laws of Motion?

The rate of change of momentum of an object is equal to the net force applied to the object.

For any force, there always is an equal and opposite reaction force.

In the absence of a net force acting on it, an object moves with constant velocity.

What goes up must come down.

What goes up must come down. his is not one of Newton’s laws, and it’s not even true. Objects with escape velocity can go up without coming back down.

Newton’s Second Law of Motion tells us that the net force applied to an object equals its _________.

mass multiplied by acceleration. We often write this fact simply as F = ma.

Suppose that two objects collide. Which of the following things is not the same both before and after the collision?

The total temperature of the objects

The total energy of the objects

The total momentum of the objects

The total angular momentum of the objects

The total temperature of the objects. To understand why temperature is not a conserved quantity, remember that temperature is a measure of only one type of energy (thermal energy, which is the kinetic energy of moving particles) and this energy can be converted into other types of energy.

When a spinning ice skater pulls in his arms, he spins faster because _________.

his angular momentum must be conserved, so reducing his radius must increase his speed of rotation. Remember that angular momentum is related to an objects mass times velocity time radius. The skater’s mass stays the same but pulling in his arms reduces his "radius," so his velocity or rotation must increase to keep his angular momentum constant.

The energy attributed to an object by virtue of its motion is known as _________.

kinetic energy. Kinetic energy is energy of motion.

Radiative energy is _________.

energy carried by light. Remember that radiation is often used as a synonym for light, so radiative energy is energy carried by light.

Absolute zero is _________.

0 Kelvin The Kelvin scale starts from absolute zero

What does the term temperature measure?

The average kinetic energy of particles in a substance. Thus, for example, air molecules are moving faster on average on a hot day than on a cool day

In the formula E = mc2, what does E represent?

The mass-energy, or potential energy stored in an object’s mass. Notice that c2 is a large number, so this formula tells us that a small amount of mass contains a large amount of energy.

According to the universal law of gravitation, if you triple the distance between two objects, then the gravitational force between them _________.

decreases by a factor of 9. Gravity follows an inverse square law, so the force goes down with the square of the distance; in this case, increasing the distance by a factor of 3 causes the force to decrease by a factor of 32 = 9.

What is the difference between a bound orbit and an unbound orbit around the Sun?

An object on a bound orbit follows the same path around the Sun over and over, whereas an object on an unbound orbit approaches the Sun just once and then never returns. This is the definition of bound and unbound orbits (around the Sun).

The allowed shapes for the orbits of objects responding only to the force of gravity are ______

ellipses, parabolas, and hyperbolas. Ellipses are bound orbits, and parabolas and hyperbolas are both unbound.

Why is Newton’s version of Kepler’s third law so useful to astronomers?

It can be used to determine the masses of many distant objects. We can apply Newton’s version of Kepler’s third law whenever we observe one object orbiting another; this is the primary way that we measure masses throughout the universe.

What do we mean by the orbital energy of an orbiting object (such as a planet, moon, or satellite)?

Orbital energy is the sum of the object’s kinetic energy and its gravitational potential energy as it moves through its orbit. Therefore, the object’s orbital energy is conserved, even though its kinetic energy and gravitational potential energy may both vary along the orbit.

Which statement must be true for a rocket to travel from Earth to another planet?

It must have large engines.

It must be launched from space, rather than from the ground.

It must carry a lot of extra fuel.

It must attain escape velocity from Earth.

It must attain escape velocity from Earth. If it does not have escape velocity, it will either fall back down or orbit Earth.

Right now, where would you find a tidal bulge on Earth?

On the portion of Earth facing directly toward the Moon and on the portion of Earth facing directly away from the Moon. These are the approximate locations of the two tidal bulges. (Approximate because Earth’s rotation pulls the bulge slightly ahead of the Earth-Moon line.)