Chapter 3

Label each mark ed position of Mars with the phrase that most accurately describes its apparent motion in our sky against the background stars.

A retrograde B prograde; moving faster c prograde; moving slower

'Oumuamua reached the orbit of Jupiter about an hour earlier than expected. What does this imply about the object's velocity?

'Oumuamua was accelerated by a force other than gravitational interaction alone.

Imagine that you are standing on the (airless) Moon, and you drop four objects, each the size of a bowling ball. Each is made of a different substance. One object is a pumpkin, and the others are made of Styrofoam, lead, or bubble wrap. In what order do they reach the ground?

None of the answers is correct; the objects all reach the ground at the same time.

When the professor tosses the ball in the air, what forces are acting on it once it leaves her hand?

gravity pulling downward

Suppose a car can go from 0 to 80.00 km/h in only 5.00 seconds. This car's acceleration is approximately:

16.00 km/h/s 80.00km/h divided by 5.00s = 16.00km/h/s

In the video, the loaded car accelerated less than the unloaded car when the same force was applied. This was noticeable because the loaded car

-reached the end of the track more slowly. -was traveling less quickly after a little time had passed.

When a ball is tossed in the air and reaches the top of its arc, which of the following quantities are zero?

-the velocity of the ball -the speed of the ball

Sort each scenario into the category indicating whether or not the car is accelerating.

Accelerating: increased speed, decreasing speed, constant speed on curve Not accelerating: constant speed

Imagine that Earth were in a stable orbit at its current distance from the Sun, but around a different star that had twice the mass of the Sun. Which of the following would have to be true?

Earth's year would be shorter.

In 1500s, Copernicus reintroduced the heliocentric model that the ancient Greeks had rejected, and it began to gain favor among some scientists. He suggested that the planets all orbit around the Sun in perfect circles, as shown in the figure to the right. This offers a much more elegant explanation of retrograde planetary motion than the geocentric model. What would be the next step in the scientific process for the Copernican heliocentric model?

Make predictions from Copernicus's model, and verify them with observations.

The planet in the diagram shown is in an elliptical orbit around the Sun. Rank the average orbital speed of the planet at each shaded orbit segment. The areas in segments A, B, and C are equal. Put the segments from highest to lowest speed

Segment A Segment B Segment C

At present, scientists' best guess as to how 'Oumuamua is accelerating is that it is due to "comet-like outgassing." Outgassing occurs when a jet of gas shoots out from the object. Considering Newton's third law, how would outgassing cause an object like 'Oumuamua to accelerate?

Similar to how a rocket operates, the outgassing pushes forward on 'Oumuamua, and 'Oumuamua pushes back on the outgassing to accelerate the object.

Newton determined that the force of gravity on an object is proportional to its mass, causing its mass to be canceled out (Newton's second law) and resulting in a constant acceleration. If the laws of physics are universal, then Newton should be able to apply his discoveries about gravity on Earth to the motion of the planets in our Solar System. Newton's third law says that for every force there is an equal reaction force in the opposite direction. What does this imply about the force of gravity associated with a person standing on the Earth?

The Earth's gravity pulls downward on the person, and the person's gravity pulls upward on the Earth

Isaac Newton developed a theory of gravity by first considering Galileo's experiment of dropping objects of different masses from the Leaning Tower of Pisa. Galileo found that, when air resistance is negligible, all objects took the same amount of time to hit the ground when dropped from the top of the tower. Watch the AstroTour on Velocity, Acceleration, and Inertia, and then choose the answer that best describes what Galileo's experiment implies.

The acceleration of all objects dropped from the tower is the same.

Newton's second law says that the acceleration (a) of an object depends on its mass (m) by the equation: a=F/m​ F is the force that is causing the object to accelerate. In the case of dropping objects from a height, the force that causes them to accelerate toward the Earth is gravity. If the acceleration of objects dropped from a height does not depend on the object's mass (as shown by Galileo's experiment), what does this imply about the force of gravity?

The force of gravity increases with increased mass.

What can you conclude from this, assuming that the geocentric model holds that the Sun, Moon, and planets like Mars all orbit around Earth in simple circles?

The geocentric model is wrong because it falsely predicts that Mars should always move in the same direction with respect to the stars.

How does the heliocentric model explain the retrograde motion of Mars?

The heliocentric model explains retrograde motion because Mars only appears to move backward as Earth passes it in its orbit around the Sun.

Which of the following most closely explains what we would see from Earth according to the geocentric model that includes epicycles?

The planets would usually move west to east through the stars, but they appear to reverse direction when they are on the part of the epicycle that has motion opposite to that of the larger circle.

Astronauts in a space shuttle can float while orbiting Earth. Why are these astronauts weightless?

They are falling around Earth at the same rate as the shuttle.

Imagine a planet moving in a perfectly circular orbit around the Sun. Is this planet experiencing acceleration?

Yes. It is changing its direction of motion all the time.

Imagine that you are floating on the International Space Station, tossing a bag of dried ice cream to a fellow astronaut. Which of the following are action-reaction pairs?

You push on the ice cream; the ice cream pushes back on you. Earth pulls on you; you pull on Earth. Earth pulls on the space station; the space station pulls on Earth.

Newton's second law of motion states that

the acceleration of an object is proportional to the net force acting on it

If you push with a different force on two cars of the same mass, which car will have the bigger acceleration?

the car you pushed with the larger force

Newton developed a universal law of gravitation that can be used under most circumstances, not just for objects on the surface of the Earth. Here is the law: F=G m1m2/r2 where G is a constant. Think about what happens when you increase and decrease each of the variables in Newton's equation for the gravitational force between two objects of mass m1 and m2, a distance r from one another. Given your results, what is the likely cause of Kepler's observation that planets travel faster when they are closer to the Sun?

the force of gravity is stronger closer to the sun

If you use the same force to push on two different cars, which will have the bigger acceleration?

the less massive one