Physics Quiz Chapter 4

The Earth rotates on its axis once every 24 hours. Due to this motion, roughly how many full hours would you expect to pass between two subsequent high tides at any given location on the Earth?

12

The difference in gravitational force (tidal forces) across the Earth causes it to stretch toward the Moon, deforming its shape. High tide on the Earth happens at the location of the Earth that is facing the Moon, as the water bulges outward from the stronger gravitational force there, and also at the opposite side. Conversely, low tide happens at the locations 90 degrees from each of the high tides, as shown in the figure below.The Earth rotates on its axis once every 24 hours. Due to this motion, roughly how many full hours would you expect to pass between two subsequent high tides at any given location on the Earth?

12

Newton's second law says that the acceleration (a) of an object depends on its mass (m) by the equation: $$a=Fm​ 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?

C. The force of gravity increases with increased mass.

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.

Given the sense of rotation in the figure, and the location of the Earth's bulge with respect to the Moon, what effect would you expect to come from this reaction force?Choose one:A. The Earth's year will become shorter with timeB. There will be no effect on the Earth from this force.C. The Earth's day will become longer with time.D. The Earth's day will become shorter with time.E. The Earth's year will become longer with time.

The Earth's day will become longer with time.

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.

If these tidal forces naturally cause synchronous rotation for each object, why is the Moon currently in synchronous rotation with the Earth, but the Earth is not in synchronous rotation with the Moon?Choose one:A. The precession of Earth's axis prevents the bulge from aligning with the Moon.B. The Moon is much less massive than the Earth, so it will take longer for it to slow the Earth into synchronous rotation.C. The Sun's gravity pulls in the opposite direction, speeding up Earth's rotation.D. Tidal forces must not be the cause—the Moon was coincidently in synchronous rotation with the Earth when it formed.

The Moon is much less massive than the Earth, so it will take longer for it to slow the Earth into synchronous rotation.

The gravity of this off-center bulge has an effect on the Moon. Study the above figure, and choose the statement below that follows from it.Choose one:A. The Moon takes less time to orbit around the Sun as time passes.B. The Moon takes longer to orbit around the Sun as time passes.C. The Moon used to be closer to Earth than it is now.D. The Moon used to be farther from Earth than it is now.

The Moon used to be closer to Earth than it is now.

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.

Subsequent semi-diurnal (twice daily) tides happen precisely once every 12 hours and 25 minutes. Given what you know of the motions of the Earth and Moon, where is this extra 25 minutes likely coming from?

the orbit of the Moon around the Earth