Physics Chapter 9

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What did Newton discover about gravity?

1) The equation for the force of gravity 2) The law of universal gravitation 3) The Newtonian synthesis

What is the Newtonian synthesis?

The union of terrestrial laws and cosmic laws

In what sense does the Moon "fall"?

It falls away from the straight path that it would follow if there were no forces acting on it.

State Newton’s law of universal gravitation in words. Then do the same with one equation

The force is proportional to the product of two masses and inversely proportional to the square of the distance between their center: F ~ m1m2/d^2

What is the magnitude of the gravitational force between two 1-kg bodies that are 1 m apart?

6.67 x 10^-11

What is the magnitude of Earth’s gravitational force on a 1-kg body at Earth’s surface?

10 kg

When G was first measured by Henry Cavendish, newspapers of the time hailed his experiment as the "weighing Earth experiment." Why?

With a known mass, a scale, and the radius of Earth, you can calculate the unknown mass of Earth if you know G.

How does the force of gravity between two bodies change when the distance between them is doubled?

The force decreases to 1/4 of its initial value

How does the thickness of paint sprayed on a surface change when the sprayer is held twice as far away?

It is 1/4 as thick

Where do you weigh more: at the bottom of Death Valley or atop one of the peaks of the Sierra Nevada? Why?

You weigh more in Death Valley because you are closer to the center of Earth

Would the springs inside a bathroom scale be more compressed or less compressed if you weighed yourself in an elevator that was accelerating upward? Downward?

More compressed while accelerating upward, but less compressed while accelerating downward

Would the springs inside a bathroom scale be more compressed or less compressed if you weighed yourself in an elevator that was moving upward at constant velocity? Downward at constant velocity?

There would be no more compression and no more expansion for both upward and downward motion

When is your weight measured as mg?

For a non-accelerating mass near the surface of Earth

Give an example of when your weight is greater than mg. Give an example of when your weight is zero.

Your weight is greater than mg when you are accelerating upward. Your weight is zero when you are in free fall

Why are occupants of the International Space Station weightless ?

They are in free fall

Do tides depend more on the strength of gravitational pull or on the difference in strengths? Explain.

Tides depend only on the difference

Describe how the gravitational forces from the Sun and the Moon compare from one side of Earth to the other

The Sun exerts stronger force on the side of Earth nearest the Sun, and the Moon exerts a stronger force on the side nearest the Moon

Which has the higher tides: spring tides or neap tides? Why?

Spring tides are higher, because the tides from the Moon and Sun add together

Do tides occur in the molten interior of Earth for the same reason that tides occur in the oceans?

yes, due to the difference in gravitational force across the core

Why are all tides greatest at the time of a full Moon or new Moon?

At full Moon and new Moon, the tides from the Moon and the Sun add because they are in line with Earth

Would a torque on the Moon occur if the Moon were spherical, with both its center of mass and center of gravity in the same location? Why?

No. The torque requires a distance between the center of mass and the center of gravity

What is a gravitational field, and how can its strength be measured?

It is a force on any body with mass. its strength is the force per unit mass on a test mass

What is the magnitude of the gravitational field at Earth center?

Zero N/kg

For a planet of uniform density, how would the magnitude of the gravitational field halfway to the center compare with the field at the surface?


What would the magnitude of the gravitational field be anywhere inside a hollow, spherical planet?

Zero N/kg

Newton viewed the curving of the path of a planet as being caused by a force acting on the planet. How did Einstein view the curved path of a planet?

Moving in curved 4-dimensional spacetime

If Earth shrank, but there was no change in its mass, then what would happen to your weight at the surface?

It would increase

What happens to the strength of the gravitational field at the surface of a star that shrinks?

It increases

Why is a black hole invisible?

The escape velocity of the black hole is greater than the speed of light

What was the cause of perturbations discovered in the orbit of the planet Uranus? What later discovery did this lead to?

Neptune caused the perturbations, and later Pluto was discovered

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