Physics Unit 9 Flashcards
Unit 9 Q
The following five diagrams show pairs of astronomical objects that are all separated by the same distance d . Assume the asteroids are all identical and relatively small, just a few kilometers across. Considering only the two objects shown in each pair, rank the strength, from strongest to weakest, of the gravitational force acting on the asteroid on the left.
Here are some comparisons for the masses of the objects on the right:
* The Sun’s mass is about 330,000 times the mass of the Earth.
* Earth’s mass is about 80 times the mass of the Moon.
* The Moon’s mass is about a million times that of a typical small asteroid.
* A typical small asteroid has a mass at least a million trillion trillion trillion times that of a hydrogen atom.
With this information and an understanding of the factors that determine the strength of gravity, you should be able to complete Part A
Unit 9 Information Question
What does the strength of gravity depend on?
The force of gravity follows an inverse square law, meaning that the strength of the force declines with the square of the distance between two masses. But if the distances between pairs of objects are all the same, as in Part A, then the strength of gravity depends only on ______.
the product of the two object masses
( M1×M2 )
Correct
Because the distance is the same for all five object pairs shown in Part A, the gravitational force depends only on the product of the masses. Moreover, notice that the asteroid on the left has the same mass in all five cases. With these facts in mind, you should be able to answer Part A.
According to Newton’s third law __________.
the strength of the force that the object on the left exerts on the object on the right has to be exactly the same (but in an opposite direction) as the force the object on the right exerts on the object on the left
Correct
Newton’s third law tells us that every force has an equal and opposite reaction force, which means the objects attract each other gravitationally; therefore, the gravitational force acting on the object on the left is always equally strong as the gravitational force acting on the object on the right (considering only these two objects). With this idea in mind, think about how your ranking for Part A is related to the ranking for Part B.
Unit 9 Q: The following diagrams are the same as those from Part A. This time, rank the pairs from left to right based on the size of the acceleration the asteroid on the left would have due to the gravitational force exerted on it by the object on the right, from largest to smallest.
According to Newton’s second law, the asteroid with the largest acceleration will be the one that has the strongest gravitational force exerted on it by the object on the right. That is why the ranking here is the same as the ranking for Part A.
Unit 9: According to Newton’s second law, the greater the force exerted on an object, the greater the object’s _____
acceleration
Unit 9 Q: Newton’s second law of motion states that force equals mass times acceleration, or F=ma. Suppose you have already calculated the gravitational force, which we will call Fg , attracting Earth and the Moon. Then the amount of acceleration of Earth due to this force is __________.
Fg divided by the mass of the Earth.
Correct
To find the acceleration of any object, we set F=m object a object. Dividing both sides by the object mass, we therefore find that the acceleration of the object is F/m object. The force in this case is the force of gravity between Earth and the Moon, so we divide this force by Earth’s mass to find Earth’s acceleration. Similarly, we would find the Moon’s acceleration by dividing the same force (because the force of gravity must be the same on both objects) by the Moon’s mass.
Unit 9 Q: Consider Earth and the Moon. As you should now realize, the gravitational force that Earth exerts on the Moon is equal and opposite to that which the Moon exerts on Earth. Therefore, according to Newton’s second law of motion __________.
the Moon has a larger acceleration than Earth, because it has a smaller mass
Correct
Newton’s second law of motion, F=ma, means that for a particular force F, the product mass x acceleration must always be the same. Therefore if mass is larger, acceleration must be smaller, and vice versa.
Unit 9 Q: The constant G in Newton’s equation _______.
makes the units of measurement consistent
Unit 9 Q: Each of the following diagrams shows a spaceship somewhere along the way between Earth and the Moon (not to scale); the midpoint of the distance is marked to make it easier to see how the locations compare. Assume the spaceship has the same mass throughout the trip (that is, it is not burning any fuel). Rank the five positions of the spaceship from left to right based on the strength of the gravitational force that Earth exerts on the spaceship, from strongest to weakest.
Gravity follows an inverse square law with distance, which means the force of gravity between Earth and the spaceship weakens as the spaceship gets farther from Earth.
