physics- who is correct?

Question

For these questions I have got different answers so I wanted to know which were correct.

1. The acceleration due to gravity is
A. Always directed toward the earth and therefore negative
B. Positive when velocity is positive
C. Negative when velocity is negative
D. B and C
E. None of these

(I got A and E)

18. Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his jello with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. With whom do you agree and why?
A. Mac because greater speed means greater inertia.
B. Tosh because inertia depends upon acceleration.
C. Tosh because mass is a measure of inertia.
D. Mac because greater speed means greater force and force is directly proportional to inertia.
E. None of these

(I got C and A)

20. If we divide a Newton by a square meter the units that result are units of
A. Speed
B. Force
C. Mass
D. Acceleration
E. Pressure

Answer 1

The force of gravity (which causes acceleration; F = m * a) acts towards the center of mass. The “negative” and “positive” directions can be arbitrarily defined as one wishes…physically it makes no difference if you call “up” positive or negative as long as you are consistent with yourself and use the appropriate signs for other directions.
Often, we call the “positive” direction “up”, and since gravity pulls objects towards the Earth’s center of mass (since the mass of the object is usually negligible by comparison), gravity pulls us “down” / in the “negative” direction.
I think the answer which question #1 is looking for is choice A, but depending on how strictly/loosely you read the question it could also be choice E….it depends on if you are comfortable with the idea that the direction pointing “towards the Earth” is always negative, when in fact this is just an arbitrarily imposed sign on our part.

We can use mass as a measure of an object’s inertia, resistance to changes in its motion.
The answer which question #18 is looking for is choice C.

Units of Force divided by units of Area gives units of Pressure.
1 Newton per square meter is equal to 1 Pascal of pressure. You may be familiar with the unit of pressure “psi”, meaning “pounds [of force] per square inch”
The answer to #20 is choice E.

Answer 2

1)E, the sign of gravity is arbitrary and depends on the particular coordinate system used. For instance we usually say up is the positive y direction. In this case the acceleration due to gravity is negative. But it is equally valid to say the down is the positive y direction. In this case the acceleration due to gravity would be in the positive direction.

18)A, inertia depends on both speed and mass. An object traveling at grater speeds will have a greater inertia and similarly a object with greater mass will have greater inertia. Mac is correct but Tosh is incorrect because he said "inertia does not depend of speed."

20) E, Pressure is defined as force per unit area, so if you divide force by area you get pressure.

Answer 3

1. D (if a rocket is moving toward the earth, then gravity adds acceleration (pos acc), and if a rocket is moving away from the earth, then gravity takes away from the acceleration (neg acc.)

18. E (Inertia means an object in motion will keep moving or not in motion will not move...nothing to do with anything...there is no equation for inertia is there?)

20. E Work, so Pressure is closest to Work.

Answer 4

A, C

Acceleration due to gravity is negative.

Mass is a measure of inertia.

Answer 5

1. D seems to make the most sense. Initially I thought A, but the "therefore negative" negates my initial impulse.
18. I would stuck between B and C, but leaning towards C.
20. E (Pressure)