ap physics questions

Question

An object’s tendency to maintain its current state of motion is due to which property:

A. speed

B.mass

C.weight

D.acceleration

E. all of these

A skydiver leaps from a plane and accelerates toward the surface of the Earth. Earth’s acceleration toward the skydiver is:

A. zero

B.much smaller than the acceleration of the skydiver

C.equal to the acceleration of the skydiver due to Newton’s 3rd Law

D.cannot be determined

Answer 1

I do agree with J. D. that the first question is about fundamentals of physics.

As for me, I do not see in multiple proposed answers to question 1 the right one.

In my opinion the confusion is stemming from English language.

First, we have a phenomenon, and this is an experimental observation: every body has a tendency to maintain its speed. And it does not matter what is the mass of the body. It could be a meteorite or it could be an elephant.

Then we have a property of the body to resist the change in its velocity. This property is different for different bodies and depends on mass of the body. It is much easier to change, say the velocity of the puppy by 1 m/s than of an elephant.

Unfortunately, for both situations we use one word in English language - "inertia". In other languages we have two separate words to distinguish the property of the body and the observed phenomenon.

Now about Newton's First law. This law is about the existence of inertial reference frames (not about the constant velocity of the body, if net force is zero - it is the consequence of Newton's second law).

We are lucky that we have these reference frames, because in inertial reference frames the motion of the body has the simplest form - rest or motion with constant speed along the straight line.

Answer 2

These are pretty fundamental concepts.

1) You have learned the Law of Inertia (Newton's 1st Law). An object maintains its velocity unless acted upon by an external net force. (I prefer to say it this way because "at rest" is just v = 0). The second Law tells you what "inertia" depends on: a (acceleration which is a change in velocity with time) = F_net/m . You can see that an object will not accelerate as much for a given force as m increases (this physical quantity, mass, is the measure of inertia). Inertia, the resistance to change in motion (velocity) is based solely on mass.

2) The 3rd Law tells you that, if the skydiver is attracted to the earth with its weight = mg, the earth must be attracted to the skydiver with the same force. However, the acceleration of the earth a_e = F/m_e whereas the a_sd = F/m_sd = mg/m = g.

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