According to Newton’s first law of motion, are you at a “state of rest” when you are lying down?

Answer 1

Depends on your bed frame of reference. (Sorry I could not resist.)

In fact, it does depend on your frame of reference. If it is your bed and you are not moving relative to it, then, yep, you are in a state of rest. So your velocity in p = mv is zero and p = mv = 0. Which means, you have no momentum relative to your bed frame.

To change that state of rest, you have to force (pun intended) yourself to move. That is, dp/dt = d(mv)/dt = mdv/dt = f; so that f/m = dv/dt <> 0 in order to change your velocity from zero to something else other than zero. f is that force acting on your mass m. dv/dt is defined as acceleration (a). And it's this acceleration that gets you going as in v = u + at; where u = 0 when you are at rest and v is your new velocity after accelerating for t time.

But look at what happens should we be interested in momentum relative to the center of Earth (a new reference frame). It takes 24 hours for one rotation of Earth, which is about 24,000 miles in circumference. So you, in your bed, are moving at 1,000 mph relative to Earth's center of rotation. Therefore, p = mv > 0 since v > 0 when measured relative to Earth's axis of spin. In which case, you are not in a "state of rest". Even so, if you wish to change that velocity (dv/dt <> 0), you still need to add some force to do that.

Good question.

Answer 2

You might be at a state of rest when you are lying down, but it has nothing with Newton's first law to do.

Answer 3

Yes, because no net force is acting on you.

Granted, gravity and the normal force of your bed are acting on you, but there is no NET force because you're not moving.