Question

Which one of these motions is not at constant acceleration?

- A rock falling from a cliff
- A dish resting on a table
- An elevator moving from the second floor to the fifth floor making stops along the way

Answer 1

because of air drag, first one is not constant

and the third one is also not constant because it changes its speed among the way.

Answer 2

but there is only one answer to this problem?

Answer 3

it is probably last one, is not it ?

Answer 4

hmm i dont know, would it be the 2nd bc its not moving at all

Answer 5

second one can not be i think. if we ignore air drag, first one is constant. Otherwise we have to accept that it is not constant acceleration. But it could be last one, if there is only choice.

Answer 6

no those are the 3 mult choices I need to pick one but cant figure it out

Answer 7

i meant that only one is correct.

Think about first one. If there is air drag the acceleration changes in time.

second one must be constant.

third one is not constant as well. Because the elevator has to stop when reach fifth floor therefore its speed changes. So the acceleration changes as well.

understood ?

Answer 8

yeah but I dont know which is correct but I do understand what you are getting at

Answer 9

You would question what constant acceleration means in an introductory course, and in an introductory course you usually neglect air drag (and changing gravity)! So, Neglect air drag (and changing gravity)!

In free fall, we have a constant acceleration of about \(-9.81\ [m/s^2]\), or about \(-10\ [m/s^2]\). The negative means down, when we say up is positive.

The dish on the table has no change in velocity with respect to time, so it has a constant \(0\) acceleration.

The elevator is stopping and starting and stopping, maybe more starting and stopping! So it gains upward velocity to start moving - positive acceleration. Then it slows to a stop - the velocity becomes more negative so that is negative acceleration. It is apparent then, that acceleration is changing if it is going from positive to negative! Haha!

Answer 10

Any questions?

Answer 11

ohh so the answer would be the elevator bc of its movement, its stopping and going.. ect

Answer 12

Right. Its not gaining velocity at a steady rate. It's gaining and losing and gaining.. etc. :) Congrats@

Answer 13

Congrats!*

Answer 14

thank you so much! can you help me on these other 2 problems I have please?

Answer 15

Post them as separate questions so others can see to help! I can take a look, and help if they don't keep me from my homework too much!

Answer 16

okay thank you!