Question

Last question! Please check my answers for physics! (I only have 1 try, and this one is confusing to me)
http://image.prntscr.com/image/9b67097a521048d793321beae8b867ca.png

Answer 1

#1 is correct.
You have the a force of gravity and an equivalent in magnitude normal force due to the surface. You have friction because as you said the surface is rough, and when you says "pushed" we assume it is moving, so the applied force is greater than friction.

Answer 2

#2, same but to the left.

Answer 3

(Oh, actually for #1 they stated it moves explicitly, no need for assumptions, apologize)

Answer 4

So far, you got #1 and #2, and I am checking the rest.

Answer 5

#3 is approximately correct.
The puck is moving along a smooth surface to the left, but there is still some friction, because friction-less surfaces don't exist.

Answer 6

So, I suppose that what they want you to assume here is that the friction is negligible.

Answer 7

So, then, yes, you are correct for #3 as well.

Answer 8

For #4 ...
You stopped pushing it, so the applied force is not there anymore.
Then, technically, we know it is going to stop at some time, so the friction is present, but having made the assumption that friction is negligible, we will not draw friction.

So, the only forces you have are the normal force and the gravitational force.

Answer 9

So, yes, #3 and #4 are correct, assuming friction is negligible (which is the assumption that they want you to make apparently).

Answer 10

questions?

Answer 11

Thank you for the thorough response! I will review your replies.

Answer 12

Actually and also, this is a good demonstration of what kind of model to use.

You have probably learned projectile motion, and in there we don't consider air resistance (thus making the velocity in the horizontal constant) ... and where would apply this kind of model? When you are flying a bullet it is reasonable! If it is a flying paper towel, then not so much.

Same here - where do you neglect friction and where you don't?
A smooth ice surface (just as it was cleared, but not like 1h later ... (I ice skate pretty often)), it is reasonable to neglect friction.

Answer 13

In any case, don't want to overtype it, although I might have already done so.
Good luck with physics!

Answer 14

Alright, thank you!

Answer 15

One last thing, I am starting to doubt my answer for question number 2. Since the puck has left the stick, the only horizontal force is friction, making it choice 7?

Answer 16

@SolomonZelman

Answer 17

@sooobored

Answer 18

ok, the arrow directions are a bit...
left right is the x-axis and up down is z-axis (or gravity?)

Answer 19

Yeah

Answer 20

ok looking at #2
after it has left the stick means that in only have a velocity in the positive direction (which is not an acceleration)
so no force in the +x direction

the puck has a mass hence gravitional force down exists
and the surface will apply a normal force upwards on it

frictional force will exist in the direction opposite to the velocity hence -x

your choice of 7 encompasses all these forces

13 doesnt make much sense since there is no surface continuously apply an extra force in the +x direction

Answer 21

Thank you, choice 7 was indeed correct.
http://image.prntscr.com/image/3f9dab3753b345278d65a211dcc8bb49.png

Answer 22

Yups, you got it all. Friction-less models:)
Physics is the study of assumptions, this is how I at times tend to think of it.
(You'll see later on in physics models assuming \(\sin(x)\approx \tan(x)\) for small \(x\),
or those that treat houses like point objects .... ah, I am not a great lover of it, though.)

Answer 23

GL:)

Answer 24

Thanks, I appreciate it!

Answer 25

It all was you who got it correctly, the only thing I did is a check:) \(y(\omega)\).