physics question

[crispygoat]

please do help me,

A boy is pushing his skateboard in a straight line path across the street. There is a total net force of 50 N acting on the skateboard in the opposite direction to the motion. Is the skateboard

a. decreasing its speed?
b. increasing its speed?
c. moving at constant speed
d. changing its direction?

[Thunderscreech]

How much force is the boy exerting on the skateboard?

And either way, if it is moving in a straight line it is obviously not doing d.

[crispygoat]

so it's b then?

[Thunderscreech]

Originally Posted by crispygoat

so it's b then?

Probably. The question seems to infer that he is applying constant force, and that he not moving backwards.


Physics, if you haven't already figured it out, is so simple it's hard.

[crispygoat]

though you have a point... if he is applying a force of 50 N on the skateboard then it's C otherwise... how am I supposed to know?

[sloasdaylight]

Net force means the total of the two forces, him pushing it, and the resistance due to friction the road provides is 50N in the direction opposite to movement, which means it looks like this nifty character diagram

50N (board)
<-----------<======>

The answer is the board is slowing down.

[Schlyder]

Originally Posted by sloasdaylight

Net force means the total of the two forces, him pushing it, and the resistance due to friction the road provides is 50N in the direction opposite to movement, which means it looks like this nifty character diagram

50N (board)
<-----------<======>

The answer is the board is slowing down.

__________________________________________________ __

you are misreading the question.

- "There is a total net force of 50 N acting on the skateboard in the opposite direction to the motion"

It is NOT the total combined forces of the pushing and resistance.
Only the total net force of the resistance opposite the motion is asked for.

[jonathan_matos5]

is this the hypothetical closed system that is perfectly level and has for some reason no other variables?
50N in opposite direction. would be slowing down given that scenario.

but honestly there is no real closed system.

there is no fixed force on a skateboard.

[Lazylion]

If he wants to avoid the oncoming bus, it better be b.

However, if he's heading for home, but knows his mom is pissed at him, it's probably a.

[jonathan_matos5]

the force is on the board and opposite to its motion.

[SnakeKappele]

If he is pushing it, I would say C. If he is riding it, I would say A, B, and C. When he pushes it increases in speed, then it slows to constant for an instant, and then decreases in speed until he pushes it again.

[sloasdaylight]

Originally Posted by Schlyder

__________________________________________________ __

you are misreading the question.

- "There is a total net force of 50 N acting on the skateboard in the opposite direction to the motion"

It is NOT the total combined forces of the pushing and resistance.
Only the total net force of the resistance opposite the motion is asked for.

Um, unless the op is withholding information in the question, like the Coefficient of friction for this problem, the mass of the kid, and maybe a few other variables I'm forgetting, if we're given only one force that's known, and it's acting opposite the direction of motion, it's slowing down the board.

where forward the forward direction = positives and rearward direction = negative, you have this.

F=MA
F=-50 (acting in the direction opposite to movement)

Suppose for the sake of argument the kid weights 100kilos, we then have
-50=100A or -50/100 = -.5m/s² which is a negative acceleration, thereby causing the board to slow down, making the answer a.

I can't see how this can be interpreted, with what we're given any other way. If some one can explain with some math please do, cause I for the life of me can't figure it out.

[Bigjohn]

a. decreasing it's speed
It is moving in one direction (direction of motion).
There is a net force acting in the opposite direction of motion.
If the net force remains the same, the skateboard will momentarily stop and reverse direction but at the moment, it is decreasing its speed.

Originally Posted by crispygoat

please do help me,

A boy is pushing his skateboard in a straight line path across the street. There is a total net force of 50 N acting on the skateboard in the opposite direction to the motion. Is the skateboard

a. decreasing its speed?
b. increasing its speed?
c. moving at constant speed
d. changing its direction?

[KillerQuebec]

The answer is "The train heading for Chicago."

[champbassist]

If the board is in motion and the net force (the word 'net' implies the sum of all forces: in the direction of motion + against the direction of motion) is against the direction of current motion, the board is surely slowing down.

Let's assume mass of skateboard+ guy on it = m and the forward force on the board = f1 =ma1. If the backward force = f2 = ma2, f2-f1 = 50N, i.e. m(a2-a1) = 50, where a2 is backward acceleration and a1 is forward acceleration. Thus, m(a2-a1)>0, i.e. a2>a1. Thus backward acceleration is more than forward acceleration, and the body is slowing down.

What's with all the math/phy questions popping up in TBOT? Give your mind some exercise, you guys! If you can't figure the thing out even after giving it considerable thought, ask your teacher. Teachers don't get paid for nothing. It's their duty to clarify your doubts.

[karrot-x]

Originally Posted by champbassist

If the board is in motion and the net force (the word 'net' implies the sum of all forces: in the direction of motion + against the direction of motion) is against the direction of current motion, the board is surely slowing down.

Let's assume mass of skateboard+ guy on it = m and the forward force on the board = f1 =ma1. If the backward force = f2 = ma2, f2-f1 = 50N, i.e. m(a2-a1) = 50, where a2 is backward acceleration and a1 is forward acceleration. Thus, m(a2-a1)>0, i.e. a2>a1. Thus backward acceleration is more than forward acceleration, and the body is slowing down.

What's with all the math/phy questions popping up in TBOT? Give your mind some exercise, you guys! If you can't figure the thing out even after giving it considerable thought, ask your teacher. Teachers don't get paid for nothing. It's their duty to clarify your doubts.

Physics professors at my University got paid to give exams. Their favorite answer to any question I ever asked was one of the following:

1. Go to the physics tutoring center
2. I don't have time for this, email me (no responses received)

[champbassist]

Originally Posted by karrot-x

Physics professors at my University got paid to give exams. Their favorite answer to any question I ever asked was one of the following:

1. Go to the physics tutoring center
2. I don't have time for this, email me (no responses received)

That's just unacceptable. I've seen a few teachers/profs of that kind, but by no means have all of them exhibited that attitude. BTW, what's 'the tutoring center'? Is there a special tutoring center at the uni?

[KarateKid25]

Originally Posted by champbassist

That's just unacceptable.

Totally.

My Maths/Physics tutors won't help unless I show them that I have at least attempted the question. Uni isn't there to spoon feed you the information, but if you can show that you've really hit a wall with your study, they will start to help.

The questions that have been popping up in TBOT are really basic questions obviously coming from high school / first year uni type courses. They should be helping with this stuff to get people up to speed for the harder times ahead.

I'd be talking to the faculty in general or a student representative about the problem if they won't help at all.

Originally Posted by champbassist

BTW, what's 'the tutoring center'? Is there a special tutoring center at the uni?

We have 'The Learning Centre' at our Uni. It is just a group of people who put out little 'cheat sheets' and calculator guides etc... Just a basic resource


EDIT: Sometimes, if I'm really stuck with something I'll visit the Physics Forums http://www.physicsforums.com/. There is a place there to post for help with assignment questions. Again though, not many people will help unless you first provide your failed attempt.

[lowfreqgeek]

Sloasdaylight and Champbassist is are right: "net" implies the sum of the total forces acting on the skateboard, so the the skateboard is slowing down.

I'd say some of my worse professors were Physics profs. Well, there was the Psychology teacher who'd only taught high-school before I got into her class. That was awful.

[KeithBMI]

How many bassists does it take to answer a simple physics problem?

There's a punchline in there somewhere...