OAPT Physics Contest 2011 - Practice

[whoareyou]

So, because a lot of people here are interested in physics and I need help practicing for the upcoming physics competition, I am going to post the 2010 contest, and any other questions that I may need help on.

This could give other members something to do in their spare time, and will also help me to study!

*EDIT: So apparently, there isn't an option to upload anything here, so I will post a link to it.
*Link to the OAPT 2010 Contest - uploaded by me: http://www.mediafire.com/?7fxmpmka8qafcgh

[whoareyou]

Question 10


It would be E right, because Fg = mg, so 12.2kg x 9.81N/kg = 120N ?

[RandomLetters]

Did you consider the verticle component of Fa?

Also, I just realized how much it sucks to have 2nd sem physics, because I am totally clueless regarding the electricity questions,

[whoareyou]

Are you writing this contest too ?

I had physics first semester, so I (think I) know everything!

And thanks for the advice - I forgot to consider the vertical component. By the way, how would you implement that into the equation? We never did any kind of force problems with angles in them.

[RandomLetters]

Forces with angles can be broken down into component vectors.

You can use trig to break down forces exactly like vectors.

Then, you would just use the verticle component separately, so Fn = Fg + Fay
(while the horizontal motion would use Fax and Ff)

[whoareyou]

That wouldn't work ... because Fg + FAy > 120N which is not an option.

[RandomLetters]

Fg is downwards, FAy is upwards, so the magnitude is subtracted, giving 70N down. Sorry, I should have said they were vectors.

[andrew.]

Basically:
F_Ay + F_N = F_g
F_Ax - F_r = ma

You only need the first equation. Solve for F_N to get F_N = 69.7 N.

[whoareyou]

Thanks for the guidance!

The following questions I am having troubles with.

QUESTION 5


QUESTION 6


QUESTION 7

I would think A or B since the passenger would exert a force downwards on the helicopter while the helicopter is flying up.

QUESTION 9

I would think A because both the dog and the flatcar are accelerating, so the dog is moving with the flatcar, so there would be no force. Although, since there is acceleration, the net force of the dog shouldn't be zero.

[A.J]

Don't merely state that you are having trouble with them. Instead, you should also include any headway you made with any of them. All the questions you posted seem to be ordinary Kinematics and Dynamics problems, not contest problems.

[DemonWasp]

Question 5: You know v1 = 0, d1 = 20. Use the equation d = v1 * t + 1/2 * a * t ^ 2 . You know that since v1 = 0, you can cross out the first term. So you have d1 = 1/2 * a * t ^ 2. If you double the time period, you have to double the value of t, so we would write d2 = 1/2 * a * (2*t) ^ 2. Solve that for d2, which you will find to be in terms of a and t. Look at the similarities between the equation you have now and the first one we wrote down, then think about what you can do with them. Remember that d2 is the distance travelled in BOTH time periods.

Question 6: Use conservation of energy for each rock, solve for v2. Compare.

Question 7: When the helicopter flies up, do you feel a force pushing you into the chair, or a force lifting you out of the chair? The answer to that should let you choose whether it's A or B.

Question 9: Since the dog is accelerating at the same rate as the flatcar (and the question doesn't mention anything else), we can assume that friction is doing all the work. Calculate the force required to produce that acceleration in the dog and you will have your answer. This is partially a trick question too, since you don't actually need the coefficients of friction. They'd be useful if the question was whether the dog slipped or not, but it isn't.

[whoareyou]

I have attempted all the questions.





For question 7, I would think this: since Fnet = ma, the mass of the of the passenger are m, and the total acceleration of the passenger would be (g+a) as vectors, so the correct answer would be A.

[DemonWasp]

Good work.

[whoareyou]

This question was taken from the 2009 OAPT Physics Contest, and my answer is wrong.

[RandomLetters]

Average velocity is:
v = Ad/At
We know that each velocity lasts for Ad/2, so we need to find At.
How do we find At, since a slower velocity takes longer to traverse Ad than a faster velocity?

(break it down to two movements for each velocity to find each At)