OMG,I'm so F*KED - PHYSICS HELP

[beer]

I know all you EE majors will laugh at me but I have my Physics I Honors (high school) final in about say 8 hours and I just downloaded a review from our Physics depts web site.

None of this makes any sense to me.
I spent a semester in this class and I can answer about 3 of 30 problems.

Here is our review.

It's just basic kinematics and dynamics.

If you all could help me by morning it will save me.

Right now I don't understand questions 8, 11 and 12 (I know its uniform circular motion, I think, but I don't know what equations to use), 13 and 14, and 18.

My answers are as follows:

1. B
2. D
3. E
4. C (not sure)
5. B
6. C
7. D
8. D (pretty sure)
9. havent worked it out yet
10. D
11. dunno
12. dunno
13. dunno
14. see above
15. omitted per teacher's request
16. omitted
17. D
18. I think E
19. B (again, not sure)
20. C
21. A
22. omitted

If anyone can help me by morning...I will erect a statue of them in my front yard, I kid you not...

 

[Passions]

my god those kind of questions give me memories. finals in college are generally 4 questions. you have it easy bro.

 

[Unsickle]

I'm pretty much 100% sure. Mental math for me (3rd year engineering major).

4. D
11. v=sqrt(2*g*h)
12. m*v^2/r+m*g
13. m1*g
14. m1*g
15. E
16. A
17. D
18. B
19. B

 

[beer]

And you're sure of 4?
Your acceleration may be down but isn't there a normal force exerted by the ramp in the direction of 3, more or less?


11 still does not make sense. I realize now that looking at it I would be using K = 1/2 mv^2 and that at point Y, as a point of reference, would be all KE and X and Z would be all PE. Thus the equation that would solve for V would be V = sqrt((2*KE)/m) correct?

The problem is how much KE do I have in the system to solve for V?

Regarding 18 - how?

So really, I understand (read: remember) everything now except 18, and 11-14.

 

[Unsickle]

Correction: 4 is C. The only acceleration is g*sin(theta) the cosine component and normal force precisely cancel out.

11. At X, all energy in PE = m*g*h. At Y, all energy in KE = (1/2)*m*v^2. Set them equal and solve for v. It's a standard factor. In fluid mechanics they even have a name for it... something Frouhard or something.

18. When you're in an elevator going up and it comes to a stop, you feel lighter. In your frame of reference the elevator is essentially accelerating downwards. That acceleration is equal to dV/dt, which in this case is (in magnitude) (8-2)/3 = 2 m/s^2. Assuming g = 10 m/s^, the elevator floor will only have to accelerate you at a = 10 - 2 = 8 m/s^2 during this interval. F=ma gives you the answer.

 

AHHH AP physics in highschool.
/me falls over on the floor and foams at the mouth

 

[Mday]

this is for during the actual test.

if you left anything blank on a multiple choice exam, you are dumb.

GUESS!!! ;-) unless it's hand graded requiring some sort of reason.

 

[callspread]

1 - A A short gap between the dots means greater change in velocity means a steeper slope on the graph.
2 - E misleading, since the acceleration does not appear to be linear
3 - E
4 - C if it were D(5) it could only travel down through the ramp
5 - E the only force present is gravity - and the block can't move lower, so it can't accelerate.
6 - C
7 - C F has a vertical component and a horizontal component, so must be larger than k which is smaller than the horizontal component alone. W must be greater than N because it has to resist the pull of N and the vertical component of F
8 - D
9 - A
10 - C since there is no friction her downwards acceleration will be the same on each, so she want to be on the longest slide - because it will take her the same amount of time no matter how long the slide.
11 - E KE = 0.5*M*V^2 PE = M*G*H(eight)
12 - E Tension = M*G
13 - B force = acceleration*mass. Speed is const, so acc. = gravity only
14 - B "
15 - E
16 - A
17 - D
18 - ?
19 - C
20 - ?
21 - A

 

[beer]

Whew
Printed it out, will go over it after my internetworking I final. Good thing that will be easy, cram for physics after.


YOU ALL F'IN RULE!!!!!!!!!!!!!!

 

[TMTCC]

Woohoo...gotta love mechanics
Here's my contribution

1) B: Ball steadily increase to a constant velocity, then the velocity drops off quickly at the end.
2) D: Acceleration is the derivative of the velocity (ie. Acceleration is given by the steepness of the velocity slope), so given B as my first answer, D is the answer for this one
3) Using my explanation for 2, E must be the answer.
4) C: Unsickle has the explanation
5) E: Callspread's explanation
6) C: Gravity is the only force acting on the block at this point
7) D: F is broken to a horizontal and verticle component. The horizontal component must equal "k" since the block isn't accelerated, so overall, F > k. Normal force cancelling gravity gives N=W.
8) D: The cylinder needs to accelerate towards the middle of the circle in order to keep the circular path. Hence, the force is also in the direction.
9) A: Rotational force (mv^2)/r must equal the frictional force (mgk). Solving for v: v = sqrt(rgk) ~ .48 m/s.
10) E: Conservation of energy
11) E: Look at unsickle's explanation
12) E: Callspread has the right idea, but I think he forgot to take into consideration the rotational force of the boy. At Y, you have gravitational force (mg) and rotational force (mv^2 / r)
13) B
14) B: Look at callspreads explanation for 13 and 14
15&16) Omitted
17) D: Equilizing and setting forces equal: ma = 3ma' and solving for a' given a = 3
18) B: look at Unsickle
19) C: need to cancal F force with equal and opposite force and create upward force, hence C
20) B: The table gives this problem a little twist, but remember, it's frictionless. Best way to conceptually see this is if you drop 2 objects in the air (say a penny and a brick). They both fall at the same rate, thus will have the same velocity at the end. KE = .5mv^2, both have the same velocity, but number II has more mass.
21) C: Both will have same velocity at the end.
22) B: Momentum = mv, so II wins given their equal velocities
23) D: Avg Acceleration is change in velocity over time, so 4 m/s over 6 sec is .67 m/s^2
24) A: There may be a quicker way to see this, but this method also works. Solve for the distances in 2 parts, from 0 to 2 sec and 2 to 6 sec. You see that the acceleration of the first part is 1 m/s^2 (an increase of 1 m/s takes 1 sec). Displacement = (initial velocity * times) + (.5 * acceleration * time^2), and if you solve for that in the first part you get 4 meters. The acceleration for part 2 is .5 m/s^2 (1 m/s increase over 2 seconds). Plug into the equation above and get 16 m. Remember to check the initial velocities.
25) A: Given 20 m from above, divide by 6 and get 3.3 m/s
26) E: Just remember that gravity is the only force acting on the ball from Y to Z. A is the incorrect statement which disqualifies D since the ball continually gets accelerated until the instant it loses contact with the spring.

Whew! Hope that helps!

 

[loogie]

lol...some of you have too much time on your hands. I just woke up and everything was spinning...it was pretty nauseating.

 

[Prodigy^]

Question: why the feck do all uni's use that dreaded .pdf format?

 

[Unsickle]

PDF is the best format for distributing documents with proper formatting. Don't give me LateX crap, or word for that matter.

PDF is the way to go, second to PS

 

[thelanx]

I'm taking Physics 1 at my high school, but I can't read .pdf format. And I'm not sure I could help you if I could.

 

[beer]

The final was actually pretty easy.

However looking at #5, no one figured out that at that point, the ramp is shaped like an arc is a circle. Thus, laws of centripital acceleration take place and the box accelerates UPWARDS.

Other than that, TMTCC did a great job. Thanks, man =)