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

beer

Lifer
Jun 27, 2000
11,169
1
0
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

Diamond Member
Feb 17, 2000
6,855
3
0
my god those kind of questions give me memories. finals in college are generally 4 questions. you have it easy bro.
 

Unsickle

Golden Member
Feb 1, 2000
1,016
0
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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

Lifer
Jun 27, 2000
11,169
1
0
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

Golden Member
Feb 1, 2000
1,016
0
0
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

Lifer
Oct 14, 1999
18,647
1
81
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

Member
Oct 13, 1999
49
0
0
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

Lifer
Jun 27, 2000
11,169
1
0
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

Member
Mar 31, 2000
152
0
76
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

Banned
Oct 18, 1999
2,478
0
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lol...some of you have too much time on your hands. :p I just woke up and everything was spinning...it was pretty nauseating.
 

Unsickle

Golden Member
Feb 1, 2000
1,016
0
0
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

Diamond Member
Jul 3, 2000
3,299
0
0
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

Lifer
Jun 27, 2000
11,169
1
0
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 =)