Physics Test - how smart are you?

Originally posted by Ramned
Keep thinking about that. Edit - you got it 🙂

[b]Next Set
1. Which has more kinetic energy: A 50 kg truck moving at 20 m/s, or a 25 kg truck moving at 40 m/s?
2. Why can no animal hear sound waves in outer space?
3. When a police car with sirens beeping comes toward you, the police in the car hears the siren and so do you. What is the difference b ...[text shortened]... 4/5 and I'll get them more challenging - next set 10 questions.
Note, m/s = meters per second[/b]

2. There are not enough particles to oscillate, thus create some sound because what we actually hear are the oscillations of our ear-drums which start to oscillate when bumped by oscillating particles. Same goes for animals.

Originally posted by Ramned
[b]Harder!

1. When looking into a concave mirror, is the image you see
A) Virtual B) Real C) Possibly both, real or virtual.
2. True or False: Convex mirrors magnify the images.
3. What produces a higher pitch at the fundamental frequency, a closed pipe or an open pipe? Why?
4. When you see a longitudinal wave, what is the relati ...[text shortened]... ) questions I can think of! Appears that AThousandYoung has common sense / is a good physicist.[/b]

1.A as the image you see is bigger than you though I have no idea what happens when you move closer to it then the focal point is. I'm crap at optics.

Harder!

1. When looking into a concave mirror, is the image you see
A) Virtual B) Real C) Possibly both, real or virtual.
2. True or False: Convex mirrors magnify the images.
3. What produces a higher pitch at the fundamental frequency, a closed pipe or an open pipe? Why?
4. When you see a longitudinal wave, what is the relation between the wave's energy and direction?
5. What is the term that describes the point on a standing wave where little or no movement occurs?
6. True or False: A car going 80 m/s on a circular racetrack is accelerating.
7. Suppose you threw a rock upward at an angle of 45 degrees. When the rock reaches the top of its projectory, are the forces acting upon the rock: A) Horizontal B) Vertical C) Horizontal and Vertical D) No forces are acting on the rock at this point.
8. Why is it colder at night when there are no clouds versus when there are clouds? Explain.
9. An elevator takes 18 passengers up 9 meters every second. The total weight of the passengers is 600 Newtons. Assuming gravity is 10 m/s, what is the Power that the elevator is using?
10. What are the 2 (or 3) factors of a string that cause different pitches when plucked?

If anyone gets all 10 of those, I will put up 5 more questions, the hardest (conceptual) questions I can think of!

So there are the questions.

1,2, 9, 10 have yet to be answered correctly. On #9, just fill in the explain how you would figure it out, don't actaully calculate.

Originally posted by Ramned
You missed about 5:

- #1-2 Mixed up concave vs convex (or guessed wrong) A virtual image is not an actual reflection - light creates the image. On number 1, explain why it is C 😉 (hint abejnood has already said why.)
- 3 - No.
- 4 - No. By Direction, I mean direction of the wave, not energy.
- 9 - Just tell the way you would figure that out.
- 10 - Length/Thickness/Material go together. One more variable.

10) tension

Originally posted by Ramned
Harder!

1. When looking into a concave mirror, is the image you see
A) Virtual B) Real C) Possibly both, real or virtual.
2. True or False: Convex mirrors magnify the images.
3. What produces a higher pitch at the fundamental frequency, a closed pipe or an open pipe? Why?
4. When you see a longitudinal wave, what is the relation between the wave's ...[text shortened]... rectly. On #9, just fill in the explain how you would figure it out, don't actaully calculate.

Well, for number 9, I have already said Power is force times velocity. Allow me to go further and show why: Power is work divided by time. Work is force times distance. Distance over time is velocity. The force is still there. So it goes back to my original equation and becomes force times velocity.

Originally posted by Ramned
Harder!

1. When looking into a concave mirror, is the image you see
A) Virtual B) Real C) Possibly both, real or virtual.
2. True or False: Convex mirrors magnify the images.
3. What produces a higher pitch at the fundamental frequency, a closed pipe or an open pipe? Why?
4. When you see a longitudinal wave, what is the relation between the wave's ...[text shortened]... rectly. On #9, just fill in the explain how you would figure it out, don't actaully calculate.

About 9. you probably mean that gravity is 10 m/s^2 and, boy, those are some small passengers you've got there, 60 kilos and 18 of them. Cats? Kittens?

Anyway, I'd say that the force is a bit more then 600 newtons because it has to be at least the weight of passengers and the lift itself and we cannot forget the frictional force. However it has to be exactly the same as all the previously mentioned forces together so there is no acceleration, because your post stated that it is linear movement.

EDIT: Damn, power, not force is what's asked.

EDIT2: P=A/t, A=S*F => P=S*F/t
S*t=v so we get that Power = Force x Velocity.
Velocity is 9 m/s so P=5400+9x where x is the weight of lift and all equipment that is being lifted and the frictional force to overcome, therefore the power is 5400+9x watts.

EDIT3: P-power, A-work, t-time, S-distance, F-force, v-velocity.

EDIT4: Eh, again, someone was faster.

If you have an object with 5 kilos mass, with how much force do you have to push it against the wall so it doesn't slide down if the friction ratio between the object and wall is 0,6?

Originally posted by kbaumen
If you have an object with 5 kilos mass, with how much force do you have to push it against the wall so it doesn't slide down if the friction ratio between the object and wall is 0,6?

Interesting problem. After some thought, it seems to me that the upward frictional force must equal the downward pull of gravity. Friction is found by the coefficient multiplied by the normal force, and in this case I'm guessing the normal force is supplied by the force we apply to it. So putting all that in equation form, and setting the two forces necessary equal, we get:

F-friction = F-gravity
(coefficient of friction) (Normal Force) = (Mass) (Acceleration due to gravity)

(.6) (N) = (5 kg) ( 9.8 m/s ^2)

N = (49 Newtons) (.6

Normal force = 29.4 Newtons.

Push against the wall with 29.4 Newtons.

Suppose the coefficient of static friction between a car's tires and the ground is 0.700. What is the steepest angle (relative to horizontal) that the car could park on without sliding away?

Originally posted by kbaumen
What to explain? He showed all the way he got to the solution as well as the formula of Kinetic Energy.

EDIT: Where did those posts go?

I editted in the details of #1 afterwards.

Originally posted by Ramned
You missed about 5:

- #1-2 Mixed up concave vs convex (or guessed wrong) A virtual image is not an actual reflection - light creates the image. On number 1, explain why it is C 😉 (hint abejnood has already said why.)
- 3 - No.
- 4 - No. By Direction, I mean direction of the wave, not energy.
- 9 - Just tell the way you would figure that out.
- 10 - Length/Thickness/Material go together. One more variable.

All longitudinal waves have their waves oscillating in the direction of the wave's motion. Therefore the energy cannot vary with the direction of the wave. There is no relationship between wave direction and energy. There is a relationship between wave magnitude and energy. Is that what you meant?

Originally posted by Ramned
Harder!

1. When looking into a concave mirror, is the image you see
A) Virtual B) Real C) Possibly both, real or virtual.
2. True or False: Convex mirrors magnify the images.
3. What produces a higher pitch at the fundamental frequency, a closed pipe or an open pipe? Why?
4. When you see a longitudinal wave, what is the relation between the wave's ...[text shortened]... rectly. On #9, just fill in the explain how you would figure it out, don't actaully calculate.

#9 - I think you changed this one for the worse. Didn't it used to read "average weight" instead of "total weight"?

Power is energy per time. In this case, it's the change in gravitational potential energy per second.

600 Newtons x 9 meters/second = 5400 J/s = 5400 Watts.

Originally posted by abejnood
Interesting problem. After some thought, it seems to me that the upward frictional force must equal the downward pull of gravity. Friction is found by the coefficient multiplied by the normal force, and in this case I'm guessing the normal force is supplied by the force we apply to it. So putting all that in equation form, and setting the two forces necessa ...[text shortened]... = (49 Newtons) (.6

Normal force = 29.4 Newtons.

Push against the wall with 29.4 Newtons.

Correct. It is really an interesting problem, though simple. I had in Mechanics test in 10th grade.

Originally posted by AThousandYoung
#9 - I think you changed this one for the worse. Didn't it used to read "average weight" instead of "total weight"?

Power is energy per time. In this case, it's the change in gravitational potential energy per second.

600 Newtons x 9 meters/second = 5400 J/s = 5400 Watts.

Yes, good.

Power = work / time. Time = 1, so here power = work.
Work = Distance * Force. Distance = 9 so Work = 9*Force
Force = Mass(Acceleration). Acceleration = 10 (gravity.) Mass = 60 (600 is weight -> /10 and 60 is mass). Force = 600.

So Work = 9*600, Work = 5400. Power = 5400. Done. (Isn't that a bit more simple, just use the Force = Mass * Acceration...no calculator necessary.)

So we still have #1-#2. (tension is the other variable in #10). Then I'll post the hardest I know.

Originally posted by Ramned

4. Why is the sky blue?

because if it was green people wouldn't know where to stop mowing their lawn.