# A Few Homework Problems To Go

1. May 9, 2006

### thunder

Going through these last few multiple choice HW problems tonight (about 50 of them total to finish tonight and I am down to these alst few)...and wondering if these are correct:

1. Newton's First law states...

a) An object can only move in a circle if external forces act on it.
b) An object can move in a circular path without external forces acting on it.
c) it does not matter whether external forces act on it.

2. If a non-zero constant net force in the horizontal direction acts on a body at rest on a horizontal frictionless surface, the body will...

a) sometimes accelerate
b) always move with constant velocity
c) always move with constant acceleration
d) accelerate when force exceeds weight

not sure if this is a trick question or not, because d) looks tempting, but I am thinking c) since it is frictionless surface and force is constant (but not sure on this one though)

3.A person stands on a scale in an elevator which is accelerating upward. The person's apparent weight as read on the scale would:

a) stay the same
b) decrease
c)increase
d)depends on other factors

I am thinking answer c) increase

4. At the top of the trajectory of a projectile, the y component of velocity is ZERO. The acceleration at that point is:

a) ZERO
b) - g
c) depends on elevation

I am thinking answer b) - g

5. At any time, the instantaneous velocity vector:

a) can point in any direction
b) always points in the direction of motion
c) always points at 90 degrees to direction of motion
d) always points in the direction of acceleration

6. A person with mass of 150 lbs. and a person with mass of 100 lbs. swing in identical swings. Once they get started, the person with the larger mass will have:
a) smaller period T
b) larger period T
c) same period T

I'm thinking answer a) larger period T

7. Two loud speakers radiate the same frequency Fo IN PHASE. A person moves about in front of the speakers until she hears a minimum in sound intensity. At this point, the difference in path length of sound from each speaker to the person's ears is:

a) 1/4 wavelength
b) 1/2 wavelength
c) wavelength
d0 3 wavelength/4

This is stumping me...but I'm thinking the difference is just one wavelength so that it is in phase.

OK, so that was 7, but I got the other 43 finished and verified those answers are correct. these 7 were the tough ones. Can you let me know how I ma doing on these? Thanks!! Hope these don't stump you all, too!!

Thanks!!

2. May 9, 2006

### lightgrav

Most okay ...
2c . weight is vertical, isn't it? (the surface keeps it from falling)

6 is like dropping a heavy object and a light one (which falls fastest?)
. . . since the driving Force is gravity, it is stronger for a larger mass
. . . but the larger mass has more INERTia to overcome.
gravity's Force F = mg , which *causes* the mass to accelerate "= ma"

7. If the waves are IN phase, both hit you with high pressure at the same time, then both pull with low pressure at the same time (half a cycle later).
You think this will result in MINIMUM Power to your eardrum? (Intensity is related to loudness or brightness Power detected).

3. May 10, 2006

### thunder

Hey thanks!!

Problem #2, I believe, is assuming an object is on perhaps an air track...(frictionless)...and moving horizontally...

So then for #2, it is answer c....??

And for #6...then then mass does not affect the frequency of the period T?? so then the correct answer for #6 would be answer c) same period T. correct??

and #7) the distance is 1/2 of the wavelength? instead of a wavelength?

correct??

Last edited: May 10, 2006
4. May 10, 2006

### thunder

**bump** will check back later~~~ thanks!! :)

5. May 10, 2006

### Hootenanny

Staff Emeritus
Yep, they're all correct.

~H