help with forces homework, check please?

[sythetickitty]

Can someone check my answers for these questions? If they are incorrect, can you explain why please? Thank you so much....stuck on homework....late late night of physics. :eek:

1. The figure below shows 4 different boxes labeled A to C. Each box is subjected to two forces as shown.The background grid provides a scale for the magnitude of the forces. The mass of each box is indicated in the figure. Which box has the greatest acceleration (in magnitude)?

Link to image:
http://img209.imageshack.us/img209/3572/quest1co4.png

a. A
b. B
c. C
d. D
e. More information is needed to answer the question

My answer: B

3. A car travels toward the east at constant velocity. The net force on the car is:

a. toward the east
b. toward the west
c. zero

My answer: B


4. Which of the following statements is incorrect?

a. My weight is 80 kg
b. My weight is 200 pounds
c. My weight is 800 N

My answer: A

7. The figure below shows 3 forces ( unlabeled black arrows). Which labeled arrow correctly represents the sum of the 3 forces?

Link to image:
http://img213.imageshack.us/img213/2875/quest7tx8.png

a. A
b. B
c. C
d. None of these. The sum of the forces is zero
e. More information is needed to answer the question

My answer: D

10. A brown box slides horizontally on a frictionless surface with a constant velocity. Which free body diagram correctly represents this situation in the figure below.

Link to image:
http://img81.imageshack.us/img81/1668/quest10kz3.jpg

a. A
b. B
c. Either A or B, they are equivalent
d. More information is needed to answer the question

My answer: A

[Stevedye56]

im almost posative #3 is zero becasue constant velocity is equlibrium

[Rozenwyn]

#1 is wrong as well. How can that box have the greatest acc. when 2 equal and opposing forces are acting on it ?

[sythetickitty]

Ah, I see. So the correct answer is is C then?

[Rozenwyn]

Correct. ;)

[Arbitrary]

I'm quite sure #10 is wrong; it's moving at constant velocity, so that means it's in equilibrium.

[Rozenwyn]

I'm quite sure #10 is wrong; it's moving at constant velocity, so that means it's in equilibrium.

Ditto, I didn't see that his answer for that question was A.

[sythetickitty]

For question #10 -- So C is the right answer? Hmm...it makes sense a bit, but I'm still kind of confused.

[sythetickitty]

A quick question (to Rozenwyn) about the help you gave me with #1. (Trying to understand it better...)
If the question was that which box is subjected to the largest NET FORCE(in magnitude) -- not which has the greatest acceleration....the correct answer would still be C?

Crossing fingers...hoping I'm right and grasping this.

Thanks!

[Rozenwyn]

Yes, remember that Force is directly proportional to acceleration. Assuming that all the boxes have the same mass, the one with greatest net force will also have the greatest rate of acceleration.

And for Q#10, if the surface is frictionless, do you need to have any net force on the box to keep it going ?