Homework Help: Double check multi-choice questions on concepts

1. Oct 23, 2013

AFSteph

Hello again! This time I have a bunch of multiple choice questions, some of which I'm really not sure on, so I'd really appreciate a little help. Questions I'm really unsure about will be marked in red.

In these questions we are to assume g = 10 m/s$^{2}$ unless otherwise specified.

"Bobsled racing is a popular Olympic sport. Questions 10-14 ask you to apply what you have learned about Newton's laws in both Part One and Part Two to this sport. In answering the questions you should neglect the force that the bobsled and the ground exert on each other."

10. "A bobsled at rest will not move until someone pushes it. This is an example of which of Newton's laws?"
(A) Newton's first law (B) Newton's second law (C) Newton's third law (D) None of these
My answer: (A) Newton's first law

11. "Two men push a bobsled for 15 meters and then jump on the bobsled at the starting line. During their first run two men push the bobsled with a force of 8N before they jump on. During their second run the same men push with a force of 10N and are going faster when they jump on than were during in their first run. This is an example of which of Newton's laws?"
(A) Newton's first law (B) Newton's second law (C) Newton's third law (D) None of these
My answer: (B) Newton's second law

12. "At the bottom of the bobsled course, the bobsled collides with straw bales, which help slow it to a stop. The force exerted by the bobsled on the bales is exactly the same as the force exerted by the bales on the bobsled. This is an example of which of Newton's laws?"
(A) Newton's first law (B) Newton's second law (C) Newton's third law (D) None of these
My answer: (C) Newton's third law

13. "If a bobsled hitting a straw bale is the action force, what is the reaction force?
(A) force of gravity on the bobsled (the bobsled's weight) (B) the mass of bobsled
(C) the force exerted by the straw bale on the bobsled (D) air resistance (E)none of the above
My answer: (C) the force exerted by the straw bale on the bobsled

14. "When the bobsled hits the straw bales, the bobsled recoils backwards, but the straw bales move only slightly. This is because (A) the bobsled exerts no force on the straw bales (B) the straw bales exert a greater force on the bobsled than the bobsled exerts on the straw bales (C) the mass of the straw bales is greater than the mass of the bobsled (D) none of these
My answer: (C) the mass of the straw bales is greater than the mass of the bobsled.

15. "The net force acting on an object results in an acceleration of 2 m/s$^{2}$. If both the net force action on the object and the mass of the object are doubled, what will be the resulting acceleration? (A) 2 m/s$^{2}$ (B) 4 m/s$^{2}$ (C) 8 m/s$^{2}$ (D) 1/2 m/s$^{2}$ (E) None of these
My answer: I think it's A but I'm not sure... a = F/m so doubling both should give the same result, right?

16. "If the net force acting on an object is 0, what can you conclude the object's momentum and acceleration?" (A) The momentum and acceleration of the object are 0. (B) The momentum of the object is constant and and the acceleration of the object is 0 (C) The momentum of the object is 0 and the acceleration is a constant 10 m/s$^{2}$. (D) None of these
My answer: I think it's B but I'm not sure... This question confuses me the most.

I have more questions but that's enough for now. Any help would be much appreciated!

2. Oct 23, 2013

Simon Bridge

That's great - for each one, please provide an example of your reasoning.
i.e. for 10. what makes you think of Newtons first law here?
Can you make a simple statement, in words, each of Newton's laws and compare them to the questions?

#13. action-reaction pairs cannot cancel out.

#15. you gave an example of your reasoning.
A good way to check is to put in the math.
In the first case you have a1=F/m
In the second case you have force 2F and mass 2m so a2=2F/2m=F/m = a1 ... so your reasoning was correct.

#16. if you showed your reasoning then I could see how you got confused, and help you with that.

3. Oct 26, 2013

AFSteph

Thank you for taking the time to comment. Now that I've mulled it over a bit, it's obvious it's B. I didn't really understand what momentum was (mass x velocity). So an object with a net force acting on it of 0 could be moving at constant velocity or at rest, have no acceleration and a constant velocity/momentum.

Next time I will make sure to include my reasoning. Thank you.

4. Oct 27, 2013

Simon Bridge

Well done.
You'll also find that having to explain your reasoning to someone else will often get you the answer to the question too.