# A few mc questions, pls help

1. Aug 13, 2009

### roy219

1. http://i159.photobucket.com/albums/t152/roy219_2007/DSC04589.jpg,
2.http://i159.photobucket.com/albums/t152/roy219_2007/DSC04590.jpg,
3.http://i159.photobucket.com/albums/t152/roy219_2007/DSC04591.jpg
http://i159.photobucket.com/albums/t152/roy219_2007/DSC04593-2.jpg

2. ft=mv-mu, mgsinX

3. for question 18, i know statement 2. when the angle decrease, mgsinX will be smaller so there is a force rise the block. for the statement one and two, i have no ideas ><

the second question is asked about resultant force. in fact, i just know it is a constant force. however, i dont know how to read these graphs.

the last question, i know water wave travel from shallow to deep region, its wavelength and velocity will change, and only frequency will stay the same, isn't it? but the ans is a both statements are correct..thy? >_<

2. Aug 13, 2009

### kuruman

For 1 you need to draw a separate free body diagram for each of the masses. The tension in the rope must be the same on either side of the pulley. Since the masses do not accelerate, the net force on each must be zero. If you look at the hanging mass, can you find the tension in the rope? Break up the weight of the mass on the incline into two components, one parallel to the incline and one perpendicular. The tension must be equal in magnitude and opposite in direction to the component of the weight parallel to the incline. The "normal reaction" is equal in magnitude and opposite in direction to the perpendicular component.

Graph 1 shows distance s vs. time. It has a constant slope. What is this slope also known as?

Graph 2 shows velocity v vs. time. It too has a constant slope. What is this slope also known as?

Graph 3 shows acceleration a vs. time. The acceleration has a constant value. What does this say about the net force?

This should get you started.

3. Aug 13, 2009

### roy219

hiiiii kuruman =)

for 1, the hanging block' mg=tension of the rope,
so tension = m2g
cause two blocks are at rest, force which is parallel to the incline is m1sinx
so would it be possible if statement one is correct?
but for question, the ans is C , just 2and 3 are correct. why??

um..for the graphs
graph 1 is st graph, that physics meaning of its constant slope means the movin object
has a constant velocity.
graph 2 is vt graph. physics meaning of its constant slope means it has a constant acceleration.
graph 3 just like graph 2, has a constant acceleration

so i assume there is a constant force acts on a odjects,i think the motion could be represented by all of them, but ans is just graphs 2 and 3
it makes me confused><

4. Aug 13, 2009

### kuruman

If θ increases, does sinθ increase or decrease? What about cosθ? Figure this out then look at

m1g sinθ = m2g

(By the way, the above is the correct expression. What you have been given is missing a "g" on the left side.)

When the angle changes, the left side increases or decreases but the right side stays the same. If the left side increases, will the hanging mass go up or down? What if the left side decreases?

For the graphs

What is the relation between resultant force and acceleration?

For graph 1 you are correct. So if the velocity is constant, what does that say about the acceleration? If the acceleration is what it is, what does that say about the resultant force?

You are right about graphs 2 and 3. The show that the acceleration is constant and not zero. Therefore, according to Newton's Second Law, is the resultant force is ...

5. Aug 13, 2009

### roy219

if the angle increase , the force which is parallel to incline is increased. cause sinX
will has a increasing value when the angle is increasing. so the hanging mass will goes up
if the angle decrease, the force which is parallel to incline is decreased, so the hanging mass will goes dwon right?

thanks you, i understand it now. i havent consider gravity in this expression before
m1g sinθ = m2g

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oh... Newtons 2 laws state that F=ma , there will be a force when the object has acceleration, right?
constant velocity means that there is no acceleration. am i right?

6. Aug 14, 2009

### kuruman

You got a little bit confused here (my underlines). When the angle increases, the sine increases that is correct. But then m1g sinθ is greater than m2g so the hanging mass will go up when the angle increases. I am sure that's what you meant to say ....

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Correct. When the acceleration is zero, the net force is zero. When the net force is zero, the acceleration is zero. I stress net force. An object may have zero acceleration but forces may still act on it. A book resting on a table has the Earth's gravity and the normal force acting on it and they are both not zero. However, the sum of the two (net force) is indeed zero.

At this point, and I hope I am not confusing you, I should add that "constant net force" means "constant acceleration". Zero for the acceleration is a perfectly good constant, just as good as 9.8 m/s2. So in that sense all three of the graphs show that the resultant force is constant, however only 2 and 3 show that the resultant force is non-zero.

7. Aug 14, 2009

### roy219

thanks, i got it.
are both of them correct?
or statement 1 is wrong and statement is correct?

8. Aug 14, 2009

### kuruman

What do you know about water waves? How are frequency and wavelength related to speed? Does speed depend on depth or does it not?

9. Aug 14, 2009

### roy219

base on my knowledge,
F depends on source,
V depends on medium,
入depends on medium,

water wave travel from shallow to deep region ,speed will increase when wavelength increases but V stay the same.
speed depend on depth !

10. Aug 14, 2009

### kuruman

Here you go from deep to shallow so the wavelength decreases. If the speed also decreases, what happens to the frequency?

11. Aug 14, 2009

### roy219

frequency should be no change?

12. Aug 14, 2009

### kuruman

Yes.

13. Aug 14, 2009

### roy219

so ans (statement 1 is worng and statement 2 is correct) ANS=D

14. Aug 14, 2009

### kuruman

I think you have enough information to make that call yourself.

15. Aug 14, 2009

### roy219

the ans is really D?? but my marking schema show that is A
both are correct and 2 is a correct explanation of 1 ?-3-?

16. Aug 14, 2009

### kuruman

Do you have an expression that relates the frequency of the waves to the wavelength and speed? Is it the harmonic relation v = λf or is it something else? I know that the speed depends on wavelength and depth for water waves, but I always thought that the frequency (or period) does not change.

17. Aug 14, 2009

### roy219

sorry , my english is not so good. do you mean that i have not read the whole question clearly?

18. Aug 14, 2009

### kuruman

No. I mean did your teacher give you an equation or equations that show how to relate the speed, wavelength and frequency for water waves? If your teacher gave you no such equation(s), are there any in the book that you are using?

Also, where did you get your marking schema? Is that in the back of a book?

19. Aug 14, 2009

### roy219

oic, you mean v=f入?
actually, it's one of the questions in my school physics pastpaper.marking schema is found in here

20. Aug 15, 2009

### kuruman

The expression v = λf says that if λ decreases by, say 10%, and v deceases by 10%, then f should stay the same. For this to happen, v and λ must be directly related. For water waves the relation is not direct, therefore the frequency changes. What I was trying to find out is whether your teacher or textbook had given you a relation between v and λ for water waves.

So both statements are "true". However, I cannot understand why (according to your marking schema) the second statement, "Refraction occurs when a water wave travels from a deep region to a shallow region" is an explanation for the first statement, "when a water wave travels from a deep region to a shallow region there is a change in the frequency of the water wave".