Physics: Post your doubts here!

[nehaoscar]

It's evaporation because evaporation is a process which depends on the velocity of the water molecules. If the molecules at the surface possess enough energy then they are able to escape the forces that hold the molecules together and then escape as vapour. But the question states that all the molecules have the same speed, hence evaporation cannot occur

So then boiling, condensation and melting don't depend on the speed of molecules? ... Then what do they depend on?

 

[nehaoscar]

Q8 pls

use s=ut + 1/2 at^2 for both motions (X-Y and Y-Z)
this will give you these:

X-Y
40 = 12u + 72a

Y-Z
40 = 6v + 18a (note that the u for this motion is the v from the motion in X-Y)

So use s = 1/2 (u+v) t
and rearrange to make v the subject and substitute the values for X-Y motion so you get v = 6.67 - u

Substitute v into the Y-Z equation

Y-Z
40 = 6(6.67 - u)+ 18a
40 = 40 - 6u + 18a
0 = -6u + 18a

Form simultaneous equations:
40 = 12u + 72a
0 = -12u + 36a (multiply the Y-Z eqn by 2 to cancel the u's)

this will give a = 0.37 = A

 

[nehaoscar]

.

It's C right? I just did this question B is wrong cause the initial force at which it is thrown is same for both, so the initial path has to be the same

 

[princess Anu]

It's C right? I just did this question B is wrong cause the initial force at which it is thrown is same for both, so the initial path has to be the same

Yeah
but why not A

 

[princess Anu]

1. help!!!

 

[eliyeap]

So then boiling, condensation and melting don't depend on the speed of molecules? ... Then what do they depend on?

1. Boiling: occurs at a fixed temperature, that is the boiling point of the liquid. Its a process that occurs throughout the liquid. Basically for boiling, you are heating up the entire liquid. The heat is being distributed evenly throughout the liquid and the speed of the molecules increases together. (Same for melting, since melting and boiling both absorb heat). Hence, at any temperature the speed of the molecules are the same.
2. But for evaporation, the process only occurs on the surface of the liquid. The molecules at the surface are absorbing heat from the surroundings and some of them will have enough energy to break their intermolecular forces and escape.
3. Condensation is mainly when hot vapour touches a cool surface and forms water droplets.

 

[nehaoscar]

Yeah
but why not A

Cause it has to be the same in both motions ; the moon doesn't have an atmosphere so there is no air resistance on the moon

 

[princess Anu]

Cause it has to be the same in both motions ; the moon doesn't have an atmosphere so there is no air resistance on the moon

can you please elaborate
the upward & downward motions?
if so, then why
isn't it the case even with air resistance too ?

 

[nehaoscar]

can you please elaborate
the upward & downward motions?
if so, then why
isn't it the case even with air resistance too ?

For upward motion you know what the path should be like (shown in A and C)
-On the earth, while going up, the golf ball is hit with a force, while coming down it falls freely but then there is air resistance opposing it so it the curve is shorter that while going up (acceleration is less than free-fall; a<9.81)
-On the moon, while going up, same as earth the ball is hit with a force
while coming down also it falls freely but this time there is no air resistance opposing it's motion so the curve is same/similar to the one while going upwards (acceleration is same as freefall on moon)

 

[princess Anu]

For upward motion you know what the path should be like (shown in A and C)
-On the earth, while going up, the golf ball is hit with a force, while coming down it falls freely but then there is air resistance opposing it so it the curve is shorter that while going up (acceleration is less than free-fall; a<9.81)
-On the moon, while going up, same as earth the ball is hit with a force
while coming down also it falls freely but this time there is no air resistance opposing it's motion so the curve is same/similar to the one while going upwards (acceleration is same as freefall on moon)

do u mean while going upwards there is no air resistance? and the acceleration is 9.81 ( on earth)? its only during downward motion that the acc decreases?

 

[nehaoscar]

do u mean while going upwards there is no air resistance? and the acceleration is 9.81 ( on earth)? its only during downward motion that the acc decreases?

No! There is air resistance while going up but an initial force is applied so it will travel further than during free fall
I'm sorry I'm not exactly sure how to explain this! :S

 

[THARINDU AWESOME]

while moving up there is air resistance thats why at highest point velocity is zero when moving down if the object reches terminal velocity zero acceleration otherwise acceleration increases

 

[THARINDU AWESOME]

does the rate of reaction insrease if we increase the concentration of the catalyst?????????

 

[nehaoscar]

does the rate of reaction insrease if we increase the concentration of the catalyst?????????

Yes; more catalysts = more active sites available = increased collisions = increased rate

 

[THARINDU AWESOME]

Yes; more catalysts = more active sites available = increased collisions = increased rate

thanks alot i been stuck like hell

 

[funky brat]

Why am I getting 13.75 instead of 15?
R= 12/3.2= 3.75
1/10+ 1/10+ 1/x= 3.75
x= 13.75

 

[funky brat]

Someone please explain how to do s11 paper 11 question 3 and 9.

 

[Ahmed Aqdam]

Why am I getting 13.75 instead of 15?
R= 12/3.2= 3.75
1/10+ 1/10+ 1/x= 3.75
x= 13.75

It should be 1/3.75 and not 3.75.
The method you're using doesn't give 13.75 either.

 

[Ahmed Aqdam]

Someone please explain how to do s11 paper 11 question 3 and 9.

3. Horizontal force is 3.0 N and vertical is 5.0 N. Apply Pythagoras. Square both and add to get 34. Take square root to get 5.8 N.

9. Momentum remains same before and after collision so momentum is mv.
Total mass will be 2m and so the velocity after collision will be 1/2 v. Apply 1/2 mv^2 to get 1/4 mv^2 as kinetic energy after collision.

 

[funky brat]

It should be 1/3.75 and not 3.75.
The method you're using doesn't give 13.75 either.

1/10 + 1/10 + 1/x = 1/3.75

Ops thank you.