How fast must you throw a snowball against a wall in order to make it melt?


by twotaileddemon
Tags: melt, order, snowball, throw, wall
twotaileddemon
twotaileddemon is offline
#1
Oct31-06, 04:28 PM
P: 263
Hi, I'm new to this forum and I have a question.

In my physics class we are discussing energy and work. We are given this problem:

"It is exactly 0*C. How fast must you throw a snowball against a wall in order to make it completely melt? (Assume all the energy is transferred to the snowball and neglect air friction)"

Now.. considering there is almost no information given, how am I supposed to solve this? I know potential energy is mgh and kinetc is .5mv^2. Work is Fs, or change in kinetic energy. It's just.. give so little details I don't know where to start. Can anyone tell me how to start the problem, or any general guidance without giving the answer? Thanks :)
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twotaileddemon
twotaileddemon is offline
#2
Oct31-06, 04:51 PM
P: 263
Hm.. judging by the other posts, even though I'm in AP physics B (12th grade level), I think this belongs in the introductory forum.. so I'll post this there. Feel free to delete this.. sorry..
cells
cells is offline
#3
Oct31-06, 06:34 PM
P: 9
its not too hard, [just thinking to myself that statement is stupid ofcourse something is not hard if you know it lol]

anyway here goes


ok kin energy = 0.5mv^2

energy needed to melt the snowball = mass of snow ball x specifc heat capacity of snow x the temp increase required to melt the snow
energy = mht where m is mass, h is heat capacity and t is temp

rearrange to get v

v^2 = 2ht v = (2ht)^0.5

so for example if the snowball specific heat capacity is 4200j/kg, and its at -20degrees centegrade. velocity would need to be 410mps.

stunner5000pt
stunner5000pt is offline
#4
Oct31-06, 06:40 PM
P: 1,445

How fast must you throw a snowball against a wall in order to make it melt?


they only asked for it to melt, not increase in temperature

snow will melt at anything above 1 really

what is the energy required to MELT (and not HEAT) the snow

think change of state - in the introductory physics forum you were told about latent heat of fusion - that is the amount of energy required per unit mass to cause a change of state.
twotaileddemon
twotaileddemon is offline
#5
Oct31-06, 11:04 PM
P: 263
Yeah, I actually found it in my textbook. It was about halfway through and we're only half way to the half way point.. if that made sense o_o"
So from what they gave me, and from what I researched, I got something along the lines of 818 m/s. I used this equation and isolated V

.5mv^2 = mL (m = mass, v = velocity, l = latent energy)
V = sq root 2L (m's cancel out)
Then all I did was put in the latent energy needed to make ice into water at 0*C, it was (not exactly sure, my notes are packed away) about 343kJ, or something to that effect (I know it's not precise, I used the correct number in my equation though) and transferred it to J, then took the square root of it (times 2). And.. I got about 818 m/s as the answer. :)

Thanks for your help! I appreciate it.


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