Question

On a cold winter day, a penny (mass 2.50 \rm g) and a nickel (mass 5.00 \rm g) are lying on the smooth (frictionless) surface of a frozen lake. With your finger, you flick the penny toward the nickel with a speed of 2.10 m/s. The coins collide elastically; calculate both their final speeds.

Answer 1

so i think you just have to use the rule of conserved momentum (and no friction coefficient) and then do some algebra, and plug in values for mass and velocity

Answer 2

do you do things like velocty initial = velocity final? or something that sounds vaguely familiar?

Answer 3

i use conservation of kinetic energy and momentum however im not getting the right answer

Answer 4

can you please post the formula you're using (i can't remember off the top of my head)

Answer 5

conservation of momentum m1U1 +m2U2 = m1V1 +m2V2
conservation of kinetic energy 1/2)m1U^21 +(1/2)m2U2^2 = (1/2)m1V^21 +(1/2)m2V2^2

Answer 6

is U velocity?

Answer 7

initial velocity

Answer 8

ok

Answer 9

is that really meant to be to the power of 21

Answer 10

the 1 is a subscript and the 2 is the power

Answer 11

oh ok, so it'd look like \[V _{2}^{2}\] etc

Answer 12

yah

Answer 13

ok well i have these two

2.5 * 2.1 + 5 * 0 = 2.5 * V1 + 5 * V2 ------- eqn 1

therefore 2.5*V1 = -5.25+5 * V2

V1 = -2.1 + 2* V2-------eqn2

and V2 = 1.05+0.5 * V1-------eqn 3

can you use that?

Answer 14

oh no, i hav eto go to work...

Answer 15

ohh

Answer 16

how did u get equation 3

Answer 17

rearranging the un numbered one, so add 5.25 to both sides, leaves
5.25+2.5 * V1 = 5* V2
then divide both sides by 5 which leaves
5.25/5 +2.5/5 * V1 = V2

Answer 18

then you might be able to sub those equations individually into the kinetic formula you get to solve for V1 then do it again to solve for V2... so use eqn 2 in kinetic for V1 and eqn 3 for V2 sorrry i can't help more, i gotta go.

Answer 19

k thanks

Answer 20

have to come back tonight... in about 12 hrs. maybe 11 if im feeling not tired at 11pm

Answer 21

if you can't get it, i'll try to finish the question with all working on here posted just leave another message or whatever and ill be back.

Answer 22

What speed did you get initially that was wrong?

Answer 23

nath aplay conservation of momentum
aplay conservation of kinetic energy
you now have 2 equations solve system of equetions to V1 and V2...


m1*v=M1*V1+m2*V2
M1( v-V1)=M2*V2 (that is eq 1)

1/2m1v^2=1/2m1V1^2+1/2 m2 V2^2
m1 V^2=m1 V1^2+m2 V2^2
m1*(V^2-V1^2)=M2*V2^2
m1*(V-V1)*(V+V1)=m2*V2^2 divide that with (1) and you have.....
V+V1=V2 (that is eq 2)
solve (1) and (2) to V1 and V2

Answer 24

or do it that way... whatever. :P