Homework Help: Speed and coefficeint of friction part 2

1. Apr 8, 2007

imy786

1. The problem statement, all variables and given/known data

Three smooth stones, A, B and C are initially at rest, in contact with each other on the smooth surface of a frozen lake. The masses of the stones are A = 400 g, B = 300 g and C = 120 g and the coefficient of sliding friction between the stones and the ice is μslide = 0.0150. An explosion between the stones causes them to fly apart across the surface of the ice. Stone A flies off due North with an initial speed of 4.50 m s−1. Stone B flies off due east with an initial speed of 6.60 m s−1.

If 20% of the total energy of the explosion was converted to the kinetic
energy of the stones, what was the total energy released in the explosion?

2. Relevant equations

KE= 1/2 * M * V^2

PE= mgh

3. The attempt at a solution

total energy= ke+ pe

work done= force*distance

force= uR

μslide = 0.0150

i dont know R. And i dont know the distance travelend to calculate work done.

2. Apr 8, 2007

Meir Achuz

1. Use cons of momentum to find the momentum and then the KE of stone C.
2. The KE of A + B + C equals 1/5 of the energy in the explosion.
You don't use PE, work done, or frliction.

3. Apr 8, 2007

imy786

why have they given friction in the quesiton at all??
i didnt use friction in the first part either.

4. Apr 8, 2007

robb_

I dont see a need for it either. (here)

5. Apr 10, 2007

Meir Achuz

Maybe it is a smokescreen to make you realize that friction plays no part in collisions.

6. May 10, 2007

imy786

stone C= m1u1 + m2u2 = m1v1 +m2v2

m1= 0.120 kg
u1= 0

m2= would it stone B + stonce c mass
u2=? i got no idea what value to put here, as i have the speed of stone b and c.

could i not just calculate the total energy by just calculating KE , or do i have to also calculate conversation of momentum?

7. May 11, 2007

Meir Achuz

Your first equation is not relevant here.
$$0=m_A{\vec v_A}+m_B{\vec v_B}+m_C{\vec v_C}.$$
to find v_c. Then use energy.

8. May 11, 2007

imy786

speed of c is aprox 36meters/sec in the south west direction.

KE after collison= 0.5mava^2 + 0.5Mbvb^2 + 0.5McVc^2
= 0.5 ( 0.4 * 4.5^2 + 0.3*6.6^2 + 0.120* 36^2)
= 0.5 (8.1+ 13.068+ 155.52)
=0.5* 176.688
=88.3 J

9. May 15, 2007

imy786

is this correct to for the calculation to work out KE

10. May 15, 2007

hage567

Are you sure that's the value you found for the speed of stone C? Looking at the other thread, I think you determined you had made a mistake with that. Double check it.

11. May 16, 2007

imy786

is the method correct though to work it calculating KE

12. May 16, 2007

hage567

It's right for finding the total kinetic energy after the explosion.