In a collision between two disks, disk 1 (inertia 0.40 kg) moves at an unknown i
ID: 1460403 • Letter: I
Question
In a collision between two disks, disk 1 (inertia 0.40 kg) moves at an unknown initial speed in the positive x direction across a low-friction horizontal surface and collides with disk 2 (inertia 0.70 kg), initially at rest. After the collision, the two (now slightly dented) disks move apart without spinning. Their final velocities are shown in the diagram below.
a. What was the initial speed of disk 1?
b. What fraction of the initial kinetic energy was converted to another form in the collision?
1f C) 20 "2 2i 50° C3 1i 2fExplanation / Answer
a) using momentum conservation,
m1 v1i + m2 v2i = m1 v1f + m2 v2f
0.40 x v + 0.70 x 0 = ( 0.40 x 1.4 ( cos20i + sin20j )) + ( 0.70 x 1.1 (cos50i - sin50j))
0.40v = 1.02i
v = 2.55 m/s .......Ans
b) initial Energy KEi = 0.40 x 2.55^2 /2 = 1.30 J
final KEf = (0.40 x 1.4^2 /2 ) + ( 0.70 x 1.1^2 /2 ) = 0.816 J
energy converted into another form = 1.30 - 0.816 = 0.484 J
fraction = 0.484 / 1.30 = 0.373