Body A of mass 4m moving with speed u collides with another body B of ...
Concept: In an elastic collision, the total kinetic energy of the system is conserved. The momentum of the system is also conserved.
Given: Body A of mass 4m moving with speed u collides with another body B of mass 2m, at rest. The collision is head-on and elastic in nature.
To find: The fraction of energy lost by the colliding body A.
Solution:
Let the final velocities of bodies A and B be v1 and v2, respectively.
By conservation of momentum:
4mu = 2mv1 + 2mv2
2mu = mv1 + mv2
Solving for v1 and v2, we get:
v1 = (2u)/3
v2 = (u)/3
Kinetic energy of body A before collision = (1/2) * 4m * u^2 = 2mu^2
Kinetic energy of body A after collision = (1/2) * 4m * v1^2 = (4/9) * mu^2
Fraction of energy lost by body A = (Initial KE - Final KE) / Initial KE
= (2mu^2 - (4/9)mu^2) / 2mu^2
= (8/9)
Therefore, the fraction of energy lost by the colliding body A is 8/9.
Answer: Option B.
Body A of mass 4m moving with speed u collides with another body B of ...
Body A of mass 4m moves with speed u and body B of mass 2m at rest.
so, initial linear momentum of system = 4mu + 2m × 0 = 4mu
Let final velocity of body A is v1 and body B is v2
then, final linear momentum of system = 4mv1 + 2mv2
as collision is elastic ,
so, v1 =
= (4m - 2m) × u/(4m + 2m) + 2(2m)×0/(4m+2m)
= 2mu/6m
= u/3
similarly, v2 =
= (2m - 4m) × 0/(4m + 2m) + 2(4m)u/(4m+ 2m)
= 8mu/6m
= 4u/3
so, initial energy of body A = 1/2 (4m) u² =2mu²
final kinetic energy of body A = 1/2 × (4m) × u²/9
= 2mu²/9
so, change in kinetic energy = final kinetic energy - initial kinetic energy
= 2mu²/9 - 2mu²
= 2mu² [ 1/9 - 1]
= -16mu²/9 [ here negative sign indicates kinetic energy lost after collision of body A]
so, fractional lost in kinetic energy = change in kinetic energy/initial kinetic energy
= (16mu²/9)/(2mu²)
= 8/9
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