# Physic- calculating velocity in elasticity?

Two students at a barbecue party put on inflatable sumo-wrestling outfits and take a run at each other. The 87.0kg student (A) runs at 1.21m/s (N) and the 73.9kg student (B) runs at 1.51m/s(S). The students are knocked off their feet by the collision. immediately after impact. Student (B) rebounds at 1.03m/s (N).

(a) Assuming the collision is completely elastic, calculated the speed of student A immediately after impact using energy consideration. ?

****** the Answer is suppose to be 1.58m/s********* I'm getting 1.42......m/s I dont know if its my calculations or the steps I'm taking ---- If someone could please explain in detail, so I can further understand the material better.

(b) How different would your answer be if only conservation of momentum were used?

### 1 Answer

- WoodsmanLv 77 years agoFavorite Answer
Two students at a barbecue party put on inflatable sumo-wrestling outfits and take a run at each other. The 87.0kg student (A) runs at 1.21m/s (N) and the 73.9kg student (B) runs at 1.51m/s(S). The students are knocked off their feet by the collision. immediately after impact. Student (B) rebounds at 1.03m/s (N).

Conserving energy, we have initial KE = final KE

87kg * (1.21m/s)² + 73.9kg * (1.51m/s)² = 87kg * Va² + 73.9kg * (1.03m/s)²

296 = 87*Va² + 78.4 → for Va in m/s

Va = 1.58 m/s

Clearly this rebound is South, since the speed is greater than that of student A.

(b) 87kg * 1.21m/s - 73.9kg * 1.51m/s = 87kg * Va + 73.9kg * 1.03m/s

Va = -0.95 m/s

Why are the answers different, you might ask? Because in a true elastic collision, B's post-collision velocity would not have been 1.03 m/s. Therefore, the energy answer is wrong, because the input data was invalid.