1D Elastic Collision between an Elephant and a Fly

[ObitoLegend]

Homework Statement

A hovering fly is approached by an enraged elephant charging at 2.1 m/s. Assuming that the collision is elastic at what speed does the fly rebound? Note that the projectile(the elephant) is much more massive than the stationary target (the fly)

Relevant Equations

1-D Elastic collisions where v2 = 0:
v'1 = ((m1-m2)/m1+m2)*v1
v'2 = (2m1/(m1+m2))*v1

1 = elephant
2 = fly
So I am trying to find v'2 which is the final velocity of the fly. I have v1 the initial velocity of the elephant 2.1m/s. So I plug it into the equation and have v'2=(2m1/(m1+m2))*2.1m/s. We are not given the masses so I just know m1>m2 but I don't understand how that will help me, or how we can get rid of the masses to solve the equation.

 

[Frabjous]

If m1>>m2, what does that say about their sum and difference?

 

[ObitoLegend]

That they are both positive?

 

[Frabjous]

That they are both positive?

What is 1 million plus or minus one? How does it compare to 1 million?

 

[ObitoLegend]

ohh so I can disregard m2 because m2 is so small compared to m1

 

[Frabjous]

ohh so I can disregard m2 because m2 is so small compared to m1

Yes. The elephant is a moving wall essentially.

 

[ObitoLegend]

got it thank you!