Collision Conservation of Energy

[jakeginobi]

Homework Statement

An 8.00-g bullet is fired horizontally into a 9.00 kg block of wood on an air table and is embedded in it. After the collision, the block and the bulet slide along a frictionless surface together with a speed of 10cm/s. What is the initial speed of the bullet?

M1 = 0.008kg
M2(block)=9.00 kg
V(total) = 0.1m/s

Homework Equations

Ek=1/2mv^2

The Attempt at a Solution

I found the total Ek after the collision 1/2(9.00kg+0.008kg)(0.1m/s)^2 = 0.04504J
but I don't know what to do afterwards. I can solve this only with using the conservation of energy, not momentum because my teacher hasn't taught momentum yet and the yet is only on energy

 

[PeroK]

The Attempt at a Solution

I found the total Ek after the collision 1/2(9.00kg+0.008kg)(0.1m/s)^2 = 0.04504J
but I don't know what to do afterwards. I can solve this only with using the conservation of energy, not momentum because my teacher hasn't taught momentum yet and the yet is only on energy

Are you sure you didn't skip the class on conservation of momentum? This is what's called an inelastic collision and energy is not conserved in such collisions. You can, in fact, use conservation of momentum to show how much energy is lost in inelastic collisions.

In short, you need conservation of momentum to solve this problem.