# Collision questions

#### mdwjun

1. An 8 g rubber bullet was fired into a 2.5 kg pendulum bob, initially at rest, and becomes embedded in it. The pendulum rises a vertical distance of 6.0 cm.Calculate the initial speed and how much kinetic energy is lost in this collision?

2. An 8 g rubber bullet was fired into a 2.5 kg pendulum bob, initially at rest, and bounces off into the opposite direction. It rises 6.0 cm vertically. (elastic case) What is the velocity of the bob after the collision? Use conservation momentum to find the change in velocity of the bullet. Use conservation of Kinetic energy to find initial velocity of bullet and final velocity of the bullet.

I heard that I needed to use the inelastic collision and the conservation of energy equation for number 1.
0.5*mass*initial velocity^2 +mass*gravitational pull* initial height=0.5*mass*final velocity^2 +mass*gravitational pull* final height.

I triedto solve but could not because i did not know the initial velocity of the bullet and the final velocity.

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#### alphysicist

Homework Helper
Hi mdwjun,

1. An 8 g rubber bullet was fired into a 2.5 kg pendulum bob, initially at rest, and becomes embedded in it. The pendulum rises a vertical distance of 6.0 cm.Calculate the initial speed and how much kinetic energy is lost in this collision?

2. An 8 g rubber bullet was fired into a 2.5 kg pendulum bob, initially at rest, and bounces off into the opposite direction. It rises 6.0 cm vertically. (elastic case) What is the velocity of the bob after the collision? Use conservation momentum to find the change in velocity of the bullet. Use conservation of Kinetic energy to find initial velocity of bullet and final velocity of the bullet.

I heard that I needed to use the inelastic collision and the conservation of energy equation for number 1.
That's true, and you'll need to separate the problem into two parts--the collision, and the swinging motion of the pendulum that occurs after the collision. What is conserved during the collision, and what is conserved during the swinging motion? That will tell you what relationship to use for each part. Also, be sure to clearly identify what "initial velocity" and "final velocity" means for each part of the motion.

Once you get the two equations for the two parts of the problem you should be able to solve it.

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