Crime Lab Physics

You are a criminologist, and have been given a gun which has been used in the commission of a crime. Both prosecution and defense claim that "muzzle velocity" (speed of the bullet as it leaves the gun) is crucial to the case. Your job...FIND IT!

Shown below is a diagram representing a "ballistic pendulum." The bullet is fired into the block, the bullet-block combination rises by a height h as shown, a little physics is applied, and ...voila!...there you have it!

h

Figure 1

Let us proceed: (correct answer choices are given on the next page)

1. The mass of the bullet is 0.01 kg and the mass of the block is 0.99 kg. The bullet is fired into the block and becomes imbedded in it. The bullet-block combination rises by 0.05 meters. What potential energy is stored in the pendulum (bullet-block combination)?

PE = mgh =

a. 50 joules

b. 5 joules

c. 0.5 joules

d. 0.05 joules

2. What was the kinetic energy of the pendulum (bullet-block combination) "immediately" after the bullet became imbedded (i.e. what was the MAXIMUM K.E. of the combination)?

KE = PE= (conservation of energy -- no calculation required!)

a. 50 joules

b. 5 joules

c. 0.5 joules

d. 0.05 joules

3. What was the velocity of the pendulum (bullet-block combination) "immediately" after the bullet became imbedded (i.e. when its K.E. was maximum)?

KE = 1 mv2

2

a. 0.5 m/sec

v2 = 2KE m b. 1.0 m/sec

v=

2KE = m c. 1.5 m/sec

d. 2.0 m/s

4. What was the momentum of the pendulum (bullet-block combination) "immediately" after the bullet became imbedded (i.e. when its K.E. was maximum)? p = mv =

a. 0 kg m/sec

b. 0.25 kg m/sec

Ballistic Pendulum.....5/15/08

c. 0.5 kg m/sec

d. 1.0 kg m/sec

1

Don Rathjen....Exploratorium Teacher Institute....3601 Lyon St., San Francisco, CA 94123...donr@exploratorium.edu

© 2008 Exploratorium, www.exploratorium

5. What was the momentum of the block before the