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Unformatted text preview: Physics 125 Discussion #13 Solution 1 Problem 1: Ballistic pendulum A ballistic pendulum is used to determine the velocity of a projectile. A 10 gram bullet is fired from a rifle at a speed of 700 m/sec into a 1 .50 kg block of a ballistic pendulum suspended by a string 4 meters long. 1. Define your coordinate system. 2. What is the momentum of the system (bullet + block) before the collision? m/s kg 7 m/s) kg)(0 50 . 1 ( m/s) 700 kg)( 01 . ( , , = + + = + = i block i bullet total p p p h h h 3. During the collision the momentum of the system is conserved. Why? The force between the bullet and the block is internal for the system. PE=0 y x Physics 125 Discussion #13 Solution 2 4. After the collision, the block and bullet are now behaving as one object. What is this objects speed right after the collision? (m/s) 64 . 4 01 . 5 . 1 7 7 7 ) ( m/s kg 7 , , = + = + = = + = + block bullet block bullet f block f bullet m m v v m m p p h h 5. Use conservation of energy to determine how high the pendulum swings. Lets define PE=0 at the level where the block is located originally (see figure). Right after the collision, the total energy is the kinetic energy, and, when the bullet-block pendulum reaches the highest point, the total energy is only the gravitational potential energy. (m) 098 . 1 8 . 9 2 64 . 4 2 ) ( ) ( 2 1 2 2 2 = = = + + = + + + = + g v h gh m m v m m KE PE KE PE block bullet block bullet f f i i 6. Compare the total KE before the collision to the total KE afterwards. Does the Compare the total KE before the collision to the total KE afterwards....
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- Fall '08