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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 bulletblock 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
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 Physics

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