Lecture11 - Racquetball Striking a Wall Collisions General...

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ollisions General Physics I Mechanics Racquetball Striking a Wall Collisions Physics 140 Mt. Etna Stewart Hall

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Collisions
Defining a Collision • In a collision, forces between two objects are large and last for a short time Before After • During that time, they are often much larger than external forces, which we can ignore • The total momentum of the system is then always conserved While a collision is occurring other forces are negligible

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Analyzing a 1D Collision 1 2 Before A ball with mass m 1 moves with v 1i . It strikes a second ball with mass m 2 . What are the final velocities? This is one equation with f f i v m v m v m 2 2 1 1 1 1 + = two unknowns: v 1f and v 2f . It has infinitely many solutions 1 2 1 2 Either of these, along with infinitely many others, would conserve momentum After
To fully predict the motion, momentum conservation is not enough. We also need to know something about what happens to the energy of the system. First consider several limiting cases. What if the balls stick together? v 1f = v 2f = v f ( 29 f i m v m m v m 1 2 1 1 1 + = ( 29 i f v m m v 2 1 + = Limiting cases: m 1 >> m 2 v f 2245 v i m 2 >> m 1 v f 2245 0 m 1 = m 2 v f = v i /2 This adds a second equation: we can now solve this! 1 2 Before 1 2 After

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• Imagine a collision like this in which two objects of the same mass stick together. How does the kinetic energy of the pair change in the collision? A. The kinetic energy increases B. The kinetic energy decreases C. The kinetic energy remains the same 1 2 Before 1 After 2 m 1 = m 2 v f = v i /2 KE i = ½ mv i 2 KE f = ½ (2 m )( v i /2) 2 KE f = ¼ mv i 2 KE f < KE i
is conserved? KE = KE 2 1 2 1 2 1 2 2 1 1 1 1 f f i v m v m v m v m v m v m + = + = To fully predict the motion, momentum conservation is not enough. We also need to know something about what happens to the energy of the system. First consider

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Lecture11 - Racquetball Striking a Wall Collisions General...

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