Energy & Momentum Problems (CP) Mixed

Rule of Thumb: If the mass of the object stays the same it is a conservation of Energy problem. If the mass of the object changes, it is a conservation of momentum problem (inelastic collision) If two objects bounce, it is a conservation of momentum problem (elastic collision) If there is a spring, it is an energy problem

V = 10.84 m/s Mass stays the same Ei = Ef Example: A block with a mass of 25 kg starts from rest 6.0 m above the ground and slides down a frictionless ramp. What is the velocity of the block at the bottom of the ramp? Draw the picture: Mass stays the same Ei = Ef V = 10.84 m/s

Vf =7.7 m/s pi = pf Mass changes Example: At the bottom of the ramp, the 25 kg block collides inelasticly with 10. kg toy car at rest. What is the final velocity of the car-block combo? Draw the picture: Mass changes V=10.84 m/s pi = pf Vf =7.7 m/s 4

Example: 2:a) A 65 kg Olympic ice skater is coasting at 5 Example: 2:a) A 65 kg Olympic ice skater is coasting at 5.0 m/s on a frozen pond when his little brother (of mass 35 kg) falls from a tree and lands in his arms. Now how fast is the skater moving while still holding his brother? (ignore any velocity from his little brother) b) The sudden appearance of his brother makes the skater hit a small stone and fall over. The friction between his jeans and the ice bring him, still holding his brother, to a stop in 1.5 m. What is the force of friction between his jeans and the ice?

Example: 2 A 0.25 kg Nerf arrow is fired from spring-loaded gun at a 10.0 kg target sitting on a frozen (frictionless) pond. After impact, the arrow and target slide across the pond at 3.7 m/s. What is the spring constant of the Nerf gun if the spring was compressed 30.0 cm when the arrow was fired? (assume the arrow does not fall during flight) (Yes, I know this is a little unrealistic) K = 64,000 N/m