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Speed and Velocity • Speed: a distance traveled over a period of time • (ex. 5 m/s, 30 mph) • Velocity is speed with direction • (ex. 5 m/s to the left, 30 mph North) • Directions could be right, left, up, down, East, West, North or South • It’s a vector (has quantity and direction) • When the velocity is to the: – – – – Right Left Up Down + – + – Which takes more force to stop? • Big player @ 2m/s Small player @ 2 m/s • Big player @ 0.6 m/s Small player @ 6 m/s • Small player @ 2 m/s Bullet @ 100 m/s • Small player @ 100 m/s Bullet @ 4 m/s Which takes more force to stop? • Big player @ 2m/s Small player @ 2 m/s Is there ever a time when it would take the same amount of force to stop both players? Momentum • Defined as “a mass in motion” (amount of motion) – Momentum equals mass times velocity – A vector quantity (has direction) • The more momentum an object has – could mean it has a larger mass or velocity – it is harder to change its motion Momentum • Momentum is different from inertia – Inertia is the resistance to the change of motion – Momentum is how much motion there is • An object not moving has inertia but zero momentum • An object moving has inertia and momentum Calculating Momentum Determine the momentum of a ... • a. 100-kg halfback moving to the right at 4 m/s. • b. 70-kg halfback moving to the left at 2 m/s. Check Your Understanding A car traveling down the road has momentum of 20,000 kg·m/s. What would be the car's new momentum if ... a.its velocity were doubled. b.its mass were tripled (by adding more passengers and a greater load) c.The car turned around. Collisions and Momentum • Objects collide • Objects push apart or explode • In any system, momentum is conserved • Total momentum is the same before and after a collision • http://youtu.be/4IYDb6K5UF8 Solving Momentum Problems • Draw pictures – Before picture (before the collision) – After picture • Need to know the momentum of each object first (p of all the objects)before = (p of all the objects)after • A hunter spots his prey off to the right. He waits patiently and does not move. When the hunter fires, the bullet has a momentum of 284 kg·m/s. The hunter feels a kickback after the bullet is fired. Determine the momentum of the hunter after he fires the bullet. • A van traveling East with a momentum of 1500 kg·m/s collides with a car moving West with a momentum of 800 kg·m/s. After they collide, they stick together and the van has a momentum of 500 kg·m/s. What is the momentum of the car? • Playing a game of pool, the cue ball is hit so it moves with a momentum of -0.90 kg·m/s. It collides with the 1-ball that was at rest. After they hit, the 1-ball has a momentum of -0.75 kg·m/s. What is the momentum of the cue ball? • Two figure skaters are standing next to each other on an ice rink. The skaters then push off from one another. One skater has a momentum of 120 kg·m/s to the right. What is the momentum of the other skater? • Playing a game of pool, the cue ball (m = 0.17 kg) is hit with a speed of 3 m/s toward the 4-ball (m = 0.16 kg) that was at rest. After the two balls collide, the 4-ball has a speed of 2 m/s. What is the speed of the cue ball after the collision? • An 80 kg hunter spots his prey off to the right. He waits patiently and does not move. When the hunter fires, the 5 gram bullet has a velocity of + 120 m/s. The hunter feels a kickback after the bullet is fired. Determine the speed of the of the hunter after he fires the bullet. • A 1000 kg van traveling East with a velocity of of 1.5 m/s collides with a 600 kg car moving West with a velocity of 1.5 m/s. After they collide, they stick together. What is the velocity of the van and the car? • Two children on ice skates start at rest and push off from each other. One has a mass of 30 kg and moves back at 2 m/sec. The other child has a mass of 15 kg, but what is their speed? • Standing on an icy pond, you throw a 0.5 kg ball at 40 m/sec. You move back at 0.4 m/sec. What is your mass?