PRACTICE PROBLEMS 1. A medicine ball with a mass of 20 kg is thrown at a velocity of 2.5 m/s. What is its momentum? What is the impulse needed to catch the ball? If the ball is in contact with its catcher for 0.7 secs before it stops, what was the average force absorbed by the catcher? (Ans: momentum = 50 kg.m/s; Impulse = -50 N.s; average force = -71.4 N)

2. Tonya Harding has a mass of 50 kg and is skating with a velocity of +6.25 m/s on the hockey rink. She decides to mix it up with Wayne Gretsky (mass = 90 kg), and hits him when he has a velocity of - 2.5 m/s. If Tonya and Wayne entangle and move as one unit after the collision, what is their velocity after impact and in which direction do they travel? Neglect any effects of air resistance or friction. (ans +0.625 m/s in the direction Tonya was initially travelling)

3. A 100 kg hockey player who is traveling at + 10 m/s jumps on the back of another 100 kg hockey player who is traveling at + 5m/s. Soon after this collision, the two players are hit by a third player who has a mass of 150 kg, and is traveling in the opposite direction with a velocity of - (negative) 12 m/s. After the collision, the three players remain entangled and continue moving together as one unit. Neglecting any effects of air resistance or friction, answer the following: a.) What is the velocity of the first two players after they collide? (ans: 7.5 m/s) b.) What is the velocity of all three players after the second collision and which way do they travel? (ans: - 0.86 m/s in the direction of the third player.)

4. After successfully clearing the bar during the pole vault, CSLA's star decathlete falls to the landing cushion which will break his fall. If his mass is 85 kg and he has a velocity of -11.5 m/s just prior to landing a). what is the linear momentum of the athlete just prior to impact? (ans: - 977.5 kg.m/s) b). how long does it take for his downward velocity to be reduced to zero by the landing cushion if the average net force on him once he hits the cushion is 2570 N? (ans: 0.38 s) c.) 0.2 seconds after making contact with the mat, what was his velocity? (- 5.45 m/s)

5. An 85 kg sprinter pushes off from the blocks and experiences an average resultant reaction force of 2000 N at an angle of 20 degrees to the horizontal. If the sprinter left the blocks with a horizontal velocity of 3 m/s, how long was sprinter in contact with the blocks? (ans: 0.14 s)

6. A gymnast dismounts from a bar and drops a distance of 4m from her peak height to the ground. If her mass is 40 kg, and her vertical velocity just before she makes impact with the ground is -8.9 m/s, what was the average net force she experienced during the landing phase if she took 0.25 seconds to reach zero velocity after making floor contact. (ans: av. net force = 1424 N)

7. Consider a gymnast (m = 70 kg) performing a giant swing on a high bar (i.e., swinging around the bar with his body fully extended). He is swinging in a counterclockwise direction with an angular velocity of 15 rad/s. What is the linear or tangential velocity of his center of gravity (CG) if his CG is lies 1.5 m from the bar (ans. 22.5 m/s).

8. A hammer thrower releases the hammer with a linear velocity of 28 m/s. If the radius of rotation was 2.1 m, what was the angular velocity of the hammer just prior to release? (ans. 13.333 rad/s).

9. A sprinter pushes against the blocks and experiences a resultant reaction force of 2500 N at an angle of 25 degrees to the horizontal. Neglecting air resistance, what is the acceleration of the sprinter's CG in both the horizontal and vertical directions? What is the resultant acceleration? (Assume the sprinter has a mass of 85 kg) (ans: aV = 2.62 m/s2; aH = 26.66 m/s2; aR = 26.79 m/s2

10. A person who weighs 170 lbs is travelling downwards in an elevator whilst stood on a pair of bathroom scales (goofy bugger!). If the scales read 180 lbs, what is the acceleration of this person? Is the person increasing or decreasing speed? (ans: 0.58 m/s2)