#### Transcript 28 Momentum and Energy Review of 37s and 38s - lindsey

Physics ρ , Work, Power, and Energy Review March 12, 2008 Please take out the 37s and 38s, Thank you! 37s 1st #1) A girl hangs from a bar. The force of gravity, her weight, is pulling her down. What is the name of the force in the ropes that holds her up? Tension 1. On April 15, 1912, Titanic sank after running into an iceberg. What was its velocity when it collided with the ice berg if it had a mass of 4.23x108 kg and a momentum of 4.9x109 kg•m/s? mv 4.9 x10 kgm / s 4.23x10 kg v 9 8 v 11.6 m / s 2. Wayne hits a stationary 0.12 kg hockey puck with a force that lasts for 0.010 seconds and makes the puck shoot across the ice with a speed of 20.0 m/s. With what force did Wayne hit the puck? Ft mv F 0.010s 0.12kg (20.0m/s) F 240 N 3. Paul has a mass of 60.0 kg and is riding in his sports car at 25.0 m/s when he slams into parked car. A seatbelt would bring his body to a stop in 0.400 seconds, but Paul was not wearing his seatbelt and was therefore stopped by the windshield in 0.001 seconds. What force would the windshield have exerted on him? How many times greater is the stopping force of the windshield than the seatbelt? mv Ft 60kg (25 0m / s ) F (0.001s) 1500 kgm / s F ( 0.001 s ) 6 F 1.5 x 10 N 3. Paul has a mass of 60.0 kg and is riding in his sports car at 25.0 m/s when he slams into parked car. A seatbelt would bring his body to a stop in 0.400 seconds, but Paul was not wearing his seatbelt and was therefore stopped by the windshield in 0.001 seconds. What force would the windshield have exerted on him? How many times greater is the stopping force of the windshield than the seatbelt? mv Ft 1500 kgm / s F ( 0.400 s ) F 3750 N 6 1.5 x10 N 400 x 3750 N 0.400 s 400 x 0.001s 4. Granny whizzes around the rink and is suddenly confronted with Adam at rest directly in her path. Rather than knock him over, she picks him up and continues in motion without braking. Find the velocity of Granny and Adam after their collision. • Granny's mass = 50 kg • Granny's initial speed = 3 m/s • Adam's mass = 25 kg • Adam's initial speed = 0 m/s mg vg ma va (mg ma )vga (50 x3) (25 x0) (50 25)vga 150 75 v v 2.0 m / s 5. When there is a car accident, some people slow down to check out what is happening, which can cause more accidents. If an 800 kg car slows to 13 m/s to check out an accident scene, but a 1200 kg truck behind the car continues traveling at 25 m/s, with what velocity will the two move if they lock bumpers after a rear-end collision. m 1 v 1 m 2 v 2 ( m 1 m 2 ) v ( 1200 x 25 ) ( 800 x 13 ) ( 1200 800 ) v 30000 10400 2000 v v 3 3 20.2 m / s 3 3 6. Lee is rolling along on his 4.0 kg skateboard with a constant velocity of 3.0 m/s when he jumps off the back and continues forward with a velocity 2.0 m/s relative to the ground. This causes the skateboard to go flying forward with a speed of 15.5 m/s relative to the ground. What is Lee’s mass? ( m ( m L L m ) v m S 4.0 ) 3.0 m 3m L L L v L m S v S 2.0 ( 4.0 x 15.5 ) 12 2 m m L L 50 kg 62 7. Luke Skywalker must swing Princess Leia across a large chasm in order to escape the Storm Troopers. If Luke and Leia’s combined mass is 145.0 kg, calculate the tension in the rope just before Luke and Leia start their swing, when the pair makes an angle of 30.0 degrees with the vertical. F F T F T T F g cos 145 kg ( 9.8 m / s 2 cos 30 1640.8 N ) 8. A tow truck is pulling a car off the road at a constant speed. If the tow line makes a 10° with the vertical as shown and the weight on the line is 10290 N, what is the tension in the line? F F T F T T F g cos 10290 N cos 10 10448.7 N 38’s 1. After firing a gun, the momentum of the gun is equal and opposite to the momentum of a bullet because of the law of conservation of momentum. A rifle recoils from firing a bullet. Why is the velocity of the gun so much smaller than velocity of the bullet? Mv = MV The gun has a larger mass. 2. A moving car has 200 J of kinetic energy. If the car’s speed doubles, what is its kinetic energy? 2v KE = ½ m v2 KE = ½ m (2v)2 KE = 4 x = 800J 3. If a skater’s speed increases so she has twice the momentum, then how much has her kinetic energy increased? mv KE 1 2 mv 2 m( 2v ) 2 KE 1 m ( 2 v ) 2 KE 4 x 2 4. How does an air bag and seat belt change the impulse in a car crash? decreases the force of impact by increases the time of impact t Δρ=F 5. A person can lift barrels a vertical distance of 1 meter or can roll them up a 2 meter long ramp to the same elevation. If they use the ramp, will they do more or less work? Same amount of work 6. What is the purpose of using a simple machine such as a pulley or lever? decrease the force by increasing the distance 7. What has more momentum a large truck with a mass of 1000 kg moving at 30 mph or small car with a mass of 600 kg moving at 60 mph? mv Small Car mv Large Truck mv 600 kg ( 60 mph ) ( 1000 )( 30 ) 36000 kgmph 30000 kgmph 8. When you jump off something high, you usually bend your knees as you reach the ground instead of landing stiff-legged. By doing this, the time of impact is increased by 10 times. How does the average force on you body change? decreased by 10 times Ft ? F ( 10 t ) F 1 10 9. Legend has it that Isaac Newton “discovered” gravity when an apple fell from a tree and hit him on the head. If a 200 g apple fell 7.0 m before hitting Newton, what was the apples change in potential energy? PE mgh m PE (.200 kg )( 9.8 s )( 7 m ) 2 PE 13.72 J 10. Bill shoots a pool ball with a mass of 1 kg off a pool table that is 0.68 m high and the ball hits the floor with a speed of 6.0 m/s. How fast was the ball moving when it left the table? (Use the law of conservation of energy) • before after • PE + KE = PE + KE 1 1 2 2 (1)9.8(.68) (1)v (1)(9.8)(0) (1)6 2 2 6.664 .5 v .5 v 2 2 0 18 11.336 v 4.76 m / s 11. How much power does it require for Phil to vertical lift a 20 kg weight 3 m off the ground in 1.5 seconds? W Fd P m W ( 20 kg )( 9.8 s W t )( 3 m ) 2 W 588 J P 588 J 1.5 s P 392 W 12. Brian has a mass of 30.0 kg. He climbs a tree 8.0 m to his tree house. How much work has Brain done? When he is in the tree house, what is his potential energy relative to the ground? W Fd PE mgh W ( 30 )( 9.8 )( 8 ) PE ( 30 )( 9.8 )( 8 ) W 2352 J PE 2352J 13. Which requires more work: lifting a 50 kg sack vertically 2 meters in 2 seconds or lifting a 25 kg sack a vertically 4 meters in 4 seconds? W Fd W ( 50 kg )( 9.8 m / s 2 )( 2 m ) W 980 J W ( 25 kg )( 9.8 m / s W 980 J 2 )( 4 m ) 14. A girl can easily lift a 5000 N car with the help of a lever. When she pushes down 25 cm with 50 N of force, how far does the car move? W in W out Fd Fd 50 N ( 0.25 m ) ( 5000 N ) d 12.5 5000 d d 0.0025 m or 2.5 mm 15. Ben, a 90 kg tackle traveling at 7.0 m/s, collides with Dan, a 45 kg quarterback running at -2.0 m/s in the other direction. After the collision Ben continues to travel forward at 1.0 m/s. How fast is Dan knocked backwards? m v m 90 ( 7 ) 45 ( 2 ) 90 ( 1 ) 45 v f D D B v m f B v m v B f 630 90 90 45 v 450 45 v v f f 10 m / s f D 16. A grey hound at a race track can run at a speed of 16.0 m/s. What is the kinetic energy of a 20.0 kg greyhound? KE 1 mv 2 2 KE 1 ( 20 kg )( 16 m / s ) 2 KE 2560 J 2 Reminders Next Tuesday – Test – Packet • 16s, 17s, 18s, 37s, 38s, EC – Great America Field Trip Form and $ due – If you will be gone for DECA – make arrangements for taking the test Work on notecard for test, POTW, extra credit