Physics Mid-­‐Term Exam

revision

8/19/14

Directions: It is important that you provide answers in your own words. Please focus only on

information from the text/eBook to create your own solutions. Please do not use direct information from an outside source (especially copying and pasting from an “answer” website). Use of direct information from an outside source is against school policy. All answers will be checked for plagiarism.

Instances of plagiarism can result in probation or possible dismissal from the school.

Grading: Please be sure to follow all guidelines (number of sentences/showing all calculations) and to provide the correct metric units of measure. All questions are 5 points (1 point for sentence number /correct units). 1. Makayla is talking to Mia about the whether we ever landed on the Moon. Mia says,” I have a theory about that.” Using 3 – 4 complete sentences, what is wrong with Mia’s use of the term “theory” in this instance?

A true scientific theory has to be based on sufficient experiments and tests that have been conducted. Mia is simply saying that she has a “thought”, more or less, because she has not conducted any reputable scientific studies. Mia’s “theory” is based on her own opinions and subjective thoughts, whereas a true theory would need to have scientific facts to back it up.

2. Choose ONE of the scenarios below, then design an experiment and describe a procedure to enable you to make an estimate of the average speed. Your description should include the materials you would need and how you would use them to collect the data you need. a. A baseball thrown from outfield to home plate b. The wind c. A raindrop d. A hand moving back and forth as fast as possible e. The tip of a swinging baseball bat f. An eye blinking g. A whisker growing An experiment to estimate the average speed of a baseball thrown from the outfield to home plate: You would a machine in the outfield to throw the baseball (to ensure the ball is thrown at relatively the same force), a person on home plate, and then a.person to measure the time. The person on home plate would need a baseball mitt, and the person measuring the time would need a stopwatch. A form of measurement would also be needed, such as measuring tape.

First, the machine throwing the baseball would need to be set in a stationary position. Then the distance between the machine and home plate would need to be measured. To determine the average speed of the baseball thrown from outfield to home plate, the person with the timer would measure the number of seconds it would take for the ball to be released from the machine and land in the person’s mitt on home plate. Doing this 5 times would give a reasonable amount of tests. Then the speeds would need to be averaged, and divided by distance from the machine in the

outfield to home plate. Thus, giving the average speed of a baseball thrown from outfield to home plate.

3. There are many familiar situations in which the net force on a body is zero and yet the body moves with a constant velocity. One example is described below: A car traveling at constant speed on a straight road has the force of the road on the tires just balanced by the force of air friction. If the gas pedal is depressed further, the tires will push against the road harder and the road will push against the tires harder; so the car will accelerate forward – until the air friction builds up enough to balance the greater drive force.

Give another example of a body moving with constant velocity under balanced forces. Specify the cause of each force on the body and explain how these forces could be changed to affects the body’ motion.

4. If a person weighs 500 N on Earth, what would she weigh on Jupiter, where the acceleration due to gravity is 26 m/s2? Show all work leading to your answer. W=m*g W=m*g 500=m*9.8 51.02kg*26 m=500/9.8 W=1326.5N

m=51.02 kg (constant on both planets)

5. If a freely falling rock were equipped with a speedometer, by how much would its speed readings increase with each second IF it were on a planet where g = 20 m/s2? Show all work leading to your answer. If g=20 m/s^2, then the rock’s speed will increase by 20 meters each second that it’s falling. Because of this, the speed of the rock will increase by 20 meters each second.

6. Cleo and Clare are looking from their balcony to a swimming pool below that is located 15 m horizontally from the bottom of their building. They estimate the balcony is 45 m high and wonder how fast they would have to jump horizontally to succeed in reaching the pool. What calculations would you show to help them determine the answer? Evaluate the practicality of their being able to succeed at jumping into the pool. First, they would need to determine how long it would take them to free-­‐fall from the balcony. 45m=(1/2)*g*t^2 45m=(1/2)*9.8*t^2 9.184=t^2 t=3.03 seconds

Then, they need to find the speed it would take to travel 15 m horizontally in 3.03 seconds. V=15m / 3.03s

V=4.95 m/s

Because they are on a balcony, I’m assuming that they will not have the available space to build up enough speed to jump 15m away from the building. Therefore, it probably would not be a good idea.

7. An airplane has a gun that fires bullets straight ahead at the speed of 600 miles per hour when tested on the ground while the plane is stationary. The plane takes off and flies east at 600 miles per hour. Explain whether each of the following is possible for the pilot to measure: a. When fired directly ahead the bullets move eastward at a speed of 1200 miles per hour This would be possible for the pilot to measure, because you would just add the two speeds together since they are both going in the same direction. b. When fired in the opposite direction (by turning the gun westward, but not the plane), the bullets drop vertically downward This would also be possible. In this scenario, the pilot would need to subtract the bullets speed from the planes speed…which would be 0. c. If fired vertically downward (by turning the gun downward, but not the plane), the bullets move eastward at 600 miles per hour while they fall. This could also be measured. Even though the horizontal speed of the bullets themselves are 0, you still have the take into account the plane’s speed, which is 600 mph.

8. A falling 50-­‐kg skydiver experiences an upward acceleration of 6.2 m/s2 when she opens her parachute. Show (by calculation) how much drag force is produced by the parachute. Total Force=mass(acceleration+gravity) F=50(6.2+9.8)= 800N

9. In three scenarios a truck is towing a boat on a trailer. Using 3 – 4 complete sentences indicate which scenario(s) (if any) is the force of the truck on the trailer greater than the force of the trailer on the truck:

Scenario A: masstruck = 1000 kg, massboat and trailer = 500 kg; velocity = 20 m/s

Scenario B: masstruck = 800 kg, massboat and trailer = 800 kg; velocity = 20 m/s

Scenario C: masstruck = 500 kg, massboat and trailer = 1000 kg; velocity = 15 m/s In none of the situations, is the force of the truck on the trailer greater than the force of the trailer on the truck. Newton’s third law states that “whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object”. Because of this law, regardless of the mass of each object or the velocity they are going, the force given by each object to the other object, will be the same. What changes the force, is the reaction of force between the truck and ground. This causes the acceleration.

10. A bug and the windshield of a fast-­‐moving car collide. Indicate whether each of the following statements is True or False (1 point each) a. The forces of impact on the bug and on the car are the same magnitude. b. The impulses on the bug and non the car are the same magnitude

c. The changes in speed of the bug and of the car are the same magnitude. d. The changes in momentum of the bug and of the car are the same magnitude. e. The accelerations of the bug and the car are the same magnitude. a. True b. True c. False d. True e. True f. False 11. Betty and Bianca weigh the same amount and they climb a flight of stairs. Betty climbs the stairs in 30 seconds. Bianca climbs the stairs in 40 seconds. In terms of force, work and power, evaluate which of the three quantities (if any) is greater for Betty or for Bianca? Justify your answers using 3 – 4 complete sentences. (1) The force of gravity acts upon them both equally (2) They both did the same amount of work. This is because they both when to the top of the stairs and weigh the same amount. (3) Power is what differs between the two. Because Betty went the same distance in less time than Bianca, Betty exerted more power than Bianca.

12. Leon’s bicycle wheels have a circumference of 2 m. What is his linear speed when the wheels rotate at 1 revolution per second? Show all work leading to your answer. A certain point on a wheel will touch the ground every time the wheel makes a revolution. So, for every revolution, the bicycle wheel will go the length of its circumference (2 meters) every second. The linear speed when the wheels rotate at 1 revolution per second, is 2 m/s.

13.

This graphic shows a scene from a trapeze act. Write 3 – 4 complete sentences to describe the physics pictured in this scene. Use the terms inertia, centripetal acceleration, force, gravity, and velocity in your description. The centripetal acceleration and force are what gives the acrobats the ability to swing towards each other on their individual ropes. As the male acrobat swings up, inertia allows his body to slow down as it reaches the highest point, allowing the female acrobat to have time to grab his hands. The female acrobat uses the centripetal acceleration from to rope to allow her to have enough velocity to act against gravity, as she lets go of her own rope to reach for the male acrobat’s hands.

14. Using 6400 km as the radius of Earth, calculate how high above Earth’s surface you would have to be in order to weigh 1/16th of your current weight. Show all work leading to your answer OR describe your solution using 3 -­‐4 complete sentences. According to the inverse square law, “the force of gravity weakens as the square of the distance”. Taking this into account, we would need to be 4 times farther away from the Earth as when we first started. So 6400 * 4= 25600. This is the distance we would need to be away from the center of the Earth, so to determine how high above the Earth’s surface we would need to be, we have to subtract 6400 from 25600, which equals 19200 km.

15. The sun’s mass is about 2.7 x 107 times greater than the moon’s mass. The sun is about 400 times farther from Earth than the moon. How does the gravitational force exerted on Earth by the sun compare with the gravitational force exerted on Earth by the moon?

16. At a particular point, a satellite in an elliptical orbit has a gravitational potential energy of 7000 MJ with respect to Earth’s surface and a kinetic energy of 4000 MJ. At another point in its orbit, the satellite’s potential energy is 2000 MJ. What is its kinetic energy at that point? Show all calculations leading to your answer. Point 1: 7000 MJ + 4000 MJ = 11000 MJ Point 2: 11000 MJ – 2000 MJ = 9000 MJ of kinetic energy 17. If a stationary cart is struck head-­‐on by a cart with twice mass of the stationary one and a velocity of 5 m/s, what will be the new velocity of the stationary cart if the collision is inelastic?

Show all calculations leading to an answer. Net momentum (before collision) = net momentum (after collision) (2m)*(5 m/s) + (m)(0 m/s) = (3m)(v) 10 = 3v v= 3.33 m/s

18. A 2.0-­‐gram bullet is shot into a tree stump. It enters at a speed of 3.00 x 104 cm/s and comes to rest after having penetrated 0.05 m in to the stump. What was the average force during the impact? Show all calculations leading to an answer. KE=(1/2)mv^2 KE=(1/2)(0.002)*300^2=90 J Force= KE/d Force=90/0.05=1800 N

19. Can you get younger by traveling at speeds near the speed of light? Explain using 3 or more complete sentences. According to the special theory of relativity, by traveling at the speed of light, you would be traveling through space and not be traveling through time at all. This doesn’t mean that you can get younger. You would technically be “ageless” and basically stay at the same age in which you started travelling through time. You would not age backwards.

20. A beam of light and a baseball are both affected by gravity. Why is the path of the baseball very different than the path of the light beam? Use 3 – 4 complete sentences in your explanation. First of all, a beam of light and a baseball vary a great deal in mass. The mass is so small in particles of light; gravitational force affects light on a much much smaller scale than it affects a baseball. A baseball will lose acceleration very quickly because the force of gravity acting against it. However with a light beam, it’s speed is so fast and it’s mass so small, that gravity has basically no effect on it.