#### Transcript Week 8 (Physics Lecture #7)

Warm-Up I: Think about This & Discuss with Neighbors Consider a trip on an elevator. You begin at rest at the bottom, start to move up, move at constant speed for a short while, then slow down to a stop at the top. Then, starting from rest at the top, you start to move down, move at constant speed for a short while, then slow down to a stop at the bottom. You’ve likely taken many such trips. Try to remember such a trip. Do you feel heavier, lighter, or normal during such a trip? At which parts? Discuss with your neighbors. Warm-Up II: Think about This & Discuss with Neighbors Consider driving in a car at constant speed over a hill. You’ve likely done this at some point. At the top of the hill, do you feel heavier, lighter, or normal? What if you are driving faster? Discuss with your neighbors Wed. May 18 – Physics Lecture #7 Forces, Motion, and Gravity 0. 1. 2. 3. 4. Announcements Forces and Motion Circular Motion Gravity Gravity and Circular Motion Uncredited images from OpenStax College Physics A ball flying in the air affected only by gravity has an acceleration that points down towards the ground as shown in the figure. What is the direction of the velocity of the ball at the instant shown? 1 4 2 5 Can’t be determined 3 a Consider the semi-circular portion of track shown. A marble follows the track, and exits at point P as shown in a top-down, bird’s eye view. Which of the sketched paths is the best prediction for the path the marble will follow after exiting the track? 3 2 4 1 5 P A net force of 10 N is applied to an object, resulting in an acceleration of 2 m/s2. What is the object’s mass? 1. 0.2 kg 2. 2 kg 3. 5 kg 4. 10 kg 5. 20 kg 6. None of the above You push straight upward with a force of 150 N on a 10 kg object. Determine the acceleration of the object. You push straight upward with a force of 50 N on a 10 kg object. Determine the acceleration of the object. You are standing in an elevator. At some instant, the magnitude of the normal force of the floor pushing up on you is greater than the magnitude of the gravity force acting down on you. At this instant, what can you say about the direction of your motion? 1. You are moving up. 2. You are moving down. 3. You are not moving. 4. Not enough information. Follow the black spot, red spot, and blue spot. 1. Black spot (innermost) 2. Red spot 3. Blue spot (outermost) 4. Same 5. Can’t answer a) Which spot has the higher speed? b) Which spot has the higher angular speed? c) Which spot has the higher acceleration? Ladybug Revolution https://phet.colorado.edu/en/simulation/rotation The figure shows a bird’s-eye view of particles attached to a string moving in horizontal circles on a table-top at constant speed. a) Which particle has the largest acceleration (in the event of a tie, hold up as many cards as needed)? b) Which particle has the largest net force acting on it (in the event of a tie, hold up as many cards as needed)? A stunt pilot performs a loop-the-loop at constant speed in a vertical circle. The diagram shows the airplane at the top and the bottom of the loop. Where does the pilot feel the larger force of the seat pushing on her? 1. At the top of the loop 2. At the bottom of the loop 3. Same at top and bottom of loop 4. Not enough information Consider Wed. May two 18 –identical Physicsobjects, Lecture #7 each of the same mass m, a distance r apart. If each mass is doubled, and the distance also doubles, by how much does the gravitational force change? 1. No change 2. Increases 2x 3. Increases 4x 4. Decreases 2x 5. Decreases 4x 6. Not enough info Determine the acceleration due to gravity at the surface of the earth (mass 5.97 x 1024 kg, radius 6.37 x 106 m). The earth (mass 5.97 x 1024 kg, radius 6.37 x 106 m) and moon (mass 7.35 x 1022 kg, radius 1.74 x 106 m) are a distance 3.84 x 108 m apart. a) Determine the force of gravity between the earth and the moon. b) Assuming a circular orbit with a stationary earth, determine the period of the moon’s orbit. Geosynchronous satellites are placed in a circular orbit around the earth (mass 5.97 x 1024 kg, radius 6.37 x 106 m) at such a distance that their orbital period is exactly equal to 24 hours. a) Why might this be useful? b) How high above the surface of the earth must such a satellite orbit? c) How fast must such a satellite orbit?