Bugge/Fitzgerald: Dynamics 4

Inertial and Non-Inertial Reference Frames 4.1 Observe and Analyze You are sitting at a light with a coffee cup sitting on the dashboard in front of you. The coffee cup is at rest. All of a sudden, the coffee cup starts sliding towards you. At the same , your friend who was waiting on the sidewalk observes that the car starts moving forward at increasing and the coffee cup remains stationary with respect to her.

a) Describe the of the cup when it starts sliding using a motion diagram for each observer: you in the car and your friend on the sidewalk. b) Explain the behavior of the cup when it starts sliding using a diagram for each observer: you in the car and your friend on the sidewalk. c) Are the force diagrams and motion diagrams for each observer consistent? Justify your answer. d) What can you say about the relationship between the unbalanced force and change in motion for the observer on the car that starts to move? e) How will you rewrite the relationship between force and change in motion to include the role of the observer? f) Why do you think your friend on the sidewalk did not see the cup starting to move (i.e. change its motion) when the car started to move? 4.2 Watch the video of the dancers. What did you observe? Identify inertial and non-inertial reference frames in the video.

4.3 Now let’s go back to the table setting. How can you describe the situation in terms of inertial and non-inertial reference frames? Why were the plates not disturbed when the tablecloth was removed?

4.4 Reason a) You are a passenger in a car. All of a sudden, your head jerks backwards. Explain this experiment from the reference frame of the car. Explain this experiment from the reference frame of the pavement. b) Describe what an observer on the ground sees when you stumble on a rock or slip on a banana peel (focus on the motion of your feet and your head, assuming that the head is only loosely attached to the body). Then describe what you observe. c) Use Newton’s first law to explain the observations on the earth-based observer and your observations for the situation described in b. Who is in the inertial reference frame? How do you know? d) Imagine that you have an infinitely long, smooth table covered in sand. A bowling ball is hit once so that it starts on the table but stops after 2 m. After removing some of the sand and repeating the experiment, the ball stops rolling after 5 m. How far do you think the ball will roll if ALL the sand is removed? e) If you take a ball whose is half the mass of the bowling ball, how will the outcome of the last experiment change? Bugge/Fitzgerald: Dynamics 4

4.5 Represent and Reason An elevator starts at rest on the ground floor of a building and stops at the top floor. The elevator then returns to the bottom floor. The elevator is our object of interest. The observer is on the ground. Complete the table that follows to determine how the force the supporting cable exerts on the elevator compares to the force Earth exerts on the elevator. The motion diagram and the force diagram should be consistent with each other and with the rule relating motion and developed in lesson 3. Experiment Sketch a motion Draw three force Check the diagram. diagrams for 3 consistency of the consecutive clock readings diagrams on the motion diagram.

(a) Elevator hangs at rest at the ground floor.

(b) Elevator starts moving upward with increasing speed.

(c) Elevator moves at a constant upward speed.

(d) Elevator slows as it approaches the top floor.

(e) Elevator starts moving down with increasing speed.

(f) Elevator moves down at constant speed.

(g) Elevator slows to a stop on ground floor.