Physics 2210 Lecture 3

1. Physics 2210Lecture 3 Today’s Concepts: a) Relative motion b) Centripetal acceleration

2. Relative Motion

3. Checkpoint vdog,belt= 8 m/s vbelt,ground= 2 m/s • A girl stands on a moving sidewalk that moves to the right at 2 m/srelative to the ground. A dog runs toward the girl in the opposite direction along the sidewalk at a speed of 8 m/s relative to the sidewalk. • What is the speed of the dog relative to the ground? A)6 m/sB)8 m/sC)10 m/s

5. Which direction should I point the toy bulldozer to get it across the cardboard fastest? A) To the left B) Straight across C) To the right Tractor Demo (moving cardboard) A B C

6. ACT • A man starts to walk along the dotted line painted on a moving sidewalk toward a fire hydrant that is directly across from him. The width of the walkway is 4 m, and it is moving at 2 m/s relative to the fire-hydrant. If his walking speed is 1 m/s, how far away will he be from the hydrant when he reaches the other side? 1 m/s • 2 m • 4 m • 6 m • 8 m Moving sidewalk 2 m/s 4 m

7. ACT y x • Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth swims perpendicular to the flow, Ann swims upstream at 30 degrees, and Carly swims downstream at 30 degrees. • Who gets across the river first? • A) AnnB) BethC) Carly Beth Ann Carly

9. Checkpoint (& Demo) A girl twirls a rock on the end of a string around in a horizontal circle above her head as shown from above in the diagram. If the string brakes at the instant shown, which of the arrows best represents the resulting path of the rock? A B C D Top view looking down

10. Centripetal Acceleration

11. Once around: v = dx /dt = 2π R / T ω = dθ /dt = 2π / T v = ω R ωis the rate at which the angleθchanges: θ R

13. We can ignore this acceleration due to Earth's rotation since its small