SPRING 2002 QUIZ 1

PHYSICS 100A

1. A bicyclist rides 150m east from the starting point, then rides 200m west as shown in the figure. The whole journey takes 55 seconds. Find his average speed and velocity during this time interval.

Solution.

We find the average speed from

                   Average speed = d/Dt = (150m + 200)/55s = 6.4m/s.

We choose the east as positive direction of the motion.

The initial position of the bicyclist is x₀ = 0m, and his final position is x_f = 150m - 200m = -50m.

We find the average velocity from

                  v_av = Dx/Dt = (-50m - 0)/55s = -0.9m/s.

Note that the negative sign means the velocity is in the opposite direction.

2. A car travels at 60km/h, all of sudden the driver brakes the car, the car comes to stop in 8 second. What is the car's average acceleration during this time interval?

Solution.

We find the average acceleration from

                               a_av = Dv/Dt = [0 - (60km/h)(1000m/km)/(3.6ks/h)]/8s = - 2.1m/s².

Note: Negative sign means that the car is decelerating.