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Physics 6A Practice Midterm1 solutions

# Physics 6A Practice Midterm1 solutions - Practice Midterm#1...

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Practice Midterm #1 Solutions Physics 6A

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1. You drive your car at a speed of 40 km/hr for 1 hour, then slow down to 30 km/hr for the next 20 km. How far did you drive, and what was your average speed? 20 km 40 km We can draw a simple diagram with distances and times. For each section we use the formula “Distance = Speed x Time” ( ) ( ) ( ) ( ) hr 3 2 t t 30 km 20 2 Section km 40 hr 1 40 x 1 Section 2 2 hr km hr km 1 = = = = 1 hr 2/3 hr The total distance is 60km. Total time is 5/3 hr. Divide these to get the average speed: hr km 3 5 36 hr km 60 speed . avg = = answer (b)
2. A speeding car is traveling at 40 m/s down a straight road. The driver slams on the brakes, and the car comes to a complete stop in 5 seconds. Assuming constant acceleration, how far did the car travel while braking? We can use one of our kinematics formulas. ( ) 0 2 0 2 x x a 2 v v + = This formula will work, but we need to find the acceleration first. We can use ( )( ) 2 s m s m 0 8 a s 5 a 40 0 at v v = + = + = Now plug into our previous equation: ( ) ( ) ( ) ( ) m 100 x x x x 8 2 40 0 x x a 2 v v 0 0 s m 2 s m 2 0 2 0 2 2 = + = + = Answer (c)

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3. A rock is thrown upward from the top of a 300-meter high cliff. The initial speed is 10 m/s. If we ignore air resistance, how long does it take for the rock to hit the ground below? y=0 y=300m v 0 =10m/s First we can draw a sketch and choose a coordinate system. I have chosen the ground as y=0. That makes the initial height y 0 = 300m. We also know the initial velocity, and the final height. We can use one of our basic kinematic equations: ( ) ( )( ) ( ) sec 9 . 6 t sec 9 . 8 t 9 . 4 2 300 9 . 4 4 10 10 t 0 300 t 10 t 9 . 4 t 8 . 9 t 10 m 300 0 t g t v y y or 2 2 2 s m 2 1 s m 2 2 1 0 0 2 = ⎯ → = ± = = + = + = This gives us a quadratic equation, which we can solve with the quadratic formula. a 2 ac 4 b b t 2 ± = Choose the positive value. Answer (b).
4. Two rocks are thrown with the same initial speed from the top of a 300-meter high cliff. Rock A is thrown upward and Rock B is thrown downward. If we ignore air resistance, which of the following is true about the speeds of the rocks just before they hit the ground? a) Rock A is faster. b) Rock B is faster. c) Rock A and Rock B have equal speeds. d) There is not enough information to determine the answer. y=0 y=300m Rock A y=0 y=300m Rock B The rocks will both land at the same speed . Rock A goes up to some high point, then turns around. By the time it passes the starting point on the way down, it is moving at the initial speed, just opposite direction. So it is just like rock B at this point. Of course rock B hits first, but they are both moving the same speed when they hit.

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Physics 6A Practice Midterm1 solutions - Practice Midterm#1...

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