Physics 271: Advanced Honors Physics I
Fall 2017

Practice 1: answers and comments

1. 2.5 s. The most efficient way to take care of the unit conversion is to convert the difference of the two velocities (137 km/h - 90 km/h = 47 km/h) to m/s.
2. 10.6 m. If you got twice this number, reread carefully what the problem asks you to find. If you found the time first, then the distance, it is correct but not efficient. Look at the constant-acceleration relations to find one that gets you directly there (without having to do an extra unnecessary computation).
3. 1.62 x 1015 m/s2.
4. (a) 3.60 m/s2 A common mistake is to think v = 0 and use a = (v2-v02)/(2L) -- BUT v is the velocity just before impact and (unfortunately for the car and driver) is NOT zero. The given time is the last bit of information you need to find the answer.
(b) 8.40 m/s
5. 41.6 s In this situation, the elevator reaches its maximum speed and travels at that speed for a while before it has to decelerate in order to arrive at the destination at rest.
7. (a) 5.4 s (b) 41 m/s
8. 12.3 m/s. First find the time for the first stone to fall.

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