Physics 271: Advanced Honors Physics I
Recitations
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.
This page is maintained by Karin
Rabe.