# Physics 216, Show My Homework 3

An object moving in one dimension starts from rest at x=0m

From t=0 s to t=10 s the object has a constant acceleration $a=+5m/s^2$

From t=11 s to t=20 s the object has a constant acceleration $a=-7m/s^2$

i) Fill in the empty cells in the table to the left showing the position and velocity of the object as a function of time

ii) Make an x-y plot of position as a function of time from t=0 to t=20s (started below)

iii) Make an x-y plot of velocity as a function of time from t=0 to t=20s (started below)

iv) Approximately when, if at all, is the object not moving?

The object is not moving around t =17 sec. Precisely the object is not moving at t=17.14 sec.  How did you determine your answer?

The object is not moving when its velocity is zero, in other words when the velocity graphs intersects the x axis.

Can an object be stationary, but accelerating?  Explain.

The answer is yes. To be stationary the condition is that its velocity is zero, to accelerate it means that its acceleration is non zero. Acceleration is different from speed (is the first derivative of speed

with respect to time) and can be nonzero is speed has only instantaneous zeros (graph of speed has

nonzero slopes).

Elapsed time in each interval (s)     Total time (s)     x (m)     v (m/s)     a (m/s2)
0     0     0     0     5
1     1     2,5     5     5
2     2     10     10     5
3     3     22,5     15     5
4     4     40     20     5
5     5     62,5     25     5
6     6     90     30     5
7     7     122,5     35     5
8     8     160     40     5
9     9     202,5     45     5
10     10     250     50     5
1     11     296,5     43     -7
2     12     336     36     -7
3     13     368,5     29     -7
4     14     394     22     -7
5     15     412,5     15     -7
6     16     424     8     -7
7     17     428,5     1     -7
8     18     426     -6     -7
9     19     416,5     -13     -7
10     20     400     -20     -7 