Kevin S. Huang

2009: Problem 10

Topic: Kinematics
Concepts: 5 kinematics equations, Free fall

Solution:

From kinematics, the trajectory of the first apple is

y_1(t)=vt-\frac{1}{2}gt^2

The trajectory of the second apple is

y_2(t)=-vt-\frac{1}{2}gt^2

The displacement between the two apples is then

\Delta y(t)=y_1(t)-y_2(t)=2vt

At $t=2\,\mathrm{s}$ , we have

\Delta y=2(7\,\mathrm{m/s})(2\,\mathrm{s})=28\,\mathrm{m}

so the answer is C.