Kevin S. Huang

2008: Problems 10-11

Topic: Dynamics
Concepts: Data expression, Newton’s laws

Solution:

By Newton’s 2nd law, we have

F-f=ma

In terms of $F$ ,

F=ma+f

so if we plot $(a, F)$ data points, we will obtain a line with slope equal to the mass $m$ . Choosing two of the data points,

m=\frac{\Delta F}{\Delta a}=\frac{5.05\,\mathrm{N}-3.05\,\mathrm{N}}{0.495\,\mathrm{m/s^2}-0.095\,\mathrm{m/s^2}}=5\,\mathrm{kg}

so the answer is B.

Topic: Dynamics
Concepts: Data expression, Forces in mechanics

Solution:

By Newton’s 2nd law, we have

F-f=ma

Since friction is kinetic,

F-\mu mg=ma

\mu=\frac{F-ma}{mg}

We found $m=5\,\mathrm{kg}$ in the previous problem, so we just need to choose a data point to get values of $F$ and $a$ ,

\mu=\frac{4.05\mathrm{N}-(5\,\mathrm{kg})(0.295\,\mathrm{m/s^2})}{(5\,\mathrm{kg})(10\,\mathrm{m/s^2})}=0.05

Thus, the answer is A.