3.6 Problem-solving strategies (2d)  (Page 2/3)

Step 3. Once a free-body diagram is drawn, Newton’s second law can be applied to solve the problem . This is done in [link] (d) for a particular situation. In general, once external forces are clearly identified in free-body diagrams, it should be a straightforward task to put them into equation form and solve for the unknown, as done in all previous examples. If the problem is one-dimensional—that is, if all forces are parallel—then they add like scalars. If the problem is two-dimensional, then it must be broken down into a pair of one-dimensional problems. This is done by projecting the force vectors onto a set of axes chosen for convenience. As seen in previous examples, the choice of axes can simplify the problem. For example, when an incline is involved, a set of axes with one axis parallel to the incline and one perpendicular to it is most convenient. It is almost always convenient to make one axis parallel to the direction of motion, if this is known.

Step 4. As always, check the solution to see whether it is reasonable . In some cases, this is obvious. For example, it is reasonable to find that friction causes an object to slide down an incline more slowly than when no friction exists. In practice, intuition develops gradually through problem solving, and with experience it becomes progressively easier to judge whether an answer is reasonable. Another way to check your solution is to check the units. If you are solving for force and end up with units of m/s, then you have made a mistake.

Section summary

• To solve problems involving Newton’s laws of motion, follow the procedure described:
  1. Draw a sketch of the problem.
  2. Identify known and unknown quantities, and identify the system of interest. Draw a free-body diagram, which is a sketch showing all of the forces acting on an object. The object is represented by a dot, and the forces are represented by vectors extending in different directions from the dot.
  3. Write Newton’s second law in the horizontal and vertical directions and add the forces acting on the object. If the object does not accelerate in a particular direction (for example, the $x$ -direction) then $F_{\text{net}x}=0$ . If the object does accelerate in that direction, $F_{\text{net}x}=\text{ma}$ .
  4. Check your answer. Is the answer reasonable? Are the units correct?

Problem exercises

A $5\text{.}\text{00}\times {\text{10}}^5\text{-kg}$ rocket is accelerating straight up. Its engines produce $1\text{.}\text{250}\times {\text{10}}^7\text{N}$ of thrust, and air resistance is $4\text{.}\text{50}\times {\text{10}}^6\text{N}$ . What is the rocket’s acceleration? Explicitly show how you follow the steps in the Problem-Solving Strategy for Newton’s laws of motion.

Using the free-body diagram:

$F_{\text{net}}=T-f-mg=\text{ma}$ ,

so that

$a=\frac{T-f-\text{mg}}{m}=\frac{1\text{.}\text{250}\times {\text{10}}^7\text{N}-4.50\times {\text{10}}^{\text{6}}N-(5.00\times {\text{10}}^5\text{kg})(9.{\text{80 m/s}}^2)}{5.00\times {\text{10}}^5\text{kg}}=\text{6.20}{\text{m/s}}^2$ .