A quicker way of solving for the acceleration is to treat the entire system (blocks
plus string) as one object. Since we are only concerned with forces acting on the
object, we can ignore tension. The string is part of the object. Then we only need to
consider forces acting in the direction of motion (Mg) and forces opposite the
direction of motion (none). Our mass in Newton’s second law becomes M + m, so
Fnet = ma becomes Mg = (M + m)a, giving us the same answer for acceleration.
- Using the diagram from the previous example, assume that m = 2
kg, M = 10 kg, and the coefficient of kinetic friction between the
small block and the tabletop is 0.5. What is the acceleration of the
blocks?
Here’s How to Crack It
Once again, draw a free-body diagram for each object. Notice that the only
difference between these diagrams and the ones in the previous example is the