equal to the block’s weight, 50 N. The friction force is μF (^) N , or 10 N.
Calling the down-and-right direction positive, we can write two equations, one for each block:
(A) To solve for acceleration, just add the two equations together. The tensions cancel. We find the
acceleration to be 1.4 m/s^2 .
(B) Plug back into either equation to find the final answer, that the tension is 17 N. This is more than
the 14 N we found for the frictionless situation, and so makes sense. We expect that it will take
more force in the rope to overcome friction on the table.
6 . Although Bert is right that acceleration is zero on the freeway, this means that the NET force is zero;
the engine still must produce a force to counteract air resistance. This is what Oscar says, so his
answer is correct. Ernie’s answer is way off—acceleration is not velocity/time, acceleration is a
CHANGE in velocity over time.
Rapid Review
• The net force on an object equals the mass of the object multiplied by the object’s acceleration.
• To solve a problem using F (^) net = ma , start by drawing a good free-body diagram. Resolve forces into
vector components. For each axis, the vector sum of forces along that axis equals ma (^) i , where a (^) i is the
acceleration of the object along that axis.
• When an object is on an inclined plane, resolve its weight into vector components that point parallel
and perpendicular to the plane.
• For problems that involve a massless pulley, remember that if there’s one rope, there’s one tension.