Calculations With Newton's Second Law of Motion
Example
Solution
Any time we are given a force problem in which the net force acting on an object is NOT equal to zero, we should immediately think to use Newton's Second Law of Motion. Newton's Second Law of Motion states that the net force acting on an object is equal to the mass of the object multiplied by the acceleration of the object. Before we start plugging values into the equation, we need to solve the equation for the quantity we wish to find. See Ms. Clanton's work at the right. We are attempting to find the acceleration of the refrigerator, and so we need to solve Newton's Second Law of Motion for acceleration. When we do, we end up with the net force acting on the refrigerator divided by the mass of the refrigerator is equal to the acceleration of the refrigerator. Next, we need to find the net force acting on the refrigerator. If we look at the image, we can see that there is an applied force of 500 Newtons to the right acting on the refrigerator and a frictional force of 600 Newtons to the left acting on the refrigerator. Thus, the net force acting on the refrigerator is 100 Newtons to the left. Now, we are ready to plug what we know into the equation and to solve for the acceleration of the refrigerator. Don't forget to include the direction and the units whenever you are solving for acceleration!

