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P115. Net Force Problems in 1-Dimension


Important notes about your solutions

Never put these forces on a force diagram: net force and apparent force (or apparent weight). Here are the reasons:

  • Net force is not a type of force in the same sense as tension, friction, normal, and weight. Rather, net force is just a name for the vector sum of all the forces acting on an object.

  • Apparent force is too general a term and can be misleading as well. One can always identify any so-called apparent force as a specific force type. For example, what the textbook author calls apparent weight on page 126 is actually a normal force. And what he calls apparent weight on page 127 is actually a tension force. (You're not supposed to read those pages anyway.)

Here is the rubric for evaluating net force solutions.

Here is a list of mistakes that students have made in the past on net force solutions.

Part A

  1. Review the method used to solve net force problems as described in the Guide to Solving Net Force Problems and this example.

  2. Download and print this template. Write your solution to the problem given on the template.

Part B

Show your work for the problems below using the same steps as on the template but do not actually write on the template form this time. Here is an example of a textbook problem solved in this style.

  1. Refer to the diagram to the right.  When an object is first dropped, air friction is very small. The main force acting is weight. The object will accelerate due to the large unbalanced force of weight. As the object falls, air friction increases. Since air friction acts opposite weight, the net force will become smaller as the air friction increases, weight remaining constant. Acceleration decreases likewise because acceleration = net force/mass. Eventually, the force of air friction will balance the weight, and the object will fall at a constant velocity called the terminal velocity.

  1. At terminal velocity, suppose the force of air friction on a skydiver is 750 N. What is the diver's mass?

  2. At an earlier time when the skydiver had an acceleration of 3.0 m/s² downward, what was the magnitude of the force of friction acting on him?

  1. A block is given a push to start it moving across a horizontal surface. Then the push is removed and the block slides to a halt due to the influence of friction. If the mass of the block is 6.0 kg and the magnitude of the friction force is 40.0 N, determine the magnitude and direction of the block's acceleration as it is slowing down. (Note the underlined phrase. This means that your force diagram must not include the push. Note also that your force diagram must include all the forces--vertical included--acting on the block.)
 


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