Tutorial 6 Homework Tutorial section Friction in Real World Physics ll. Pushing

Question

Tutorial 6 Homework Tutorial section Friction in Real World Physics ll. Pushing a Couch A student is trying to push a heavy couch across the floor. She pushes and pushes until she finally gets the couch moving at a steady pace. Below is a graph of the force of friction from the surface on the couch versus fsc I. Use the letters on the graph to describe when the motion of the couch begins. Explain 2. Use the letters on the graph to describe the force applied by the student onto the couch during each period of time. 3. What could the student do to make moving the couch easier? Explain. Florida International University Physics Education Research Group, Spring 2010

Explanation / Answer

This question requires familiarity with the frictional force, and the coefficients of static and kinetic friction.

Question 1) The motion of the couch begins at points B/C because there is a sudden change in the force of friction there. That is important because the coefficient of kinetic friction is usually less than the coefficient of static friction. That means that more force is needed to get an object to begin to move than it takes to keep it moving.

Question 2) From point A to point B, the student is applying more and more force in order to get the couch to move. That is why the force keeps increasing in that period.

From point B to point C, the couch begins to move, and the student suddenly needs less force in order to keep the couch moving.

From point C to point D, the student is moving the couch with a constant force that is less than what was needed in order to get it moving, since the coefficient of kinetic friction is less than that of static friction.

Question 3) In order to make moving the couch easier, the student can also apply an upwards force to the couch while they are trying to move it. That is because the frictional force is proportional not only to the coefficient of friction, but also to the normal force on the couch. If no other vertical forces act on the couch, the normal force is just equal to the weight of the couch. But if an upwards force is applied to the couch, there will be less net force on the ground (since it would subtract from the weight), and therefore a lower normal force. That means that less horizontal force will need to be applied on the couch in order to overcome the frictional force.

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