Lesson Plans

Physics 4th Edition ©2002

by Jerry Wilson and Anthony Buffa

Week 4

Chapter 4: Force and Motion

College Board Performance Objectives:

• Demonstrate by definition and example an understanding of the distinction between mass and weight.
• Define equilibrium.
• Define equilibrant.
• State the conditions necessary for equilibrium.
• Demonstrate an understanding of Static Equilibrium.
• Define the units newton and slug and to be able to express them in SI and English units.
• Discuss the difference between Static Equilibrium and Translational Equilibrium.
• Make a mathematical statement of the First Condition of Equilibrium and give several physical examples.
• Experimentally demonstrate an understanding of Newton's First Law of Motion.
• State specific examples to illustrate an understanding of Newton's Third Law of Motion.
• Draw a free-body diagram representing all forces acting on an object that is in static equilibrium.
• Draw a free-body diagram representing all forces acting on an object that is in translational equilibrium.
• Apply the First Condition of Equilibrium to write two equations involving components of given vectors along the x-axis and the y-axis of a frame of reference.
• Solve simultaneous equations derived from the First Condition for unknown forces.
• Discuss the forces of kinetic and static friction and suggest a means of measuring them.
• Define the limiting angle of repose for the two surfaces involved along an inclined plane.

College Board Lab Objectives:

• Measure coefficients of static and kinetic friction using a wooden block on an inclined plane.
• Show that the coefficients of static and kinetic friction are both independent of the normal.
• Show a given set of surfaces that the coefficient of static friction is greater than for kinetic friction.
• Demonstrate that the frictional force is independent of the area of contact.
• Determine the relationship between the inertial mass of a body and its gravitational mass.

Suggested Labs:

• Inertial Balance
• Static and Kinetic Friction
• Static Equilibrium
• Inclined Plane

Resources:

• Student Edition — pp. 99–130
• Student Study Guide — pp. 40–56
• Instructor's Solution Manual — pp. 47–64
• Test Items File — pp. 57–75

Pacing Guide:

• The Concepts of Force and Net Force—day 1
• Inertia and Newton's First Law of Motion—days 1, 2, and 3
• Newton's Third Law of Motion—days 1 and 2
• Free Body Diagrams, Static Equilibrium, and Translational Equilibrium—days 2, 3, and 4
• Conditions Necessary for Equilibrium—days 2, 3, and 4
• Friction in a Mechanical System—days 3 and 4
• The Inclined Plane as a Physical System with Zero Acceleration—days 3 and 4
• Lab—day 5
• Block Scheduling
  Force, inertia, free body diagrams, and Newton's First and Third Laws of Motion and applications require two blocks. Spend the last block on friction and further application of the first and third laws. Spend as much time as possible with examples showing different situations and applications of equilibrium and the first and third laws.

Key Words:

Critical Thinking Questions:

1. A 25 kg wooden crate is pulled at constant speed across a horizontal floor by a rope making an angle of 30° with the floor. The coefficient of friction between the surfaces is 0.4. What is the normal force acting on the crate? What is the tension in the rope?
2. An inclined plane is inclined at some angle. A wood box rests midway along the plane in a State of Impending Motion. When the box is touched it begins to slide down the plane. The coefficient of static friction for the surfaces involved is known to be 0.25. Determine the angle.
3. A wall meets a ceiling in the normal perpendicular fashion. A length of rope, rope 1, is attached to a hook on a ceiling. The free end is attached to a 50 kg mass. A second rope, rope 2, is attached to the mass. Rope 2 is pulled to the right and is tied to a wall hook that is below the mass. With the system in static equilibrium, rope 1 is measured to make an angle of 60° with the ceiling and rope 2 makes an angle of 70° with respect to the wall. What is the tension in each rope?
4. A 20 kg mass experiences twice the gravitational force as a 10 kg mass. Why doesn't the 20 kg mass fall faster than the 10 kg mass?

Troubleshooting Tips/Error Traps:

• In problem solving, make free-body diagrams, attach a frame of reference, and label all vectors.
• Recognizing Newton's First Law is limited to zero acceleration where the velocity of the body is either zero or it is constant.
• Students may not be careful in considering vector behavior. Free-body diagrams can present problems. Using examples, the teacher should emphasize free-body diagrams with properly identified forces. The student should practice making these diagrams.
• Some students develop a habit of making the normal force vertical. It must be emphasized that normal means perpendicular. It is the perpendicular force the surface exerts on the body. Regardless of the force applied, surfaces only know how to "push back" in a perpendicular manner.
• Students find motion up or down inclined planes to be confusing. Emphasizing a frame of reference being attached to the plane will help reduce the confusion. Set the x-axis parallel to the plane making down as positive and up as negative. The y-axis will be perpendicular to the plane where up is positive and down is negative. The normal force will be along the y-axis.
• Help students in making the proper free-body diagrams with the directions of the force properly labeled. Stress the importance of the frame of reference and labeling the vectors.
• Help students identify the direction of friction vectors. Emphasize that frictional forces will always oppose the direction of motion or the tendency of motion.

End of Chapter Activity:

1. Place a wooden block on a table so its greatest area contacts the table. Use a spring scale to drag a wooden block across the table with constant velocity to study the effects of friction. Add masses to the block and repeat the activity. How does the addition effect the normal? How does the addition of the masses effect friction?
2. Arrange the block so it rests on edge. Repeat the above activity. How do the results compare? What conclusion can you make?
3. Repeat the first activity on a glass surface. What happens? What conclusions do you reach? Wet the glass surface. What happens?
4. Place a 3 by 5 card over the mouth of a cup. Place a quarter at the center of the card. Quickly, flick the card with your index finger. What happens? Why does this happen?

Suggested Problem Assignments: