Tutorials in Introductory Physics

1. Tutorials in Introductory Physics Chapter 4 – Forces and the Laws of Motion Mrs. Dimler General Physics HTHS

2. IDENTIFYING FORCES • Two people are attempting to move a large block. The block, however, does not move. Chris is pushing on the block. Pam is pulling on a rope attached to the block. ACTIVITY: On your white board, draw a large dot to represent the block. Draw vectors with their “tails” on the dot to show the forces exerted on the block. Label each vector and write a brief description of that force next to the vector.

3. NEWTONIAN PHYSICS • In Newtonian physics, all forces are considered as arising from an interaction between two objects. • Forces are specified by identifying the object on which the force is exerted and the object that is exerting the force. For example, in this situation, there is a gravitational force exerted on the block by the Earth. ACTIVITY: Describe the remaining forces you have indicated on your white board in a similar fashion.

4. FREE-BODY DIAGRAMS • The diagram you have just drawn on your white board is called a free-body diagram. A free-body diagram should show only the forces exerted on the object or system of interest (in this case, forces exerted on the block). Activity: Check your free-body diagram and, if necessary, modify it accordingly.

5. FREE-BODY DIAGRAMS CONT. • A proper free-body diagram should not have anything on it except a representation of the object and the labeled forces exerted on that object. • A free-body diagram never includes: • forces exerted by the object of interest on other objects • sketches of other objects that exert forces on the object of interest.

6. Contact Forces vs. Non-Contact Forces • All forces arise from interactions between objects, but the interactions can take different forms. Activity: Which of the forces exerted on the block require direct contact between the block and the object exerting the force? Which of the forces exerted on the block do not arise from direct contact between the block and the object exerting the force? • We will call forces that depend on contact between two objects contact forces. We will call forces that do not arise from contact between two objects non-contact forces (or field forces, or “at-a-distance” forces).

7. TYPES OF FORCES • There are many different types of forces, including: • Friction • Tension • Magnetic Forces • Normal Forces • Weight (Gravitational Force) • Electrical Force • Applied Force • Spring Force • Air Resistance Activity: Categorize the above forces according to whether they are contact or non-contact forces.

8. Consider the following discussion between two students… • Student 1: “I think the free-body diagram for the block should have a force by Chris, a force by the rope, and a force by Pam.” • Student 2: “I don’t think the diagram should show a force by Pam. People can’t exert forces on blocks without touching them.” Activity: With which student do you agree? Explain your reasoning.

9. LABELING FORCES AND FBD’S • It is often useful to label forces in a way that makes clear (1) the object on which the force is exerted, and (2) the object exerting the force. For example, the gravitational force exerted on the block by the Earth might be labeled Fgravity-on-block Activity: Label each of the forces on your free-body diagram in the manner described above.

10. ACTIVITY: DEFINE “FORCE” • A force is a push or pull on an object. • A force is an action exerted on an object which may change the object’s state of rest or motion. • Forces can cause changes in an object’s velocity with respect to time. • Forces can cause accelerations. • A force can change the state, size or shape of an object.

11. ACTIVITY: WHAT ARE THE UNITS OF FORCE? • The SI Unit of force is the Newton (named after Sir Isaac Newton, whose work contributed much to the modern understanding of force and motion). • 1N = • How much is a Newton? • A Newton is roughly equivalent to the weight of a medium-sized apple. • 1 Newton = .225 lb • 1 lb = 4.448 Newtons

12. FORCE IS A VECTOR • Because the effect of a force depends on both magnitude and direction, force is a vector quantity. Activity: Write a symbol you would use to represent FORCE and the fact that it is a vector quantity.

13. DIAGRAMS SHOWING FORCES • Diagrams that show force vectors as arrows are called force diagrams, and isolating one object and showing all forces acting on that “free-body” are called free-body diagrams.

14. ACTIVITY: FREE-BODY DIAGRAM PRACTICE • A book is at rest on a table top. • An egg is free-falling from a nest in a tree. Neglect air resistance. • A flying squirrel is gliding from a tree to the ground at constant velocity. Consider air resistance. • A rightward force is applied to a book in order to move it across a desk. Consider frictional forces. Neglect air resistance. • A skydiver is descending with a constant velocity. Consider air resistance. • A man drags a sled across loosely packed snow with a rightward acceleration. • A football is moving upwards toward its peak after having been booted by the punter.

15. Activity: Conversion Practice • Jonathan Vilma weighs 230 lbs. How much does he weigh in Newtons? How much mass does he have? How much does he weigh on the moon (in pounds)? Note: Acceleration due to gravity on the moon is 1.6 m/s2. GO SAINTS!