BELLWORK 10/26/15  What mechanism causes motion? (What do you think causes objects to move?)

4.1 The First Law: Force and Inertia Objects in motion remain in motion at a constant speed in the same direction and objects at rest remain at rest unless acted upon by an unbalanced force.

4.1 Force  Force is an action that can change motion.  A force is what we call a push or a pull, or any action that has the ability to change an object’s motion.  Forces can be used to increase the speed of an object, decrease the speed of an object, or change the direction in which an object is moving.

FORCE (FREE-BODY) DIAGRAMS  Because the effect of force depends on both magnitude and direction, force is a vector quantity.  Diagrams that show force vectors as arrows are called free-body diagrams.  Force is measured in units of Newtons.  1 N = 1kg x 1m/s 2

 A force diagram, which is also known as a free body diagram, is a sketch in which all the force vectors acting on an object are drawn with their initial points at the center of the object.

Problem 1 A book is at rest on a table top. Diagram the forces acting on the book. In this diagram, there are normal and gravitational forces on the book.

Problem 2 An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

ANSWER Gravity is the only force acting on the egg as it falls.

Balanced Forces  There are 2 cases when all of the forces acting on an object are equal:  1. When an object is not moving  2. When an object is moving with constant velocity.

Problem #4  A man is pulling a sled. The magnitudes of the forces acting on the sled are 60N by the string, 130N by the Earth, and 90N upward by the ground. Draw and label the free-body diagram.

Classwork / Homework  Page 124 Section Review #’s 1-5 except #2

Bellwork 10/27/15  Get a book and complete #’s 1 and 2 under Practice A on Page 124..

VIDEO

4.2 OBJECTIVES  1. Explain the meaning of inertia  2. Describe how net forces acting on an object allow it to move.

4.2 Inertia  Inertia is a term used to measure the ability of an object to resist a change in its state of motion.  An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop.  The word “inertia” comes from the Latin word inertus, which can be translated to mean “lazy.”

Explain this trick using Newton’s 1 st Law of Motion 

BELLWORK 10/29/2015  Explain the Physics behind Newton’s Law of Inertia for the index card and penny trick.  Why does having more pennies stacked on top of each other make it easier to pull the index card out?

REAL-LIFE APPLICATION OF NEWTON’S 1 ST LAW

 Describe what is meant by “net force?”

What is Net Force?  The net force is described as the total of all forces acting on an object.

Net Force (cont) Adding up forces can be different from simply adding or subtracting numbers because the direction of the forces have to be considered

Adding Forces  When two forces are acting in the same direction, the net force is the sum of the two.

Subtracting Forces  When two forces are acting in opposite directions, you subtract the two.

Evaluate  Homework:  Complete the Free-Body Diagram Wksht and the Net Force Worksheet