TAKEN FROM - CHAPTER 10, SEC. 1 Forces

What is a force?  A soccer ball is kicked toward the net, and the goalie stops the ball just in time by kicking it back in the opposite direction.  What caused the ball to start moving, stop moving, and change direction?  The answer is force!  Force is a push or a pull that causes an object to start moving, stop moving, or change direction.

How do we describe force? Force is described by its strength and direction, much like velocity and acceleration. The strength of force is measured in the SI Unit called the newton.  Named after Sir Isaac Newton The symbol for a newton is (N).

Strength and Direction of force The strength and direction of force is represented by an arrow. The direction of the arrow tells you in which direction the force is acting. The length of the arrow tells you how strong the force is.  The longer the arrow, the greater the force.

Combining Forces Most of the time, more than one force acts on an object at one time. The combination of all the forces acting on an object is called the net force. When forces act in the same direction the forces are added together. 5N + 5N = 10N When the forces act in opposite directions, the forces are subtracted, much like adding a positive and a negative number. 10N + 5N = 5N When equal forces act in opposite directions they cancel each other out – leaving a net force of zero. + = 0

Balanced and Unbalanced Forces Unbalanced forces occur whenever there is a net force acting on an object. Unbalanced forces ALWAYS cause a change in the object’s motion.  The object may start moving, stop moving, or change direction. Balanced forces occur when equal forces act on an object in opposite directions.  Each force is balanced by the other.  Balanced forces do not cause an object’s motion to change.

Calculating Net Force Practice force problem: (chp.10, p.337)  You exert a force of 120N on a desk. Your friend exerts a force of 150N in the same direction. What is the net force you and your friend put on the desk? The net force is 270N

Exit Slip How do balanced forces acting on an object affect the object’s motion? How do unbalanced forces acting on an object affect the object’s motion? Balanced forces cancel each other out and do not change the object’s motion. Unbalanced forces cause the object’s motion to change by either causing it to start moving, stop moving, or change direction.