Chapter 4

1642, several months after Galileo died, Isacc Newton was born. Newton is the father to: Classical Mechanics Born prematurely and barely survived on Christmas Day, Wrote one of the greatest scientific books ever: Principia Mathematica Philosophiae Naturalis

Definition of Force A force is a push or pull on an object in a particular direction. Forces affect how objects move. They can make objects: – start or stop moving – move faster or slower – change direction – change shape.

Definition of Forces If forces cause objects to speed up, slow down, and change direction then that means that they affect _____________. If they affect velocity then they affect _______________. Therefore forces cause ACCELERATION.

Definition of Forces There are a lot of misconceptions about what a force is and what it isn’t.

Definition of Force It is important to remember that FORCES do not exert forces, objects exert forces. You cannot say that the force on an object is because of these: You can use these terms: Centripetal force Friction Acceleration Gravity Inertia Velocity Tension Drag String Earth Hand Table Floor Air Water Magnet

Definition of Forces Example: – When a ball is kicked only the earth and air act upon the object not gravity and drag. – The road, through friction makes a car move forward not friction, velocity, or momentum. When in doubt just remember…

Free – Body Diagrams When representing the forces acting upon an object, it is important to use a free-body diagram. – A well made diagram makes the problem simpler and minimizes the negatives in an equation.

Free – Body Diagrams 1.Use arrows to show the direction and magnitude of the force applied. 2.Label the forces. 3.Choose a positive and negative direction. –O–Often the positive direction will be the larger of the two forces.

Free – Body Diagram Example 1.Use arrows to show the direction and magnitude of the force applied. 2.Label the forces. 3.Choose a positive and negative direction. F Earth on people F people on Earth + -

Free – Body Diagram Practice For each of the following draw a free-body diagram. A flowerpot falls freely from a windowsill. A car traveling at a constant velocity crashes into a brick wall. F Earth on flowers F air on flower + - F wall on car F car on wall - +