1. Section 1 Forces change motion Pg. 345-351 (Outline)
Chapter 11: Forces
Section 1
Forces change motion Pg
(Outline)

2. We’ll learn more about these in chapter 12
A force is a push or a pull.
Types of Forces
Contact force: when one object pushes or pull another object by touching it. Ex: your hand exerts a contact force to turn a page in your book
Gravity: the force of attraction between two masses. The force of gravity depends on two thing: mass and distance between the objects
Friction: a force that resists motion between two surfaces that are pressed together
We’ll learn more about these in chapter 12

3. Size and Direction of Forces
Force is a vector, so it has size and direction.
Ex: basketball requires a force of the correct size (not too hard or too soft) and in the right direction. **prove this in class***
In your book, force is show with red arrows. The longer the arrow, the stronger the force. The direction of the red arrow is the direction of the force. Mass is shown with a blue box.
Look at the forces (red arrows) acting on the skater on pg Which force is the strongest? Which force is the weakest? How do you know?

4. Balanced and Unbalanced Forces
Net force: the overall force acting on an object when all the forces are combined. An object will move in the direction of the net force.
Balanced forces: occurs when two or more forces applied on a object are equal in strength but in opposite directions. The net force will be zero and the motion of the object will not change. Ex: basketball players on bottom of pg. 347
Unbalanced forces: one force is stronger than another force, causing an object to move in the direction of the stronger force.

5. Forces on Moving Objects
If an unbalanced force is applied to the moving object, the object can
Speed up
Slow down
Change direction
***caption question pg. 347: compare the net force on the balls in these two photographs. Which photograph shows a net force of zero?****

6. ?????Questions?????
Does an object always move when a force acts on it?
What forces are acting on the book on your table?
Are these forces balanced or unbalanced?
If you are holding a book so that it does not move, what can be said about the force of gravity on the book and the force of your hand on the book?
When you lift and turn a page, how do unbalanced forces change its motion?

7. Newton’s first law relates force and motion
Galileo’s Thought Experiment
Ancient Greeks thought that for an object to keep moving a force had to be continuously applied.
Galileo hypothesized that if there was no friction, a object that was moving would continue moving even if a net force was NOT continuously applied.
Galileo said a force had to be applied to cause an object to stop moving (friction)
Ex: closing the door

8. Newton’s First Law
Newton used Galileo’s ideas in his first law of motion.
States that an object at rest tends to stay at rest and an object in motion tends to stay in motion at a constant velocity unless an unbalanced force is applied.
Question: can anyone really create a perpetual motion machine?

9. Inertia
The resistance of an object to a change in speed or direction.
The greater the mass of an object, the greater the object’s inertia (resistance to change).
Inertia is what causes you to keep moving forward if your car stops suddenly (pictures on pg. 351).
**If a car makes a sudden stop, what happens to a passenger riding in the back seat who is not wearing a seat belt?