1. Section 2.1 Outline
Forces Change Motion

2. I. A Force is a Push or a Pull
FORCE: a push or a pull
Example: picking up your backpack

3. A. Types of forces 1) Contact force: when an object pushes or pulls
an object by touching it
Example: skater applies CONTACT FORCE to the ground when skating
2) Gravity: force of attraction between two masses
Example: Earth’s GRAVITY pulls skater to the ground
3) Friction: a force that resists motion between two surfaces that are pressed together
Example: ground’s surface resists the skater’s forward motion

4. B. Size and Direction of Forces
1) like velocity, force is a vector which means the force has both size and direction
2) example: shooting a basketball (needs both direction and amount of force)

5. C. Balanced and Unbalanced Forces
1) NET FORCE: overall force acting on an object when all forces are combined
2) If the net force on an object is zero, the forces are balanced; balanced forces have the same effect as no force at all
3) Only an unbalanced force can change an object’s motion; it does not matter whether the object started at rest or was already moving

6. D. Forces on Moving Objects
1) An object with forces acting on it can be moving at a constant velocity as long as those forces are balanced
2) Balanced forces CANNOT change an object’s speed or direction
3) An unbalanced force is needed to change an object’s motion

7. II. Newton’s First Law Relates to Force and Motion
1) Ancient Greeks concluded that it was necessary to apply a constant force to keep an object in motion
2) Greeks reasoned that an object stops moving because you stop pushing it

8. A. Galileo’s Thought Experiment
1) Galileo concluded that it does not take a force to keep an object moving; it takes a force (friction) to stop an object that is already moving
2) Objects at rest and objects in motion both resist changes in motion
3) Objects at rest tend to stay at rest and objects in motion tend to continue moving unless a force acts on them

9. B. Newton’s First Law
NEWTON’S FIRST LAW: states that objects at rest remain at rest, objects in motion remain in motion, unless acted upon by an unbalanced force
2) example: a soccer ball will continue moving until stopped by a goalie or net

10. C. Inertia
INERTIA: the resistance of an object to change in the speed or direction of its motion
Inertia is closely related to mass; when measuring the mass of an object, you also measure its inertia
It is harder to change the motion of an object (inertia) if it has more mass