1. Ch 10.3 Forces • Force: action on an object to change state of rest or motion (accelerate) • (push or pull on object … “baby” answer) • Objects change motion because of forces • 2 kinds: • Contact forces are touching, like when you push something (friction, applied force) • Field forces do not touch, like gravity or magnetism

2. Net forces • Net force: total of all forces acting on object • if net force = 0 • forces are balanced • object does not change rest/motion • if net force is not 0 • forces are unbalanced • object changes state of rest/motion (accelerates)

3. Net forces • Important distinction: • Not true that something that has balanced forces isn’t moving. It could be moving at a constant velocity (speed and direction) • True that something with balanced forces is not accelerating (changing speed, direction or both) • Balanced forces = constant motion • Unbalanced forces = acceleration (change in motion)

4. Balanced and Unbalanced • So the reason things move differently, in a scientific sense, is that it’s being acted on by unbalanced forces. • Can something be moving when the forces on it are balanced? • Yes…but it won’t accelerate. It is moving at a constant speed and direction.

5. Balanced or unbalanced? • Car moving in a straight line, 65 mi/hr • Car slowing down • Car parked • Car turning a corner • Car speeding up • Car stopped at a stoplight with engine running

6. Balanced or unbalanced?

7. The Force of Friction • Friction: acts against motion of an object (opposite direction of motion) • static: when objects not in motion (parked car on hill) • kinetic: when things are moving (tires on road around turn) • Can be rolling or sliding • Air resistance: form of friction with air molecules

8. Friction and Motion • Friction can be good or bad • Bad: parts in engine rub together. Decrease by: • lubricant (oil) • replace sliding with rolling (wheels, ball bearings) • smooth surfaces • Good: keeps car on road. Increase by adding • more force • rougher surface