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