1. Motion & Forces Chapter 11, Section 3
Force: Any action that can change the state of motion of an object. Force has magnitude & direction.
Motion: An object’s change in position relative to a reference point.

2. 4 Fundamental Forces in Nature
*Gravity: Experience it daily (force that pulls us toward Earth).
*Electromagnetic Force: Experience it daily-Friction.
Strong Nuclear Force: Holds together nucleus of atom (proton & neutrons).
Weak Nuclear Force: Holds electrons in atom.

3. Forces: Contact & Field
If you push a desk it moves. If you catch a baseball it stops moving = contact forces. Move something by physically touching it.
Field Forces: Attraction of gravity (pulls us toward Earth, keeps us from floating away) and repulsion between two north pole of magnets. Does not require that the objects touch each other.
Does my desk have forces acting on it?

4. Net Force: Balanced & Unbalance Forces
Net force: Combination of all the forces acting on the object. Net force determines if the object will change its motion.
Net force = O (zero) the object will NOT move.
If there is a net force acting on the object it will accelerate in the direction of the net force.
*Spring Scale exercise.

5. Balanced Forces
When the forces applied to an object produces a net force of zero = balanced forces.
Balanced forces do not cause an object at rest to start moving.
Balanced forces do not cause a change in the motion of a moving object.
A light hanging from the ceiling. A book on the desk.
Two people exercise.

6. Unbalanced Forces
When the net force on the object is greater than zero the object will accelerate in the direction of the greater force.
Think about two people pushing a desk in the same direction.
Think about two people unequally pushing a desk toward each other (it will accelerate).
If a person pushes a desk north and the other person pushes the desk east the desk will move in a north-easterly direction.
Tug of war activity.

7. Friction
Friction: A force that opposes motion between two surfaces that are in contact.
Friction occurs because the surface of any object is rough.
When two surfaces are touching the hills and valleys of one surface stick to the hills and valleys of the other surface.

8. Types of Friction
Static Friction: Friction between surfaces that are stationary. The force that resists the initiation of sliding motion between two surfaces that are in contact & at rest.
Kinetic Friction: Friction between surfaces that are moving over each other.
Which is greater? Is it harder to move an object that is sitting still or an object that is already moving?
Static friction is greater than kinetic friction.

9. Types of Kinetic Friction
Sliding Friction: When objects slide past each other (slide hands together).
Rolling Friction: When a rounded object rolls over a flat surface. i.e. a bowling ball
Which friction is greater?
Friction activity.

10. Unwanted vs. Helpful Friction
Friction is necessary for many everyday tasks to work correctly. Ex= walking, driving, gripping the steering wheel to turn it, writing with a pencil.
Sometimes friction is unwanted. Lubricants and low-friction materials help reduce friction.
Non stick cooking spray
Wax
Motor Oil
Grease
Air on an air-hockey tabel

11. Helpful Friction Increase friction by making surfaces rougher.
Sand on icy roads
Texture on a football, baseball batting glove
Grips on a golf club
Friction increases when the force pushing the surfaces together is increased.
Cars can not move without friction. Without friction between tires and the road the car could not move forward.