Forces Force – any push or pull acting on an object Net force – the total of all forces acting on an object Balanced forces – when the net force = 0 N, forces are considered balanced, and there is no change in motion Unbalanced forces – when the net force </> 0 N, the forces are unbalanced and can cause acceleration

Four Basic Forces Gravitational force – acts on matter & light Electromagnetic force – attraction & repulsion of electric charges and magnetic fields Strong nuclear force – binds protons and neutrons together Weak nuclear force – some interactions between some subatomic particles

What forces are interacting?

What forces are interacting?

What forces are interacting?

Same direction? Add forces.

Opposite direction? Subtract them.

Remember… Objects can continue moving in a straight line without applying force Forces act on an object at rest A force is necessary to change the direction of the force

Friction – the force that opposes motion Internal friction – friction between a substance’s molecules, can create heat External friction – friction between outer surfaces of two objects Static – no movement due to balanced force Kinetic – friction between moving objects Includes rolling and sliding

Friction: Helpful or harmful? Demo 1 – books on rollers Demo 2 – lubricated jar lid

Factors Surface texture Force pushing the surfaces together Presence or absence of lubricants

Weight affects friction

Friction doesn’t change if weight doesn’t change

Static Friction = no movement

Kinetic friction = movement

Gravity Gravitational attraction exists between two objects How strong the attraction is depends on mass and distance Because Earth is so “big” relative to everything on it, everything accelerates towards Earth at the same rate, 9.8 m/s2

Gravity depends on mass

Gravity depends on distance

Mass vs Weight