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