Forces Force any push or pull acting on

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/s 2

Gravity depends on mass

Gravity depends on distance

Mass vs Weight