Forces & Motion

What is a Force?•Force: push or pull

•Unit: Newton (N) Kg x m/s2

•Vector: has both magnitude & direction

Combining Forces•Use vector addition•Net Force: the total force acting on an

object

•Balanced Forces: when the net force is zero▫No motion occurs

•Unbalanced Forces: when the net force is not zero▫Motion occurs (acceleration or change in

direction)

Balanced Forces:

Unbalanced Forces:

Friction•Friction: a force that opposes (acts

opposite to) motion

•Four types:▫Static▫Sliding▫Rolling▫Fluid

Types of Friction•Static: friction force on objects that are

not moving (greatest friction force)•Sliding: friction force on an object as it

slides over another•Rolling: friction force on an object as it

rolls over another (ex. Ball bearings)•Fluid: friction force on an object in a fluid

(liquid or gas) (ex. Air resistance)

Gravity•Gravity: downward force pulling objects

toward the center of Earth

•Acceleration on Earth due to gravity = 9.8 m/s2

•Terminal velocity: constant velocity that a falling object reaches when the force of gravity is equal to the air resistance

Projectile Motion•An object thrown forward will follow a

curved path due to gravity

•The combination of an initial forward velocity and the downward vertical force of gravity causes the object to follow a curved path

Projectile Motion

Scientists who contributed to the ideas about Force & Motion

•Aristotle: incorrectly proposed that force is required to keep an object moving at a constant speed

•Galileo: studied constant acceleration due to gravity

•Newton: defined force & mass; introduced 3 laws of motion

Newton’s First Law of Motion•Law of Inertia

•An object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force

•If the net force = 0, then the object’s state motion will remain unchanged

Newton’s Second Law of Motion•The force acting on an object is equal to

the mass of the object times the object’s acceleration

•Force = mass x acceleration (or F = m x a)

•Units: N = kg x m/s2

Weight & Mass•Weight: the force of gravity acting on an

object W = m x g same equation as F = m x a Changes depending on the gravitational force

•Mass: a measure of inertia on an object (how much matter is within an

object) Constant no matter what the gravity

Newton’s Third Law of Motion•For every action there is an equal and

opposite reaction▫Forces occur in pairs

•Not all forces create motion

Momentum•The product of an objects mass times its

velocity▫An object with large momentum is hard to

stop

•Momentum = mass x velocity ▫M = m x v▫Units kg*m/s = kg x m/s

Momentum

Law of Conservation of Momentum•In a closed system, the loss of momentum

of one object equals the gain in momentum of another object▫Momentum is conserved

Universal Forces•Electromagnetic Forces: forces

associated with charged particles (electric & magnetic)

•Nuclear Forces: forces that occur within the nucleus of an atom

•Gravitational Forces: an attractive force between any two masses

Newton’s Law of Universal Gravitation•Every object in the universe attracts

every other object▫Closer distance = greater force▫Larger mass = greater force