Chapter 8 Section 3 NotesChapter 8 Section 3 Notes

Motion and ForceMotion and Force

What is Force?What is Force?

□A forceforce is the cause of acceleration or change in an object’s velocity

Fundamental ForcesFundamental Forces

□There are 4 fundamental forces4 fundamental forces in nature:□Gravity□Electromagnetic force□Strong nuclear force□Weak nuclear force

Fundamental ForcesFundamental Forces

□Gravity: very, very weak force; this force acts over a long distance; much weaker than the electromagnetic force; field force

□Electromagnetic force: moderate force; acts over long distances; about 1/100 the strength of the strong force; field force

Fundamental ForcesFundamental Forces

□Strong nuclear force: strongest of all forces, but acts over a very short distance; only acts over distances the size of the atomic nucleus□Holds together protons and neutrons in

the nuclei of atoms

□Weak nuclear force: associated with nuclear decay

Fundamental ForcesFundamental Forces

Types of ForcesTypes of Forces

□Contact forces: caused when objects touch□Examples: when you push a shopping cart or

catch a baseball and it stops moving□Field forces: do not require that objects touch

each other□Examples: attraction of gravity or repulsion

between 2 north poles of a magnet□Both contact and field forces can cause an

object to move or stop moving!

Balanced and Unbalanced Balanced and Unbalanced ForcesForces

□Net force: the sum of all the forces acting on an object□Whenever there is a net force acting on

an object, the object accelerates in the direction of the net force.

Balanced and Unbalanced Balanced and Unbalanced ForcesForces

□An object will not accelerate if the net force is equal to zero.

□Balanced forces: net force of zero□Do not cause a change in motion of an

object.

Balanced and Unbalanced Balanced and Unbalanced ForcesForces

□Example of Balanced Forces:□When 2 teams are playing tug of war

and both teams are exerting equal amounts of pressure, there is no acceleration, and neither team moves.

Balanced and Unbalanced Balanced and Unbalanced ForcesForces

□Unbalanced forces: occurs when forces acting on an object don’t cancel out; there is a greater force acting on 1 side of the object□Example: When 2 teams are playing tug

of war and one team exerts a greater force and pulls the other team forward

Balanced and Unbalanced Balanced and Unbalanced ForcesForces

□When an object is at rest, are there still forces acting on it?□Yes! It is just not moving because the forces

are balanced.

□What happens if forces act in different directions that are not opposite one another?□The combination of forces acts like a single

force and cause acceleration in a combined direction

□Example: You push your book east and your friend pushes the book north; the book will move northeast

FrictionFriction

□Friction: force between 2 objects that opposes the motion of either object□Example: Imagine a car rolling on a flat, evenly

paved street. Eventually, the car will stop due to the force of friction.

□Friction depends on surfaces in contact:□Frictional forces are greater when both

surfaces are rough□Example: new tennis shoes walking along a carpeted

floor will keep you from falling down.

FrictionFriction

□Two types of friction:□Static Friction: when 2 objects touch but

do not move□Example: A filing cabinet sitting on the floor

FrictionFriction

□Kinetic Friction: friction between moving surfaces. Two types of kinetic friction:□Sliding friction: when 1 object slides over

another□Example: Sliding a box across the counter

□Rolling friction: when 1 object rolls across another

□Example: Rollerblading

FrictionFriction

□The force necessary to make a stationary object start moving is usually more than the force necessary to keep it moving; therefore, static friction is usually greater than kinetic friction.

□Rolling friction is usually less than sliding friction.

Friction and MotionFriction and Motion

□Friction is not all bad; it is actually required for many everyday tasks to work correctly.□Examples:

□Cars could not move without friction; as a car’s wheels turn, they push against the road and as a reaction, the road pushes forward on the car.

□You would not be able to write down these notes either!

Friction and AccelerationFriction and Acceleration

Air ResistanceAir Resistance

□Air Resistance: a type of friction□Air Resistance is caused by the

interaction between the surface of a moving object and the air molecules

□Air Resistance depends on 3 factors:□Size of the object□Shape of the object□Speed of the object

GravityGravity

□Gravity: force of attraction between 2 particles of matter due to their mass□Gravity is different from other forces

because: gravity acts on an object even when the objects don’t touch

□All objects in the universe attract each other through the force of gravity!

GravityGravity

□Gravity depends on two factors:□The mass of the

objects: the greater the mass of the object, the larger the gravitational force

□The distance between the objects: as the distance between 2 objects increases, the gravitational force decreases.