FORCES Chapter 5

Mechanics The study of Motion Isaac Newton, 1600’s The father of mechanics

Kinematics The branch of Mechanics describing motion with out explaining the causes

Dynamics The branch of Mechanics that studies the forces that cause motion

Force A push or a pull

Forces are vectors Includes magnitude and direction

The Four Fundamental Forces

Gravity An attractive force between all objects Weakest of the 4 forces

Electromagnetic Force From the forces between charges When in motion produce a magnetic force Holds atoms and molecules together Photons and light

Strong Nuclear Force Holds the nucleus together against forces of repulsion Strongest of the four – only acts over a very small distance

Weak Nuclear Force Responsible for radioactive decay (including nuclear fusion in stars) Linked to the EM force

Grand Unification Theories The idea that all forces are aspects of a single force

String Theory or Superstring Theory AKA M-Theory Particles are replaced by one-dimensional “strings” The math requires 11 space-time dimensions Allows for multiverses

Inertia An object’s resistance to a change in its motion Determined by its mass

Newton’s First Law of Motion Law of Inertia An object with no net force acting on it remains at rest or moves with constant velocity in a straight line

Net Force The sum of all forces acting on an object

Newton’s Second Law of Motion The greater the force, the greater the acceleration (directly proportional) The greater the mass, the lesser the acceleration (Inversely proportional)

The Unit of Force F=ma a newton (N) The force required to accelerate a 1 kg mass 1 m/s in 1 second

Newton’s Third Law of Motion When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude but opposite in direction.

USING NEWTON’S LAWS 5.2

Weight and Mass Weight - The gravitational force on an object Mass - The amount of matter in an object

Weight is a Force W = mg An apple weighs about 1 newton

Normal Force The force pushing up on the object, perpendicular to the surface The book pushes on the table and the table pushes back on the book

Two ways to measure Mass Inertial Mass - The greater the mass, the greater force it takes to change its motion (acceleration) Gravitational Mass - Measured with a balance

Friction The force that opposes the motion between two surfaces that are in contact

Static Friction The force that opposes the start of motion The box will move when the force of the push is greater than the force of the static friction

Sliding Friction The force between surfaces in relative motion (aka kinetic friction) It is less then static friction

Coefficient of Friction How large is the force of sliding friction? – It depends on The Normal Force and the coefficient of Friction (mu) of the surface A constant that depends of the surfaces

Force of Tension Force of Tension = force of gravity = weight = mg F T = F g = mg The hanging mass is 500 grams. What is the force of tension on the string?

Net Force Causes Acceleration Net force is the sum of all forces Including friction and gravity

If the object is moving, what is the force of Tension? An elevator with a mass of 1400 kg accelerates upward at 0.39 m/s/s. What is the force (tension) acting on the support cable? F T = F net + F g (weight) F net = ma F g = mg

The larger mass (on the on the frictionless table surface) has a weight of 22 N. The smaller hanging mass has a weight of 16 N. What is the acceleration of the two boxes? What force is exerted on the string? F = ma a = F/m The force is 16 N The mass is the total mass of the two boxes F = ma Acceleration is the answer from above Mass is the mass of the hanging box

Equilibrium Force The force that produces equilibrium (net force = 0) Equal in magnitude, opposite in direction

Air Resistance Friction with the air (drag) Terminal Velocity – The friction force is equal to the force of gravity – So the net force is 0 – Acceleration stops