Ch 4 Forces in One Dimension 4-1 Force and Motion

Force A push or a pull Forces (F) are vectors Magnitude and direction https://www.youtube.com/watch?v=bOIe0DIMbI8 Despicable Me

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

Net Force and Unbalanced Forces The sum of all forces acting on an object Unbalanced forces  The net force is not zero Unbalanced forces cause acceleration What about constant velocity?

System Object or objects of interest

Contact forces A force applied to an object in contact Objects are touching, in contact

Field Forces Exerted without contact Gravity Magnetism

Agents The cause of the force When you push a textbook, your hand (agent) exerts a force on the textbook (system)

Free-body diagrams A physical representation that shows the forces acting on a system See page 92 Object represented by a dot Use F for force with subscript Force vector has proportional length and always points away

Net Force The vector sum of all the forces acting on an object Net Force Causes Acceleration

Acceleration and Force Applying a constant force results in constant acceleration NOT constant velocity

Interpreting Slope Acceleration-Force Graph What would happen if the force is applied to double the mass? Results in the following equation

Newton’s Second Law of Motion

Inertia An object’s resistance to a change in its motion What happens if the net force acting on an object is zero? 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

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

4.2 Weight and Drag force

Weight is a Force An apple weighs about 1 newton The gravitational force on an object An apple weighs about 1 newton

Weight and Mass Weight - The gravitational force on an object Measured with a ___ Mass - The amount of matter in an object Measured with a ___

Apparent Weight Fscale = ma + mg Fscale = Fnet + Fg (weight) Fnet = ma What an object appears to weigh on a scale A measure of the support force Imagine standing on a scale while being accelerated up or down Fscale = Fnet + Fg (weight) Fnet = ma Fg = mg Fscale = ma + mg

Weightlessness Imagine you are in freefall, what would the scale read? The support force is zero Why does an astronaut experience weightlessness https://www.youtube.com/watch?v=WmIn1htRavk Howard Describes zero gravity

Drag Force The force exerted by a fluid on an object opposing motion through the fluid Magnitude depends on the velocity and shape or surface area of the object Also the density and viscosity of the fluid

Terminal Velocity The drag force is equal to the force of gravity So the net force is 0 Acceleration stops

4-3 Newton’s Third Law

Interaction Pairs Two forces acting in opposite directions, have equal magnitude, and act on different objects Aka action-reaction pair, but is misleading, one doesn’t cause the other

Newton’s Third Law of Motion All forces come in pairs and act on different objects (boy on toy and toy on boy)

Tension FT = Fg = mg The force that a string or rope exerts Force of Tension = force of gravity = weight = mg FT = Fg = mg The hanging mass is 1000 grams. What is the force of tension on the string?

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 Equal to the weight, but opposite in direction

FT = Fnet + Fg (weight) Fnet = ma Fg = mg 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? FT = Fnet + Fg (weight) Fnet = ma Fg = mg

The larger mass (on the on the frictionless table surface) has a weight of 32 N. The smaller hanging mass has a weight of 26 N. What is the acceleration of the two boxes? F = ma a = F/m The force is 26 N The mass is the total mass of the two boxes What force is exerted on the string? F = ma Acceleration is the answer from above Mass is the mass of the hanging box

Net Force Fnet = Fapplied + Ff Ff = Force of Friction (negative) When sliding or air resistance Fnet = Fapplied + Ff Ff = Force of Friction (negative)

Net Force Fnet = Fapplied + Fg When against gravity Fnet = Fapplied + Fg Fg = Force of gravity or weight (negative)

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

The Four Fundamental Forces

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

Electromagnetic (EM) 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

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 https://www.youtube.com/watch?v=UqL-jHZy27s https://www.youtube.com/watch?v=zRDP5pOiN1c