Dynamics

Dynamics  Dynamics

is the study of forces which causes MOTION.

A

Force is a push or a pull.  Metric Units are Newtons  English Units are ?  How Big is a NEWTON ?

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Forces  All

forces act through a distance  Some at short distances = contact forces  Some at long distances = long range forces

The Four Forces Gravitational

Forces Electromagnetic Forces Strong Force Weak Force DEB – Rank the forces from weakest to strongest.

Galileo’s Experiment Ball

rolling down a ramp in the diagram. How high will it rise on the other side?

Galileo’s Experiment Ball

rolling down a ramp in the diagram. How high will it rise on the other side?

Galileo’s Experiment  Ball

rolling down a ramp in the diagram.  How high will it rise on the other side?

Newton 1642-1727 Newton

was born on Christmas Day in 1642, the same year that Galileo died.

First Law Also

called the LAW of INERTIA

Inertia

is the property that an object has that causes it to resist changes in its position

First Law LAW

of INERTIA

Inertia

is an another word for MASS. So the more mass an object has, the more inertia it has.

Newton’s First Law of Motion A

body at rest will remain at rest…..

And

A body in motion will remain in motion…

Unless………..

DEB – Finish the Sentence

Newton’s First Law of Motion  The

University of Nebraska Football team can demonstrate the first law of motion called the law of inertia. Check out their web site.  http://physics.unl.edu/fp_media/inertia.html

Newton 1642-1727  Newton

was asked, “Why don’t the planets stop moving around the sun?”  His reply, “Why should they!”  An object at rest is the same case as an object at constant velocity. (Only 2 types of motion  a or no a changing v or constant v

1 LAW Demonstrations st

Bowling

Ball

Steel

ball

A

Roll of Toilet Paper

Embroidery

Hoop

 DEB

- Which is harder to start moving? A Truck or a car?  Why??  DEB - Which is harder to stop moving? A Truck or a car?  Why??

Forces Forces

always have agents(the immediate cause) such as  kicking the ball, desk holding up the book, gravity pulling the moon, pushing the broom, pulling on the rope, etc. Next – SOLVING DYNAMICS PROBLEMS

How to Draw a FREE BODY DIAGRAM - FBD a) sketch the situation b) circle the system c) identify where the system touches the environment. d) replace the object by a dot.

How to Draw a FREE BODY DIAGRAM - FBD e) represent each force by a vector with its head in the direction of the force and drawn to the correct scale. Note- the tail of the force vector is always on the particle, even if the force is a push. f) label the force including units N , Ff, Fsp , T, Fthrust , W

A

book is at rest on the table. It's just as unreasonable to expect the book to start moving. A book in motion(constant velocity) it is unreasonable to expect it to stop or change velocity. If the forces add to zero(no unbalanced force) then an object in motion will remain in motion - which means no acceleration. 

3 Equivalent Statements ΣF

= 0, a = 0, constant velocity.

Misconceptions about FORCES 1. Throw a ball and the forces of the hand remain with the ball  2. A force is needed to keep the ball moving  3. Inertia is a force - velocity is a force - momentum is a force  4. Air does not exert a force  5. ma is a force

FBD practice worksheet DEB Draw a FBD for a 2 kg book resting on a table.

FBD practice Draw a FBD for a 2 kg book resting on a table.

N = 20 N BOOK

W = 20 N

FBD practice Draw a FBD for a 5 kg crate resting on the ground. It is being pushed to the left by Sima with a force of 20 N and Kevin is pushing it to the right with a force of 20 N.

FBD practice N

W

Newton’s 2nd Law What

will happen if one of the forces in the FBD is bigger than the others and is not balanced? When an unbalanced force acts on an object, it will accelerate in the direction of the force.  ∑F = ma

Newton’s 2nd Law What

will happen if one of the forces in the FBD is bigger than the others and is not balanced? When an unbalanced force acts on an object, it will accelerate in the direction of the force.  ∑F = ma

DEB

– What is the relationship between the Force and the Acceleration if the Mass remains constant? DEB – If the force doubles, what happens to the acceleration? DEB – Sketch the graph of F vs a

Throw

a ball up DEB – Which direction is the net F, a, v ?

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Throw

a ball

down DEB – Which direction is the net F, a, v ?

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DEB

– What is the relationship between the Mass and the acceleration, if F remains constant? DEB – If the Mass doubles, what happens to the acceleration? DEB – Sketch the graph of M vs a

DEB

– What is the relationship between the Force and the acceleration, if mass remains constant? DEB – If the Force doubles, what happens to the acceleration? DEB – Sketch the graph of F vs a What does the slope represent?

Double the Force, Double the acceleration

Double the Mass, Half the acceleration