Download Unit 3 - Forces

Unit 3 - Forces
Newton’s Laws of Motion
Different Forces
Applying Newton’s Laws
Work and Energy
Momentum and Collisions
Using Mass & Weight
• With study groups, measure weight of 3 different
masses ( using force scale and mass hangers )
– 100, 200, & 300 g
• Plot weight measured ( in Newtons ) vs mass
• Objective: what is the proportionality constant?
• Use to discuss Newton’s Laws
Free Body Diagrams
• First, draw a sketch in your notes of a mass
hanging from a force scale
• What are the forces acting on the mass?
• What are the forces acting at your shoulder?
Measure and Plot ( Notes )
• Get 3 100 g masses – will measure 100, 200, 300 g
with force scale ( make a table with “g” and “N” )
– MAKE SURE FORCE SCALE IS INITIALLY READING ZERO
• Sketch a graph in your notes, plotting weight
measured vs. mass hung on force scale
• What is the slope?
• Formulate an equation for the line plotted
Force and Mass
Force: push or pull
Force is a vector – it has magnitude and direction
Force and Mass
Mass is the measure of how
hard it is to change an
object’s velocity ( INERTIA ).
Mass can also be thought of
as a measure of the
quantity of matter in an
object.
Newton’s First Law of Motion
If you stop pushing an object, does it stop moving?
Newton’s First Law - In the absence of any net external
force, an object will keep moving at a constant speed in a
straight line, or remain at rest.
This is also known as the law of inertia.
In order to change the velocity of an object – magnitude or
direction – a net force is required.
Hanging Mass
• Is there a NON-ZERO net force acting on the
hanging mass?
• So, is the mass in motion?
Newton’s Second Law of Motion
Two equal weights exert twice the force of one; this can
be used for calibration of a spring:
Newton’s Second Law of Motion
What if we drag an object with the force scale?
Acceleration is proportional to force:
Newton’s Second Law of Motion
Acceleration is inversely proportional to mass:
Newton’s Second Law of Motion
Combining these two observations gives
Or, more familiarly,
Newton’s Second Law of Motion
An object may have several forces acting on it; the
acceleration is due to the net force:
(5-1)
Newton’s Second Law of Motion
Free-body diagrams:
A free-body diagram shows every force acting on an
object.
• Sketch the forces
• Isolate the object of interest
• Choose a convenient coordinate system
• Resolve the forces into components
• Apply Newton’s second law to each coordinate
direction
Newton’s Second Law of Motion
Example of a free-body diagram:
Newton’s Third Law of Motion
Forces always come in pairs, acting on different objects:
r
r
If object 1 exerts a forceF on object 2, then object 2 exerts a force –F on object 1.
These forces are called action-reaction pairs.
What was the action-reaction pair on the hanging
mass?
What about on your shoulder as you hold the force
scale?
Newton’s Third Law of Motion
Every action has an equal and opposite reaction
Newton’s Third Law of Motion
Some action-reaction pairs: