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Chapter 3: Forces and Motion
First law of motion:
-first concluded by Galileo, stated here in Newton’s words
*An object at rest will remain at rest and an object in motion
will continue in motion at a constant velocity (constant speed
in a straight line) in the absence of any interaction with
something else
inertia – the apparent resistance of an object to change its state of rest or
uniform motion
mass – a measure of the inertia of an object at rest
*the more something resists being set in motion the greater its
Compare the mass of a bowling ball to a basketball
- an experiment of 2 different masses both attached to the same spring will
move away with different velocities
- if different springs are used the ratios of velocities will stay the same
ma = va
*the object of lower mass will always have greater speed
mb vb
the next step would be to have something like a standard unit of measure. In
France there is the 1kg Pt cylinder that defines 1 kg
Why do we measure mass this way?
What does mass measure?
*amount of matter
- somewhat vague definition
- spring method is much more convenient
ex hitting a ball with a bat, the result is a change in velocity (direction)
*an interaction can lead to a change in magnitude or direction
A force is any influence that can change the velocity of an object.
*this definition agrees with the idea of forces as “pushes” or “pulls”
contact force arise from physical contact
pushing, pulling, hitting, friction
field forces (action-at-a distance) when forces exert forces on each other even
though they are not touching.
Magnetic and electric fields
?? Would something move if there were 2 exactly opposite forces acting on it??
a net force is needed for a velocity change. also called and unbalanced force
equilibrium is when an object is acted on by a set of forces whose resultant is
zero. these forces all cancel.
the first laws restated in terms of net force:
*if no net force acts on it, an object will remain at rest and an object in motion
will stay in motion at constant velocity
Second Law of Motion
*the net force on an object equals the product of the mass and the acceleration
of the object. The direction of the force is the same direction of the
F = ma (net force) = (mass) * (acceleration)
Forces are vectors, therefore they must be added “vectorally”
ΣFx = max = net forcex
ΣFy = max = net forcey
Honors Homework:
Read Chapter 4 Sections 4.1-4.5
Problems: Pg 125 1 & pg 127 3-10