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Forces and Newton’s Laws
Reading Notes
Section 4-1: Force
What is force?
Give an example of a force you have experienced continuously all your life.
Give an example of a situation where an object is given a force but is not moved.
What object do we use to measure force?
Section 4-2: Newton’s First Law of Motion
Explain Aristotle’s (wrong) idea about force. What effect confused Aristotle into having this view?
State Newton’s First Law.
What does the term “net force” mean?
What is inertia?
What is an inertial reference frame?
Section 4-3: Mass
Give TWO definitions of “mass”.
What units do we use to measure mass?
Section 4-4: Newton’s Second Law of Motion
What does a net (or unbalanced) force cause? What are the units with which we measure force?
Write Newton’s Second Law as a sentence.
Write Newton’s Second Law as an equation and explain what the  symbol means.
Section 4-5: Newton’s Third Law of Motion
State Newton’s Third Law of Motion
What is very important to remember about the “action” and “reaction” forces of an action/reaction pair?
Test Your Understanding:
When you stand, the floor exerts an
upward force on your feet. What is
the reaction force?
The Earth exerts a downward
gravitational force on you. What is
the reaction force?
Name the action and reaction
forces that occur when you walk.
Test Your Understanding: A person pushes a desk across the floor. Draw all four forces acting on the person,
and all four forces acting on the desk.


Person
Desk
Compare the horizontal forces on the desk (which is stronger, or are they equal) when the desk is:
Speeding up
Slowing down
Moving with constant speed.
Compare the horizontal forces on the person (which is stronger, or are they equal) when the person is:
Speeding up
Slowing down
Moving with constant speed.
Name two horizontal forces that must be the same strength at all times. Explain your reasoning.
Section 4-6: Weight—The Force of Gravity; and Normal Force
What is the equation for the force of weight on an object? __________
What is a contact force?
When you stand still on the floor, your weight force of gravity is equal and opposite to the normal force the
floor exerts on you. Explain whether this is an action/reaction pair.
What is an object’s “inertial mass”?
What is an object’s “gravitational mass”?
How are “inertial mass” and “gravitational mass” related for all objects?
Describe a simple experiment that would prove what you said for the relationship between gravitational and
inertial mass. What would be observed? What equation could be written that justified this result?
Section 4-8: Applications Involving Friction, Inclines
What is “kinetic friction”?
What is the friction force proportional to?
What is the equation for kinetic friction? ________ What does the symbol k stand for? _________________
What does the k depend on?
What is static friction?
What is the equation for static friction? ________ What does the symbol s stand for? __________________
Why is there a  sign in that equation?
Example: A 50-kg student stands on a scale in an elevator. The scale reading on the elevator is shown below
as a function of time between elevator stops. (Use g = 9.8 m/s2.)
Draw the forces acting on the student.
Write an equation that relates the forces that you drew
to the mass and acceleration of the student.
Find the acceleration (magnitude and direction) of the elevator at
Time t = 7 seconds
Time t = 63 seconds
Explain how the data can be used to determine the top speed of the elevator’s motion.
Forces At An Angle
Suppose an object has a force that is at an “oblique angle”. This means that the force makes an angle that is not
parallel and not perpendicular to the other forces acting. What should we do?
When an object is on an inclined plane, the _______________ force is the one that makes the oblique angle
with the acceleration.
The block’s mass is m. No Friction.
The block’s mass is m. Coefficient of Friction is .
What is the normal force on the block?
What is the normal force on the block?
What is the acceleration of the block?
What is the acceleration of the block?
What conclusion can you draw about the acceleration
of the block from your previous answer?
What conclusion can you draw about the acceleration
of the block from your previous answer?
Example: The box shown experiences no friction. The box is released from
rest at the top of the incline.
What is the acceleration of the box?
What is the speed of the box at the bottom of the
incline?
How much time does it take for the box to go from the
top to the bottom of the incline?