Forces and the
Laws of Motion
Chapter 4:
pp 123 -165
Essential Question(s):
Objective(s):
 Review Newton’s three laws and their
application.
Agenda:
 Review Homework from 3.3 Projectile Motion
 Discuss week ahead. Reminder: Lab Reports due
tomorrow.
 Bad news: I have a meeting Wednesday afternoon.
 Notes:
 Newton’s Three Laws
 Everyday Forces
 Problem Solving
 Pass back book reports tomorrow.
What is Force?
 a push or a pull.
 causes a change in velocity
 Units: Newtons. The amount of force required to
increase the speed of a 1-kg object by 1 meter
per second in 1 second.
 1 N = 1 kg*m/s2
Newton’s First Law
 “An object at rest remains at rest, and an object
in motion continues in motion with constant
velocity ( that is constant speed in a straight line)
unless the object experiences a net external
force.”
 Net external force: sum of all the forces acting
on an object.
 For example!
Equilibrium
 Equilibrium: net external force is zero.
 Does not mean there are no forces acting on the
object, only that forces are totally balanced.
 Does not mean there is no motion, only no
acceleration, no change in velocity.
 Sum of the forces in each direction is zero!
 ΣFx = 0
 ΣFy = 0
Newton’s Second Law
 Force is proportional to mass and accleration, or
in other words…
 “The acceleration of an object is directly
proportional to the net external force acting on
the object and inversely proportional to the
object’s mass.”
 ΣF = ma
 If there is a net force on an object in some
direction, the object will accelerate in that
direction.
Newton’s Third Law
 “If two objects interact, the
magnitude of the force exerted on
object 1 by object 2 is equal to the
magnitude of the force
simultaneously exerted on object 2
by object 1, and these two forces
are opposite in direction.”
 Forces always exist in pairs.
 Action reaction forces each act on
different objects.
Weight
 The force of gravity acting on the mass of an
object.
 Weight = Fg = mg = mass * acc. due to gravity
 Mass is inherent, it will not change. Weight may
change based on location (e.g. the moon).
 In FBDs, weight always points down! toward the
center of the earth.
The Normal Force
 In plain speak: the force exerted by surfaces
supporting objects.
 … is perpendicular to the contact surface and is
not always opposite in direction to the force of
gravity.
The Force of Friction
 Force of friction opposes other forces (applied,
gravity, etc.)
 Static friction (Fs): force resisting motion when
object is not moving.
 Kinetic (sliding) friction (Fk): force resisting motion
when object is moving.
The Force of Friction Cont’d
 As long as object isn’t moving, Fs = - Fapplied
 Kinetic friction is less than static friction.
The Force of Friction Cont’d
 Friction is proportional to the normal force.
 Depends on what surfaces are in contact.
 Static Friction: Fs = μsFN
 Kinetic Friction: FK = μKFN
Solving Chapter 4 Problems
 Make a free-body diagram!
 Choose a coordinate system (generally parallel
or perpendicular to the most forces)
 Consider all forces in x-direction.
 Consider all forces in y-direction.
 Solve!
Newton’s Second Law Practice
 A 2.0-kg otter starts from rest at the top of a
muddy incline 85 cm long and slides down to the
bottom in 0.50 seconds. What net external force
acts on the otter along the incline?
 Need the acceleration, and you know distance
and time. Now you know mass and acceleration.
 F = 14 N along the incline.
Overcoming Friction Problem
 A student moves a box of books down the hall by
pulling on a rope attached to the box. The student
pulls with a force of 185 N at an angle of 25.0° above
the horizontal. The box has a mass of 35.0 kg, and μk
between the box and the floor is 0.27. Find the
acceleration of the box.
 Steps:
 Find the normal force.
 Use the normal force to find the force of friction.
 Find the sum of the forces in the x-direction.
 ax = 2.7 m/s2
Homework
 If we got this far…
 p154 #54a
 p155 #57