Download Forces in 1D

Why do people “skip” more when they walk
on the moon versus when they walk on
Earth?
Objectives
 Define force.
 Explain Newton’s laws.
 Apply Newton’s laws to solve problems.
What is a force?
 An object that
experiences a push or
pull has a force exerted
on it.
 Have both magnitude
and direction
 Forces acting upon an
object can be added
using vector addition to
find the net force.
Newton’s First Law
An object at rest will stay at rest, and an object that is
moving will continue to move in a straight line with
constant speed, if and only if the net force acting on that
object is zero.
Newton’s Second Law
Acceleration of a system equals the net force acting on it,
divided by its mass.
Let’s practice!
 So Ms. Carlin weighs 165
lbs (on a good day),
which is 734 N.
 What is Ms. Carlin’s
mass?
 What is your mass?
 How many pounds in 1
Newton? The answer is
0.225.
Newton’s Third Law
Two forces that make up an interaction pair of forces are
equal in magnitude, but opposite in direction and act on
different objects.
IN Question!
Give an example of Newton’s
Second Law.
New rule: NO empty seats in
the front of the classroom
Inertia
 Newtons First Law is sometimes called Law of Inertia
 Inertia is the tendency of an object to resist change.
Free Body Diagram
 Physical model that represents the forces acting on a
system.
 Helps show the vectors of each of the forces!
Normal force
 Perpendicular contact force exerted by a surface on
another object.
Draw the free-body diagram
 A flowerpot falls freely from a windowsill.
 A cable pulls a crate across a horizontal surface.
 A rope lifts a bucket at a constant speed.
 A rope lowers a bucket at a constant speed.
Practice Problems!
 Two horizontal forces, 225 N and 165 N, are exerted on
a canoe. If these forces are applied in the same
direction, find the horizontal force.
 If the same two forces are exerted on the canoe in
opposite directions, what is the net horizontal force?
More practice
 4.2 Section Review