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Newton’s Laws
Chapter 4 – Force and
Motion
Newton 1
 Law
of Inertia
 “In the absence of an unbalanced
force (Fnet = 0) a body at rest
remains at rest and a body in motion
remains in motion with a constant
velocity.
 Inertia – the tendency of objects to
resist changes in motion
Newton 2
 Fnet
= ma
 A net unbalanced force changes a
body’s velocity.
 The acceleration caused by an
unbalanced force is directly
proportional to the net force and
inversely proportional to the mass of
the body.
Weight



Weight is the force of gravity pulling a
body towards Earth.
Since the Earth accelerates all objects
near the surface at g = -9.8 m/s2,
weight is calculated using Newton’s
second law:
F = ma

W = mg
Newton 3
 For
every force (action) there is an
equal and opposite force (reaction).
 Forces
 The
come in pairs!
normal force is the reaction to
an object’s weight.
Action Reaction Pairs
 When
considering the motion of an
object, we consider only forces
acting on the object and ignore the
reaction pair..
 Identify
the action reaction pairs on
page 114
Normal Force Calculations



Figure 4.11 page 113
To find the normal force, resolve all forces
acting on a body into components both
parallel and perpendicular to the surface
on which the body rests.
The normal force is found by ensuring an
equilibrium of all components
perpendicular to the surface.
Free Body Diagrams
 Force
vectors are sometimes drawn
to indicate their point of application
on an object.
 In a free-body diagram, all forces are
shown as emanating from a common
point (the origin)
 Forces don’t have to be drawn to
scale but the diagram should show
whether there is a net force.
Drawing Free Body Diagrams
1.
2.
3.
4.
5.
Sketch the situation and identify the forces
acting on each body of the system.
Isolate the body for which the free-body
diagram is to be constructed. Draw a set of
axes through the body, with one axis
aligned with the object’s acceleration.
Draw properly oriented force vectors
(include angles) emanating from the origin.
Resolve vectors into components.
See Example 4.6 page 117