Transcript Chapter 4 - cloudfront.net

Chapter 4
4-2 Newton’s first law
Newton’s first law
 An
object at rest remains at rest , and a
object in motion continue in motion with
constant velocity unless the object
experiences a net external force.
Law of Inertia
 What
is Inertia?
 Is the tendency of an object not to
accelerates.
 Newton’s first law is often referred to us as
the law of inertia. Because it states that in
the absence of a net force, a body will
preserve its state of motion.
Newton’s first law
 In
other words, Newton’s first law states
that when the net external force on an
object is zero, the object’s acceleration is
zero.
Net force
 What
is a net force
 Answer: Is a single force whose external
effects on a rigid body are the same as
the effects of several actual forces acting
on a body.
 In other words a net force is the vector
sum of all forces acting on an object
Determine Net Force
 Derek
leaves his physics book on top of a
drafting table that is inclined at a 35°
angle. The free-body diagram below
shows the forces acting on the book. Find
the net force acting on the book.
Example#1
1. Define the problem, and identify the
variables.
Given:
Fgravity-on-book = Fg = 22 N
Ffriction = Ff = 11 N
Ftable-on-book = Ft = 18 N
Unknown:

Fnet = ?
continue
 2.
Select a coordinate system, and apply
it to the free-body diagram.
Choose the x-axis parallel to and the yaxis perpendicular to the incline of the
table, as shown in (a). This coordinate
system is the most convenient because
only one force needs to be resolved into x
and y components.
continue
 Find
the x and y components of all
vectors.
 Draw a sketch, as shown in (b), to help
find the components of the vector Fg. The
angle  is equal to 180– 90 – 35 = 55.
continue
x-coordinate
𝑐𝑜𝑠𝜃 = 𝑓𝑔𝑥/𝑓𝑔
𝑓𝑔𝑥 = 𝑓𝑔𝑐𝑜𝑠𝜃
𝑓𝑔𝑥 = 22 𝑐𝑜𝑠55 = 13𝑁
Y-coordinate
𝑠𝑖𝑛𝜃 =
𝑓𝑔𝑦
𝑓𝑔
𝑓𝑔𝑦 = 𝑓𝑔𝑠𝑖𝑛𝜃
𝑓𝑔𝑥 = 22 𝑠𝑖𝑛55 = 18𝑁
continue
 4.
Find the net force in both the x and y
directions.
 Diagram (d) shows another free-body
diagram of the book, now with forces
acting only along the x- and y-axes.
continue
For the x direction:
Fx = Fg,x – Ff
Fx = 13 N – 11 N
Fx = 2 N
For the y direction:
Fy = Ft – Fg,y
Fy = 18 N – 18 N
Fy = 0 N
continue
Find the net force.
Add the net forces in the x and y
directions together as vectors to find the
total net force. In this case, Fnet = 2 N in the
+x direction, as shown in (e). Thus, the
book accelerates down the incline.
Equilibrium
 Objects
that are either at rest or moving
with constant velocity are said to be in
equilibrium.
What is equilibrium?
Equilibrium is the state in which the net
force on an object is zero.
Equilibrium
 Newton’s
first law describes objects in
equilibrium.
Tip: To determine whether a body is in
equilibrium, find the net force. If the net
force is zero, the body is in equilibrium. If
there is a net force, a second force equal
and opposite to this net force will put the
body in equilibrium.