Newton’s 1st Law – Objectives:
1. Explain the relationship between the motion of an
object and the net external force acting on it.
2. Determine the net external force on an object.
3. Calculate the force required to bring an object into
equilibrium.
Homework – castle learning
Newton's First Law
• Newton's first law of motion: An object at rest stays at rest
and an object in motion stays in motion with the same speed
and in the same direction unless acted upon by an
unbalanced force.
Newton’s First Law
Also known as the “Law of Inertia”
Inertia
Tendency of an object to maintain its STATE OF
MOTION
Forces Don't Keep Objects Moving
Newton’s 1st law – the law of inertia
• The state of motion of an object is defined by its
velocity - the speed with a direction.
• Inertia: tendency of an object to resist changes in its
velocity.
• Inertia: tendency of an object to resist accelerations.
Forces Don't Keep Objects Moving
Everyday Applications of Newton's First Law
Inertia is proportional to MASS
Do these guys have a lot of inertia?
MORE MASS
means
MORE INERTIA
LOTS OF INERTIA
hard to…
GET MOVING or
STOP
Balanced and Unbalanced Forces
If two individual forces are of equal
magnitude and opposite direction, then
the forces are said to be balanced.
When only balanced forces act on an
object, the object is said to be at
equilibrium.
Unbalanced forces
Acceleration is determined by net
external force
• Net external force: the resultant force from a
vector additions of all external forces acting
on an object.
• Unbalanced force means there is net external
force.
• Net external force produce acceleration.
• Newton’s 1st law states that an object will not
accelerate if the net external force is zero.
Equilibrium
• Equilibrium is the state in which there is no
change in a body’ motion.
• Newton’s 1st law states one condition that
must be true for equilibrium: the net external
force acting on a body in equilbrium must be
equal to zero.
Determining Net External Force
1. Draw a free body diagram (FBD)
2. Select a coordinate system, and apply it to the FBD.
3. Find the x and y components of all vectors.
Ax=AcosθA;
Ay=AsinθA;
Bx=BcosθB;
By=BsinθB …
4. Find the x and y component of the resultant vector:
Rx=Ax + Bx …;
Ry=Ay + By …
5. Use the Pythagorean theorem to find the magnitude of the
resultant vector.
2
2
R  Rx  Ry
6. Use tan-1 function to find the angle the resultant vector makes
Ry
with the x-axis.
1
 R  tan ( )
Rx
7. Evaluate your answer.
Determining the Net Force
•The net force is the vector sum of all the forces that act upon an object.
A
400 N up
30 N
B
C
200 N
down
20 N left
R2 = (30N)2 + (40N)2
θ = tan-1(40/30) = 53.1o
40 N
Net force is 50 N at 53.1o West of North
Example 4A
• Joe leaves his physics book on top of drafting table that is
inclined at a 35o angle. The FBD shows the forces acting on
the book. Find the net external force acting on the book, and
determine whether the book will remain at rest in this
position.
Fn
Ff
Fg
R
X (N)
Y (N)
0
18 N
-11
0
22cos(-55o)
22sin(-55o)
2
0
The net external force is 2 N in the
positive x direction, the book will not
remain at rest.
-55o
Class work
• Page 133 – practice 4F
• Page 135 – section review #1-5