3. OBJECTIVES
•State Newton’s first law and explain its meaning.
•Calculate the effect of forces on objects using the law of inertia.
•Explain conceptually why moving objects do not always appear to
obey the first law.
4. NEWTON'S LAWS OF MOTION
Two of the most important quantities in physics are force and
acceleration.
As you have learned, acceleration is the rate at which the velocity
changes with time.
Force is, quite simply, a push or a pull.
Two quantities characterize a force:
the strength, or magnitude of the force
the direction in which the force acts
5. NEWTON'S LAWS OF MOTION
Objects don't start or stop moving on their own.
This observation is the essence of Newton's first law
of motion:
7. NEWTON'S FIRST LAW OF MOTION
A glass is placed on a board and the board is
jerked quickly to the right. The glass tends to
remain at rest while the board is removed.
8. NEWTON'S FIRST LAW OF MOTION
Assume glass and board move together at
constant speed. If the board stops suddenly,
the glass tends to maintain its constant speed.
10. 1st law: law of inertia
A body at rest tends to stay at rest, a body in
motion tends to keep moving along at a constant
speed and in a straight-line path unless
interfered with by some external forces.
13. NEWTON'S FIRST LAW OF MOTION
Newton's first law of motion contains the phrase "no net force." What
does this mean?
The net force is the vector sum of all the individual forces acting on an
object.
When you sit in a chair, there are essentially two forces acting on you:
the upward push of the chair and the downward pull of gravity. Since
you are at rest, the two forces must cancel out. Therefore, the vector
sum of the forces, or net force, acting on you is equal to zero.
15. INERTIA
Newton's first law is sometimes referred to as the law of inertia.
Loosely speaking, inertia means laziness. Objects may be thought of
as lazy because they don't change their motion unless forced to do
so.
The tendency of an object to resist any change in its motion is
referred to as its inertia.
16. INERTIA
Every object resists change to its state of motion/velocity. To change it,
the force must act on it. We call this resistance INERTIA.
Inertia is resistance an object has to a change of velocity.
• sort of laziness (inerzia – laziness in Italian)
inertia increases with mass
17. RESULTANT FORCE
The net force – resultant force
is the vector sum of all forces acting on ONE object.
the object accelerates as if only one force – net force is applied
Fnet or ΣF
20. EQUILIBRIUM
First Condition for Equilibrium
If the Net Force acting on the object is
zero
FNET = 0 a = 0
The object is either stationary (v = 0) or
traveling with a constant velocity
(v = constant)
21. 1 - QUIZ
A book is at rest on a table top. Diagram
the forces acting on the book.
22. 2 - QUIZ
An egg is free-falling from a nest in a tree.
Neglect air resistance. Draw a free-body diagram
showing the forces involved.
23. 3 - QUIZ
A rightward force is applied to a book at rest,
in order to move it across a desk. Consider
frictional forces. Neglect air resistance.
Construct a free-body diagram for the book.
24. 4 - QUIZ
Two forces, 3 newtons (N) and 2 N, act to the right.
Calculate the resultant force.
25. 5 - QUIZ
A force of 5 N acts to the right, and a force of 3 N act
to the left. Calculate the resultant force.
