Download Chapter 10.3 Newton`s 1st & 2nd Laws of Motion

Chapter 10 Forces - Section 3:
Newton’s First and Second Laws
Key Concepts:
What is Newton’s first law of motion?
What is Newton’s second law of motion?
Newton’s First Law
An object at rest tends to stay
at rest and an object in motion
tends to stay in motion with the
same speed and in the same
direction unless acted upon by
an unbalanced force.
So, an object at rest will remain at rest...
and
An object in motion will remain in
motion at a constant velocity…………
Unless acted upon by an unbalanced force
 An
unbalanced
force will
cause an
object to
speed up,
slow down, or
change
direction.
Inertia

Inertia is the tendency of an object to resist a change in
motion.
 Newton’s first law of motion is also called the “law of inertia.”
 If you don’t want to move, someone may call you “lazy” or
“inactive”, this is what inertia means in Latin.
Inertia

Inertia depends on mass…..the greater the mass, the
greater the inertia.
Inertia
Newton’s Second Law
The
force exerted on an
object is equal to the
acceleration multiplied by
the mass of the object on
which the force is acting.”
Newton’s Second Law of Motion

Acceleration depends on the net force acting on the
object and the object’s mass.
 More mass – More Force needed.
 Less Mass – Less Force needed.

Formula is:
Net Force = Mass x Acceleration
Newton’s 2nd Law Basics:
 Acceleration
is measured in meters per
second per second (m/s2)
 Net force is measured in newtons (N)
 Mass is measured in kilograms (kg)
 Acceleration = Net Force
Mass
 Net Force = Mass x Acceleration
 1 Newton is the force required to give 1
kg mass an acceleration of 1 m/s2
Calculating Force





A speedboat pulls a 55-kg water-skier. The skier
accelerates at 2.0 m/s2. Calculate the net force
that causes this acceleration.
Read and Understand
What information have you been given?
Mass of the water-skier (m) = 55 kg
Acceleration of the water-skier (a) = 2.0 m/s2
Part 1
Calculating Force

Problem: A speedboat pulls a 55-kg water-skier. The skier accelerates
at 2.0 m/s2. Calculate the net force that causes this acceleration.

Plan and Solve
 What quantity are you trying to calculate?
 The net force F = ?
 What formula contains the given quantities
 F=mxa
 Perform the calculation.
 F = m x a = 55 kg x 2.0 m/s2
 F = 110 kg • m/s2
 F = 110 N
Part 2
Calculating Force

Practice Problem

What is the net force on a 1,000-kg object
accelerating at 3 m/s2?

F=mxa
 F = 1,000 kg x 3 m/s2
.
 F = 3,000 kg m/s2

F = 3,000 N