Newton’s First and
Second Laws of
Motion
Chapter 12 Section 2
Newton’s First Law
Also known as The Law of Inertia
O The state of motion of an object does
not change as long as the net force
acting on the object is zero.
Newton’s 1st law
O An object in motion tends to stay in motion
O In a straight line at a constant speed
O And an object at rest tends to stay at rest
O Unless acted upon by an outside force
O It requires a push or pull in order to change
an objects state of motion!!!
Newton’s 1st law
O Inertia is the tendency of an object to
resist a change in motion.
O ESA Video
Newton’s 2nd Law
O The acceleration of an object is equal to the
force you apply divided by the mass of the
object.
Newton's Second Law
O If you apply
more force to an
object, it
accelerates at a
higher rate.
O Force is directly
proportional to
acceleration.
Newton's Second Law
O If an object has more
mass it accelerates at
a lower rate because
mass has inertia.
O The more mass the
less acceleration;
mass is indirectly
proportional to
acceleration.
Newton’s 2nd Law
O ESA video #2
Newton's Second Law
Newton’s 2nd Law
• How much force is needed to accelerate a 1400
•
•
•
•
•
kilogram car 2 meters per second/per second?
Write the formula
F=mxa
Fill in given numbers and units
F = 1400 kg x 2 meters per second/second
Solve for the unknown
• 2800 kg-meters/second/second or
N
2800
Practice Problems
• 1. What acceleration will result when a 12 N net
force applied to a 3 kg object? A 6 kg object?
• 2. A net force of 16 N causes a mass to
accelerate at a rate of 5 m/s2. Determine the
mass.
• 3. How much force is needed to accelerate a 66
kg skier 1 m/sec/sec?
• 4. What is the force on a 1000 kg elevator that
is falling freely at 9.8 m/sec/sec?
Practice Problems-ANSWERS
• 1. What acceleration will result when a 12 N net
force applied to a 3 kg object? A 6 kg object?
• 4m/s 2; 2m/s 2
•
2. A net force of 16 N causes a mass to
accelerate at a rate of 5 m/s2. Determine the
mass.
• 3.2 kg
• 3. How much force is needed to accelerate a 66
kg skier 1 m/sec/sec?
• 66N
• 4. What is the force on a 1000 kg elevator that
is falling freely at 9.8 m/sec/sec?
• 9800N
Now Try on your own!!!
Review:
O If mass stays the same, and you INCREASE
the force applied to an object, what happens
to the acceleration of the object?
O If the force applied to an object stays the
same, but the mass is increased, what
happens to the acceleration?
Mass Vs. Weight
O Mass is the amount of matter present in
an object. Measured in Kg.
O Weight is the effect of gravity on an
object’s mass. Measured in Newtons
O Gravity on Earth is 9.8 m/s2 or 9.8 N/Kg)
O Mass does NOT change.
O Weight DOES change as gravity changes
O Weight equals mass times the
acceleration due to gravity.
O W=mxg
O Remember, gravity is different on other
planets!