Newton’s Second Law of Motion
How fast does it go?
Acceleration
• An unbalanced force causes something to
accelerate.
• Remember acceleration is any change in
speed or direction!
Acceleration
• Acceleration is directly related to the size
of the force (mass) and the direction of the
force.
• Objects accelerate in the direction you push
or pull them.
In other words….
Large Force
= Large Acceleration
F
a
Remember that…
• The more MASS an
object has, the more
INERTIA the object
has.
Which vehicle has more
inertia?
• Bigger objects are
harder to start &
stop!
Slide from www.science-class.net
Newton’s Second Law
• Newton, that brilliant genius, observed those
“rules” of acceleration and came up with his
second law of motion. It is both a formula & a
law.
Newton’s Second Law
• The acceleration of an object depends on
the net force acting on the object and the
mass of the object!
• Force = Mass x Acceleration
• aka…
F = ma and “The Law of Force”
• Measured in Newtons (N).
Units!
• Lets review some of our units…
– The units used for mass are kilograms (kg)
– The acceleration units are meters per second
squared (m/sec2).
– One Newton = the force required to accelerate 1kg
of mass at 1 meter per second per second…..
– *So…
1N = 1kg x 1m/sec2
GUiDeD PrAcTiCe!
• A 52kg water-skier is being pulled by a
speedboat. The force causes her to
accelerate at 2 m/sec2 . Calculate the force
that causes this acceleration.
• Force = Mass x Acceleration in Newtons
• F = 52kg x 2 m/sec2
• F = 104 kg x m/sec2
• F = 104 N
INDEPENDENT PrAcTiCe!
• 1. What is the force on a 1,000 kg
elevator acceleration at 2 m/sec2 ?
1,000 x 2 = 2,000 N
• 2. How much force is needed to
accelerate a 55 kg cart at 15 m/sec2?
• 55 x 15 = 825 N
• 3. When the force on an object increases,
how does the objects acceleration
change? It increases.
• 4. Suppose you know the acceleration of
a shopping cart as it rolls down an aisle.
You want to know the force it was
pushed with. What other information do
you need to know to determine this?
The mass.
• 5. Suppose you doubled the force acting
on an object. In what way could you
change its mass to keep its acceleration
constant?
Double it.