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Newton’s Second Law of Motion
How fast does it accelerate?
Acceleration
• The rate at which velocity changes
In other words…
Acceleration is a change in velocity
– Speeding up
• Positive acceleration
– Slowing down
• Negative acceleration
• Deceleration
– Changing direction
Acceleration
• An unbalanced force causes something to
accelerate.
Acceleration
• Acceleration is directly related to the size of
the force and the direction of the force.
• An object accelerates in the direction you
push or pull it.
In other words….
Large Force
F
= Large Acceleration
a
In other words….
Small Force
F
= Small Acceleration
a
So….if you push twice as hard, it accelerates twice as much.
But there is a twist….
• Acceleration is INVERSELY related to the mass
of the object.
In other words…..using the same
amount of force….
F
Small acceleration
Large Mass
a
Large acceleration
F
Small Mass
a
Newton’s Second Law
• Newton, observed these “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 is directly
proportional to the net force & inversely
proportional to it’s mass.
• F = ma
• Force = Mass x Acceleration
Calculating Acceleration
• Acceleration Equation
(final speed (in m/s) – initial speed (in m/s))
Acceleration =
Time (in seconds)
A=
(sf-si)
t
A few more things to know…
• First, you need to know the units of Force,
Mass & Acceleration.
– The units used for force are Newtons (N)
– The units used for mass are kilograms (kg)
– The acceleration units are meters per second
squared (m/sec2).
What is a Newton?
1 Newton is the force it would take to accelerate
a 1 kilogram mass to 1 m/s2
Let’s practice…
•
Rob is really bored one Saturday night and
goes outside to study the nocturnal habits of
mice in the hayfield. He sees a mouse
sniffing along at 0.1 m/s. but it hears Rob
and starts to scurry for safety. In just 3.7 s it
accelerates to 0.9 m/s. Find its acceleration.
Calculating Acceleration
• Acceleration Equation
(final speed (in m/s) – initial speed (in m/s))
Acceleration =
Time (in seconds)
A=
(sf-si)
t
Oh… I get it now!
Let’s Practice
(final speed (in m/s) – initial speed (in m/s))
Acceleration =
A=
A=
(sf-si)
t
0.9m/s – 0.1 m/s
3.7s
A=
0.8m/s
3.7s
A= 0.22 m/s2
Time (in seconds)
Let’s Practice again…
•
A roach moves down the hall at 1.2 m/s.
When he sees the janitor coming down the
hall, he begins to run. After 3.2 s, he is
moving at 3.6 m/s. What is his acceleration?
Now that we’re pros at Acceleration…
A quick review problem of Net Force:
The net force in this case is:
275 N + 395 N – 560 N = 110 N to the right