Download Forces, Motion and Roller Coasters!

Newton’s Second Law of
Motion
Newton’s Second Law – the Law of
Acceleration

The acceleration (a) of an object
depends on the objects mass (m) and
the amount of force (F) applied to the
object.
Remember, acceleration occurs
when an object…
1. Speeds up
3. Changes
Direction
2. Slows Down
What does this look like?
Most familiar version:
Actual definition of
Newton’s 2nd Law:
F = m•a
a = F/m
Force
Other
Manipulation:
m = F/a
mass
X
acceleration
F=mXA
1.
Practice!
A = F/m
What is the acceleration of a 15 kg mass
if a force of 45 Newton’s is used to move
it?
- Which formula will you use?
A = F/m
A = 45 N / 15 kg
A = 3 m/s2
F=mXA
With a partner!
A=F/m
What force is necessary to accelerate 200 kg
object at a rate of 40 m/s2?
F=mXA
On your own…
A=F/m
What is the force necessary to move an
object with a mass of 25 kg at an
acceleration of 5 m/s2?
Net Forces
• Total of forces acting on
an object
• the refrigerator has
many forces acting on
it, but when added
together they cancel
out.
• therefore there is no
change in it’s motion
Forces in the Same Direction
– In this example,
there are two
forces acting on
the object in the
same direction.
– These two forces
are added
together.
– The objects
resulting motion is
to the right.
15 N
20 N
Resulting Net Force=
35 N
Net Force of Zero
– Because the
forces from Man
A and Man B are
equal but
opposite in
direction, there
is no change in
the objects
motion.
A
B
Net Force Other Than Zero

If there is a difference
between two
opposing forces, the
resulting change in
motion is in the
direction of the larger
force.
– The force of the car is
10 N to the right while
the parachute provides
7 N of force in the
opposite direction.
 Net Force = 10 N – 7 N
 F = 3 N to the right
Practice
 What
would the
resulting motion
be from this
group of forces?
More Practice

The mass of the
car and the driver
equals 240 kg.
How fast will the
driver and his car
accelerate?
?
7N
85 kg

10 N
15 N
75 kg
90 kg
How much force
would the man on the
left need to provide
in order to stay in the
same position?

3N
55 kg
50 kg
If the man on the
right was able to
provide 30 N of force,
what would the
resulting acceleration
be?
Always remember…
If Force remains the same and
 mass increases, then acceleration will
decrease.
 Acceleration increases, then the mass will
have to decrease.
2nd Law Video
Newton’s 2nd Law and trains