Newton's second law of motion

1. Let’s Recall!
• Net force
• Mass
• Acceleration

3. Suppose you hit these balls
with the same amount of
force, which would accelerate
faster? Why?

4. Newton’s Second
Law of Motion

5. 2nd
Law

6. Force and Acceleration
• Force is a push or a pull.
•Acceleration is when the motion of an object
changes.
Examples:
Speed up
Slow down
Changes direction
•Mass is the amount of matter in an object.

7. Force and Acceleration
• In this equation, a is the acceleration, m is the
mass, and Fnet is the net force.
• If both sides of the above equation are
multiplied by the mass, the equation can be
written this way:

8. Formula Practice
A book with a mass of 2.0kg is pushed
along a table. If the net force on the book
is 1.0N, what is the book’s acceleration?
Answer: .5m/s2

9. Force and Acceleration
• Units of Force = Newtons (N)
SI units for mass is kg
SI units for acceleration is m/s2
1 Newton (N) = 1kg * m/s2

10. Gravity & Weight
• Gravity is the force of attraction that exists
between any two objects that have mass.
• The force of
gravity depends
on the mass of
the objects and
the distance
between them.

11. Gravity & Weight
• Weight is a force, like the push of your
hand is a force, and is measured in
Newtons.
• The force of gravity causes all objects near
Earth’s surface to fall with an acceleration
of 9.8 m/s².
• Your weight on Earth is the gravitational
force between you and Earth.

12. Gravity and Weight
• How are weight and mass different?
• Weight is a force, like the push of your
hand is a force, and is measured in newtons.
• Mass is the amount of matter in an object,
and doesn’t depend on location.
• Weight will vary with location, but mass
will remain constant.

13. Newton’s 2nd
Law
• Newton’s Second Law states: an object
acted upon by an unbalanced force will
accelerate in the direction of the force.
• If you kick the
ball, it starts
moving.
• The ball
accelerates only
while your foot is
in contact with the
ball.

14. 2nd
Law
The net force of an object is
equal to the product of its mass
and acceleration, or F=ma.

15. Newton’s 2nd
Law
• Newton’s second law of motion can be used
to calculate acceleration.
• For example, suppose you pull a 10-kg sled
so that the net force on the sled is 5 N.
• The acceleration can be found as follows:

16. If mass remains constant, doubling the acceleration, doubles the force. If force
remains constant, doubling the mass, halves the acceleration.

17. Newton’s 2nd
Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd
Law we know
that they don’t hit the
ground with the same
force.
F = maF = ma
98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s
F = maF = ma
9.8 N = 1 kg x 9.89.8 N = 1 kg x 9.8
m/s/sm/s/s

19. Problems:
1. How much force is needed to accelerate a
1400 kilogram car at 2 m/s2
?
2. How much force is needed to accelerate a
1000-kg car at a rate of 3 m/s2
?
3. A dancer lifts his partner above his head
with an acceleration of 2.5 m/s2
. The
dancer exerts a force of 200 N. What is
the mass of the partner?

20. Section Check
Question 1
Explain Newton’s second law of motion.
Answer
An object acted upon by an unbalanced force
will accelerate in the direction of that force.
Newton’s Second Law says that the
acceleration of an object is equal to the net
force on it divided by its mass.

21. Section Check
Question 2
Your _______ on Earth is the gravitational
force between you and Earth.
A. acceleration
B. inertia
C. mass
D. weight

22. Section Check
Answer
The answer is D. Since weight is dependant on
gravity, your weight would be different if you
were standing on a planet other than Earth.

23. Section Check
Explain
A thin old man moves slowly despite
his little mass. Why?

24. Section Check
Value Focus
How is the law applied in
accomplishing a task in relation to a
person’s ability and the effort
exerted?

25. Assignment: ½
crosswise
A. Solve the following word problems. Show
your solution.
1.What net force is required to accelerate a
car at a rate of 2m/s2
if the car has a mass of
3,000 kg?
2. A 10 kg bowling ball would require what
force to accelerate down an alley at a rate of
3 m/s2
?

26. B. Explain your answer. The net force on an
object and its resulting acceleration are
always in the same direction. To
demonstrate this relationship, a spool of
thread is pulled horizontally to the right. In
which direction will it roll? Will it make a
difference whether the string is on top or at
the bottom when it was pulled?

27. C. by group: Bring the following materials on
July 4, Monday:
•5 pcs. Party balloon (unblown)
•Adhesive tape/scotch tape
•2 m string/yarn
•Scissors
•2 pcs. Drinking straw