1. Force and mass determine acceleration Or Newton’s Second Law of Motion
Chapter 11: Forces
Section 2 Outline
Force and mass determine acceleration Or
Newton’s Second Law of Motion pg

2. First, let’s review
Newton’s first law of motion: An object at rest tends to stay at rest and an object in motion tends to stay in motion at a constant velocity unless an unbalanced force acts on it.
Inertia: an object’s resistance to a change in speed or direction.
Greater mass = more inertia!

3. Open your interactive notebooks to pg. 33.
You’re going to read more about force, mass, and acceleration. Now we’re going to see how they affect each other in real life.
Open your interactive notebooks to pg. 33.
Turn to pg. 343 in your science book.
Read the directions for “Popping Ping-Pong Balls”. DON’T DO ANYTHING!!!
Write the name of the activity (Popping Ping-Pong Balls) in the center of the top line in your interactive notebook.
Under the name of the activity, write the page number in your book (pg. 343).
Under the page number, write today’s date (1-8-13).

4. Copy these questions in your notebook:
Which parts of the experiment stay the same?
Which part of the experiment do you change (the manipulated variable)?_________________
What will you measure (the responding variable)___________
Now write your hypothesis:
If you pop a ping-pong ball and a golf ball with a ruler using the same amount of force, then the ________________ will go farther.
Do the experiment, being careful NOT to break the ruler!
Write and answer the two observe and think questions.

5. You’ve read about force, mass, and acceleration
You’ve read about force, mass, and acceleration. Now we’re going to see how they affect each other in real life.
Get out your interactive notebooks. I will show you which page to write on.
Turn to pg. 353 in your book.
Read the three steps listed under PROCEDURE.
In the small cup on your desk are the supplies that you need. Follow directions for the activity.
After you’ve completed the activity, turn to the correct page in your interactive notebook.
Write the name of the activity (How are force and acceleration related?) in the center of the top line in your interactive notebook.
Under the name of the activity, write the page number in your book (pg. 353).
Under the page number, write today’s date ( ).
Skip two lines, and then write and answer the two WHAT DO YOU THINK? questions in your interactive notebook.

6. What did you discover?
The more paper clips on the other end of the thread, the faster the motion of the paper clip on the table.

7. II. Force and mass determine acceleration.
Newton’s second law relates force, mass, and acceleration.
Newton’s Second Law
The more force you apply to an object, the greater the acceleration of the object.
The more mass the object has, the slower the object will accelerate.
The direction the object accelerates in is the same as the direction of the force.

8.
Teachers: To open, right-click on the link, then choose open hyperlink
Students: you can visit this website from any computer. Go to classzone.com and follow the directions!

9. Force = mass x acceleration F = ma
Force Equals Mass Times Acceleration
Newton’s second law can be written as an equation
If you are given two of the three factors (F, m, or a) you can calculate the missing factor.
Force is measured in units called Newtons (after Sir Isaac Newton).
One Newton = 1 kg x m/s2
Force = mass x acceleration F = ma

10. The object with less mass will have the greater acceleration.
Check Your Reading
If the same force is applied to two objects of different mass, which object will have the greater acceleration?
The object with less mass will have the greater acceleration.

11. Calculating Force Sample Problem
What force is needed to accelerate a 10 kg shopping cart 3 m/s2?
What do you have to calculate? Force
What are you given? Mass = 10 kg
acceleration = 3 m/s2
Which formula will you use? F = ma
F = (10kg) (3 m/s2)
F = 30 kg m/s2
F = 30 N

12. If a 5 kg ball is accelerating 1.2 m/s2, what is the force on it?
On a new sheet of paper, work the two practice the math problems on pg. 355.
If a 5 kg ball is accelerating 1.2 m/s2, what is the force on it?
What do you have to calculate? Force
What do you know? Mass and acceleration
Which formula will you use?
F = ma
F = (5 kg) (1.2 m/s2)
F = 6 kg m/s2
F = 6 N

13. What do you have to calculate? force
2. A person on a scooter is accelerating 2 m/s2. If the person has a mass of 50 kg, how much force is acting on that person?
What do you have to calculate? force
What are you given? mass and acceleration
Which formula will you use?
F = ma
F = (50 kg)(2 m/s2)
F = 100 kg m/s2
F = 100 N

14. Calculating Acceleration Sample Problem
If a team pulls with a combined force of 9000 N on an airplane with a mass of 30,000 kg, what is the acceleration of the airplane?
What do you need to calculate? Acceleration
What are you given? Force and mass
Which formula will you use? F = ma
But you need to change the formula around to calculate acceleration….. F/m = a
9000 N
= a
30,000 kg
9000 (kg)(m/s2)
= a
30,000 kg
0.3 m/s2 = a

15. On the same sheet of paper that you used for the first two practice the math problems, complete the two Practice the Math problems on the bottom of pg. 356.

16. Half the people on the team decide not to pull the airplane
Half the people on the team decide not to pull the airplane. The combined force of those left is 4500 N, while the airplane’s mass is still 30,000 kg. What will the acceleration be?
What do you have to calculate? Acceleration
What are you given? Force and mass
Which formula will you lose? a = F/m
a = 4500 N/30,000 kg
a = 4500 (kg)(m/s2)/30,000 kg
a = 0.15 m/s2

17. What do you have to calculate? acceleration
2. A girl pulls a wheeled backpack with a force of 3 N If the backpack has a mass of 6 kg, what is its acceleration?
What do you have to calculate? acceleration
What are you given? force and mass
Which formula will you use? a = F/m
a = 3 N/6 kg
a = 3 (kg)(m/s2)/6 kg
a = 0.5 m/s2

18. 3. Mass and Acceleration
If the same force acts on two objects, the object with less mass will accelerate more.
If an object loses mass and the force stays the same, the object will accelerate more.
Look at the picture of the NASA launch rocket on pg As the rocket loses fuel, will it accelerate more or less? Why?

19. Calculating Mass Sample Problem
A model rocket is accelerating at 2 m/s2. The force on it is 1 N. What is the mass of the rocket?
What do you need to calculate? mass
What do you know? acceleration and force
Which formula will you use? F = ma
You have to rearrange the formula to F/a = m
m = 1 N
2 m/s2
m = 1 (kg)(m/s2)
m = 0.5 kg

20. Add the two ‘practice the math’ problems on pg
Add the two ‘practice the math’ problems on pg. 357 to your sheet of problems.
Another model rocket is accelerating at a rate of 3 m/s2 with a force of 1 N. What is the mass of the rocket?
What do you need to calculate? Mass
What do you know? Force and acceleration
Which formula will you use? M = F/a
m = 1 N
3 m/s2
m = 1 (kg)(m/s2)
m = 0.3 kg

21. What do you need to calculate? mass
2. A boy pushes a shopping cart with a force of 10 N, and the cart accelerates 1 m/s2. What is the mass of the cart?
What do you need to calculate? mass
What are you given? Force and acceleration
Which formula will you use? m = F/a
m = 10 N
1 m/s2
m = 10 (kg)(m/s2)
m = 10 kg

22. Explore Activity: Popping Ping-Pong Balls
Open your book to pg. 343.
Read ALL directions completely and carefully.
When you are doing this activity, do NOT break my rulers!!!

23. After you have completed the activity using both a ping-pong ball and a golf ball, turn to the first empty page in your interactive notebook.
In the center of the top line, write the name of the activity (Popping Ping-Pong Balls)
Below the name of the activity, write the page number (pg. 343).
Below that, write today’s date ( )
Skip two lines, and then write and answer the OBSERVE AND THINK questions.

24. II. Forces can change the direction of motion.
Remember, there are three ways to accelerate; speed up, slow down, or change directions.
Centripetal force
Any force that keeps an object moving in a circle
Without the centripetal force, the object would go flying off in a straight line.

25. CHECK YOUR READING (bottom of pg. 358)
How does centripetal force change the motion of an object?
It consistently changes its direction.

26. Direction the object wants to travel
Direction of centripetal force

27. List as many examples as you can of centripetal force:
The moon’s rotation around the earth is caused by the gravitational attraction between the moon and the earth.
The way your back ‘sticks’ to the Gravitron at the seafood festival (don’t puke!)
Twirling a paper clip on the end of a string.
Twirling a bucket of water (if you’re brave!)
Playing crack the whip! What is the difference in the acceleration of the person in the center of the ‘whip’ and the person on the outside edge of the ‘whip’?

28. C. Circular Motion and Newton’s Second Law
To increase acceleration of an object moving in a circle, you have to apply more force.
This is just another example of Newton’s second law (F = ma).
To get greater acceleration (to get the object to move faster), you have to apply more centripetal force.
An object with more mass will take more centripetal force to get it to move around a circle at a certain speed.

29. Its acceleration increases!!!
CHECK YOUR READING
How does increasing the centripetal force on an object affect its acceleration?
Its acceleration increases!!!

30. Activity: What affects circular motion?
Open your books to pg. 358.
Open your interactive notebook to the next empty page (on the left).
In the center of the top line, write the title of the activity (What affects circular motion?).
On the next line, write the page number (pg. 358).
Under the page number, write today’s date ( )
Skip two lines, then copy the first ‘What do you think?’ question (Did your observations support your hypothesis?).
Read the activity; What affects circular motion?
We’re NOT going to use paint in the classroom (I’ve scrubbed paint out of the carpet more than enough this year!).
However, I will give you a foam plate and a marble so you can see centripetal force in a real-life situation.

31. Since you followed directions inside of the classroom, let’s move outside for some fun!
We’re going to play ‘crack the whip’ to let you feel centripetal force. If you don’t want to play, that’s okay.
If someone is really brave (and has some extra clothes for PE) you can use a bucket of water to demonstrate centripetal force to the class.

32. Then we’re going to use water balloons and string to test the power of centripetal force. But of course there have to be some rules involved!
Don’t do anything unless you’re told to do so.
Do NOT aim your water balloon at another person. You will be written up if you do. No excuses!
Twirl your water balloon by your side to get a chance to feel the centripetal force pulling from your hand, through the string, to the water balloon.
If you would like to, you can test the hypothesis we made in class today: if there was no centripetal force acting on the water balloon then it would keep going in a straight line because of Newton’s first law.
To test your hypothesis, form a line parallel to the brick wall.
Start your water balloon moving in a circular motion, then release your centripetal force (string).
Your water balloon accelerated in a straight line until an unbalanced force made it stop moving (Newton’s first law!!!!)
Make sure someone isn’t aiming a force in your direction and then go and pick up your string and what’s left of your water balloon.
No littering!
No horseplay!!!!
Your behavior today determines if we do film-canister rockets later this week!!!!!