Chapter 11: Forces

Section 2 Outline

Force and mass determine acceleration

Or

Newton’s Second Law of Motion

pg. 353-359

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!

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).

• Copy these questions in your notebook:

1. Which parts of the experiment stay the same?

2. Which part of the experiment do you change (the

manipulated variable)?_________________

3. 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.

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 (12-9-10).• Skip two lines, and then write and answer the two WHAT DO YOU

THINK? questions in your interactive notebook.

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.

II. Force and mass determine acceleration.

A. Newton’s second law relates force, mass, and acceleration.

1. Newton’s Second Lawa. The more force you apply to an object, the

greater the acceleration of the object.

b. The more mass the object has, the slower the object will accelerate.

c. The direction the object accelerates in is the same as the direction of the force.

http://www.classzone.com/books/ml_science_share/vis_sim/mfm05_pg50_newton/mfm05_pg50_newton.html

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!

2. Force Equals Mass Times Accelerationa. Newton’s second law can be written as an

equation

b. If you are given two of the three factors (F, m, or a) you can calculate the missing factor.

c. Force is measured in units called Newtons (after Sir Isaac Newton).

d. One Newton = 1 kg x m/s2

Force = mass x accelerationF = ma

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.

Calculating ForceCalculating ForceSample 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

On a new sheet of paper, work the two practice the math problems on pg. 355.

1. 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 = maF = (5 kg) (1.2 m/s2)F = 6 kg m/s2

F = 6 N

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? forceWhat are you given? mass and accelerationWhich formula will you use?

F = ma

F = (50 kg)(2 m/s2)

F = 100 kg m/s2

F = 100 N

Calculating AccelerationSample 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 = a30,000 kg

9000 (kg)(m/s2) = a 30,000 kg

0.3 m/s2 = a

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.

1. 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

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

3. Mass and Acceleration

a. If the same force acts on two objects, the object with less mass will accelerate more.

b. If an object loses mass and the force stays the same, the object will accelerate more.

c. Look at the picture of the NASA launch rocket on pg. 357. As the rocket loses fuel, will it accelerate more or less? Why?

Calculating MassSample 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)

2 m/s2

m = 0.5 kg

Add the two ‘practice the math’ problems on pg. 357 to your sheet of problems.

1. 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) 3 m/s2

m = 0.3 kg

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? massWhat are you given? Force and accelerationWhich formula will you use? m = F/a

m = 10 N 1 m/s2

m = 10 (kg)(m/s2) 1 m/s2

m = 10 kg

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!!!

• 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 (12-9-10)• Skip two lines, and then write and answer

the OBSERVE AND THINK questions.

II. Forces can change the direction of motion.

A. Remember, there are three ways to accelerate; speed up, slow down, or change directions.

B. Centripetal force1. Any force that keeps an object moving in a

circle

2. Without the centripetal force, the object would go flying off in a straight line.

CHECK YOUR READING

(bottom of pg. 358)

How does centripetal force change the motion of an object?

It consistently changes its direction.

Direction the object wants to travel Direction of centripetal force

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’?

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.

CHECK YOURREADING

• How does increasing the centripetal force on an object affect its acceleration?

Its acceleration increases!!!

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 (12-10-10)• 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.

• 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.

• 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!

1. Don’t do anything unless you’re told to do so.2. Do NOT aim your water balloon at another person. You will be written

up if you do. No excuses!3. 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.

4. 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.

5. To test your hypothesis, form a line parallel to the brick wall.6. Start your water balloon moving in a circular motion, then release your

centripetal force (string).7. Your water balloon accelerated in a straight line until an unbalanced

force made it stop moving (Newton’s first law!!!!)8. 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. 9. No littering!10. No horseplay!!!!11. Your behavior today determines if we do film-canister rockets later this

week!!!!!