SUBTOPIC: Demonstration of Newton's Three Laws of Motion and the Law of Gravitation

OBJECTIVES:

The students will:

1) give examples of each of Newton's three laws as they occur in everyday experiences

2) visualize and differentiate the difference between a direct proportion and an inverse proportion using the formula F=ma

3) understand how the gravitational law affects the tides of the earth

4) calculate the actual gravitational force between the sun and earth and the moon and earth to conclude which has the stronger influence

Isaac Newton summed up motion in three laws. Today we take these laws for granted as we grow up assuming they are true. We do not realize the struggles scientists went through in attempt to understand the world around them. The following activities use brainstorming, discussion, and simple labs to illustrate the laws.

Newton's Three Laws:

1) An object which is moving at a constant velocity or at a state of rest does not change its state unless a force acts upon it.

2) Acceleration of an object increases as the amount of force causing the acceleration increases when mass is constant.

3) For every force, there is an equal and opposite force.

ACTIVITY #1:

MATERIALS: CHALK AND BLACKBOARD

PROCEDURE:

1) Brainstorm everyday examples of the first law.

2) Present a lecture to students, including the following necessary background information:

Just prior to Newton's time Galileo had worked with the idea of acceleration. Galileo could only guess about time since precise clocks had not been invented. This is why he rolled metal balls down smooth ramps. Since he noticed how a ball slowed when rolling across the floor, he concluded that friction was the cause. Thus friction was responsible for the idea that objects in motion naturally come to rest. But 'rest' is just one kind of constant velocity. The concept of inertia and Newton's 1st law emerged from this insight.

3) Use some of the following examples to explain to the students how Newton's first law occurs in everyday events:

a) car suddenly stops and you strain against the seat belt

b) when riding a horse, the horse suddenly stops and you fly over its head

c) the magician pulls the tablecloth out from under a table full of dishes

d) the difficulty of pushing a dead car

e) lawn bowling on a cut and rolled lawn verses an uncut lawn

f) car turns left and you appear to slide to the right

ACTIVITY #2 :

MATERIALS: METER STICKS, INDEX CARDS, TAPE, MARKERS

PROCEDURE:

1) Newton's second law deals with F=MA. When written A = F/M on sees that the acceleration will vary directly with the force applied and inversely with the mass of the body. Since students have trouble with these terms, a simple visual aid can help them.

2) Take three index cards and write an A, F, and M on them, and then tape the F card to a meter stick at the 50 cm mark so that it hangs down. Next tape the A card at 0 cm and the M card at the 100 cm mark.

3) Explain to the students that if the force is constant (either flip the card up or cover it with your hand), when acceleration increases (raise the 0 cm end of the meter stick at a 30 angle) mass must decrease.

4) Note that the 100 cm end now angles down. This shows an inverse proportional relationship.

5) Now cover the acceleration card with your hand. When force or mass increases or decreases the other variable will do the same. This shows the direct proportional relationship.

6) Lastly, do the same for the M card.

7) Plug in numbers and work through some simple F=MA problems.

8) Use the meter stick to help visualize what the answer will be (greater or smaller). Finally brainstorm everyday applications, some examples are listed below.

a) hitting a baseball, the harder the hit, the faster the ball goes

b) accelerating or decelerating a car

c) The positioning of football players - massive players on the line with lighter (faster to accelerate) players in the backfield

d) a loaded versus an unloaded truck

ACTIVITY #3:

MATERIALS: SKATEBOARD AND SPRING SCALE

PROCEDURE:

1) Have a student bring in a skateboard.

2) Have one student stand on the skateboard at the front of the class and hold one end of the spring scale.

3) Another student should pull the first student at a constant force of 10 newtons.

4) Observe the speed of the students as they keep the force constant.

5) Explain that this shows the direct relationship between force and acceleration.

ACTIVITY #4:

MATERIALS: CHALK AND BLACKBOARD

PROCEDURE:

1) Brainstorm everyday examples of the third law with the class. Listed below are some examples:

a) rockets leaving earth--many physicists of the nineteen hundreds (Goddard's time) said that rockets could never leave the earth. Discuss how a spaceship flies in space.

b) guns being fired- discuss why they kick in proportion to the size of the bullet. Why is the stock of the rifle so large? What would happen if the stock of a shotgun came back to a point shape?

c) two cars hit head on

d) astronauts in space

e) pool or billiards

f) jumping out of a boat onto the dock

g) sprinklers rotating

ACTIVITY #5:

MATERIALS: 3 LONG BALLOONS, 1 PLASTIC STRAW, 60 CM (OR MORE) OF FINE WIRE OR FISHING LINE, TAPE, MODEL AIRPLANE (BALSA WOOD), MARKER

PROCEDURE:

1) Have the students follow the procedures listed below:

a) Blow up balloons, fasten them with rubber bands, and label them A, B, and C.

b) Tape the straw lengthwise to Balloon B and run the wire through the straw.

c) Tape Balloon C to the top of the fuselage of the model airplane, placing the balloon opening toward the tail of the airplane.

d) Loosen the rubber band on Balloon A very slowly and record the speed and direction of movement.

e) Now, on Balloon B, have a partner hold each end of the wire through the straw and keep the wire tight.

Cut the rubber band quickly and observe the balloon. Record observations. 

f) Draw a sketch showing the direction the air in the balloon moved. Also, on the same sketch, draw a diagram of how the balloon moved along the wire.

g) To test Balloon C, have a partner hold the airplane loosely. Cut the rubber band as your partner releases the airplane. Record the flight. You may wish to stage olympic type competitions between lab partners.

Hint: Try different shapes of balloons. Elongated should work the best. Also a round trip rocket could be designed.

a) Describe the reaction of the rubber band when it was cut.

b) Describe the flight of Balloon B.

c) What was the force that moved the Balloon B?

d) Why did Balloon B move differently from Balloon A?

e) Why did Balloon B and Balloon C move more rapidly than Balloon A?

f) State Newton's third law and explain how this activity illustrates it.

ACTIVITY #6:

MATERIALS: PENCIL AND PAPER

PROCEDURE:

1) Ask the students to write a 2 to 3 page science fiction story describing what differences we would observe if the opposite of Newton's three laws were true on earth. For example, guns would not have recoil, and a cannon's mass would not have to be greater than a cannon ball. You would also not be pushed back in your seat when undergoing acceleration in a car.

2) As an alternative, you may wish to do a verbal brainstorming of how things on earth would be different if we lived under the reverse of Newton's laws.

ACTIVITY #7:

MATERIALS: CHALK AND BLACKBOARD

The gravitational force of the moon and sun play an important role in the tides. When the sun, earth, and moon are in a straight line, their combined gravitational pull causes extra high and low tides known as spring tides. Whenever there is a full or new moon this occurs. The neap tides form when the sun, the earth, and the moon form a right angle, causing a half moon. The question is which, the sun or the moon, has the stronger gravitational pull?

PROCEDURE:

1) Using Newton's gravitational formula, have the students research (homework) the data needed and do a class project at the board doing the calculations.

2) Depending on the ability of the students, each student may do their own calculations.
Mass of Earth 5.98 x 10²⁴ Kg

Mass of Sun 1.98 x 10³⁰ Kg

Mass of Moon 7.36 x 10²² Kg

Distance - Earth to Sun 1.50 x 10¹¹ m

Distance - Earth to Moon 3.84 x 10⁸ m

F = G m₁m₂ where G = 6.67 x 10^-11 m³

d² Kg sec₂
 

1) Sun to Earth

F = 6.67 x 10^-11 m³ x 1.98 x 10³⁰ Kg x5.98 x 10²⁴ Kg

Kg sec² (1.55 x 10¹¹ m)²
 

F = 3.51 x 10²² m/sec²
 

2) Moon to Earth

F = 6.67 x 10^-11 m³ x 7.36 x 10²² Kg x 5.98 x 10²⁴ Kg

Kg sec² (3.84 x 10⁸ m)²

F = 1.99 x 10¹⁹ m/sec²

The pull between the sun and the earth is about 180 times stronger than the pull between the moon and the earth. So our calculations are correct, but why doesn't the sun cause tides 180 times greater? Because of the sun's great distance from the earth, there is not much difference in the distances from the sun to the earth's near and far side. This means that there is not much difference in the gravitational pull of the sun on the ocean nearest it and on the ocean furthest from it. The relatively small difference in pulls on the opposite sides of the earth only slightly elongates the earth's shape. Thus the sun produces tidal bulges less than those of the moon.

The tilt of the earth also affects tides. The tilt causes the 2 daily high tides experienced in most parts of the ocean to be unequal in height.

ACTIVITY #8:

PROCEDURE:

1) Since Jupiter is 7.8 x 10¹¹m from the sun and has a mass of 1.8 x 10²⁷Kg. Have the students calculate Jupiter's gravitational force, and determine if the sun produces tides on Jupiter.

F = 6.67 x 10^-11 m³x 1.98 x 10³⁰ Kg x 1.8 x 10²⁷ Kg

Kg sec² (7.8 x 10¹¹ m)²

F = 3.9 x 10²³ m/sec²

ACTIVITY #9:

MATERIALS: WATER, SMALL ROUND BALLOONS, AND STRING

PROCEDURE:

1) Demonstrate the shape of the earth by first filling a balloon with water. It might be best to consider performing this outside, in the event that the balloon breaks.

2) Next, tie it shut and attach a string securely.

3) Swing the balloon around over head and observe the shape of the balloon. It should look elongated.

4) Explain that this is the same process which occurs on earth while it is rotating around the sun. The water covering the earth is distorted and will bulge like the balloon.

5) Read about the debate over the shape of the earth between the followers of Newton and those of Descartes in Tom B. Jones, The Figure of the Earth, 1967.

ACTIVITY #10:

MATERIALS: 2 CARTS, 2 PULLEYS, 2 HOOK MASS HOLDERS, 2 BARBIES, STRING, 2 BLOCKS OF WOOD (2X4X8")

PROCEDURE:

1) Place a Barbie doll on each cart. On one of the carts, use a rubber band to securely attach the Barbie (seat belt).

2) Attach 2 meters of string to each cart. Attach 200 g. to the hook mass holder. Attach the pulleys to the table edge.

3) Place a block of wood in front of the pulley and place the string over the pulley.

4) Now attach the mass holder to the string while someone holds the cart in place.

5) Pull the carts back and allow the weight to accelerate the carts.

ACTIVITY #11:

MATERIALS: 500 ML FLASK, 8 INCH EMBROIDERY HOOP, 10- 1/8 INCH NUTS

PROCEDURE:

1) Balance an embroidery hoop vertically on the flask's mouth.

2) Stack nuts on the top of the hoop. Using one hand, snatch the hoop away quickly so that the nuts will fall into the flask.

3) Have students perform the activity and create a contest to see who can get the most nuts at once into the flask.

4) Relate this to Newton's first law and the famous magician's act of pulling the tablecloth out from under the dishes.

Hewitt, Paul G. Conceptual Physics.
Jones, Tom B. The Figure of the Earth. 1967.
Jones and Childers. Contemporary College Physics.
Ross, David A. Introduction to Oceanography.