# Newton's First and Second Laws

 Materials: Computer and School Network Time Allotment: 3 Class Days

### Purpose:

The purpose of this activity is to study Newton's conception of the relation between force and motion and to compare his conception to other popular misconceptions.

1. Log on to the school server in the usual manner.
2. Open the Physics Explorer application (One Body model) by double-clicking on its icon; it is found in the directory Science-Math/Science Apps/Physics.
3. Choose Open... from the File menu and navigate to the Unit 2 folder.
4. Open the file titled Exploring One Body. A multi-part Physics Explorer simulation is opened. Open the simulation for this lab by clicking on the "Newton's Laws" button. Check that the Aristotle's view window appears as shown at the right.

### Background Information:

In these days of space travel, you have probably seen television pictures of objects in a space craft continuing in motion virtually indefinitely. For hundreds of years, however, people believed Aristotle's idea (from the fourth century B.C.) that a body needs to be constantly pushed or pulled in order to continue in motion. Aristotle's view of the causes of motion are still believed by many today. Perhaps you can evaluate through this activity if you believe in Aristotle's view.

### Newton's First Law:

There are many situations in which Aristotle's view (that a force is required to sustain a motion) appears correct. This activity simulates a boat moving on a lake, being pushed by its motor with a constant force.

Aristotle's View:

1. Test the validity of Aristotle's view of motion by running the simulation.
1. Does the case of a boat moving on a lake (beng propelled by its motor) support Aristotle's view that a force is required to keep an object in motion? _______ Explain your answer.

2. Predict the effect of an increasing motor force upon the velocity of the boat?

3. Test your prediction by running several simulations. What happens?

2. Click on the "Show Speed-Time Graph" button.
1. On the axis below, predict the appearance of a velocity-time graph if you reduce the motor force to 20 N.

2. Run the simulation with a reduced force of 20 N to check your answer. Were you correct? ________ If incorrect, then sketch the corrected answer and label the correct and incorrect appearances on the graph above.

What does the shape of the velocity-time graph tell you about the motion of the boat for this physical situation?

Newton's View:

1. In the 17th century A.D., Isaac Newton proposed his first law of motion, saying that the velocity of an object will remain constant unless acted upon by an unbalanced force. Newton claimed that when the forces acting on an object are balanced, the object in motion continues in motion with a constant velocity. On the other hand, the presence of an unbalanced force causes an object to either slow down or speed up (depending on whether the unbalanced force opposes or supports the motion of the object). To simulate Newton's proposition, click on the "To Newton's View" button. The friction coefficient simulates the resistance of the water to the boat's motion (sometimes referred to as "water drag").
1. Run multiple simulations with a motor force of 20 N, an initial velocity of 10m/s and several different values of friction (ranging from 0 N*s/m to 5 N*s/m). Observe the graphical representation of each motion by clicking on the "Show Speed-Time Graph." Does this case support Aristotle's view or Newton's view of the relation between forces and motion? Explain.

2. What happens when the friction is reduced to 0? Run the simulation, study the velocity-time graph and respond here.

2. Today it is accepted that Newton's law is correct. Change the friction factor and run the simulation once more.

Sketch the shape of the velocity-time graph and explain what the features of the velocity-time graph reveal about the motion of the object.

Run the simulation with a motor force of 20 N, an initial speed of 0 m/s and zero friction. How could you describe the motion? Is there an acceleration or a constant velocity? __________ If there is an acceleration, iss there a constant acceleration value or a changing acceleration value? ___________ Explain how the velocity-time graph helps you to determine this.

3. What made the boat travel at constant velocity when the motor exerts a force?

### Newton's Second Law:

You just saw that an object changes its velocity if there is an unbalanced force acting on it. You can now investigate more exactly how the velocity changes with time.

1. Click on the "To Newton's 2nd Law" button to obtain the following window.

Press the GO tool and note how the velocity of the "boat" appears to change. You can use the "Single Step" button to slow down the simulation to help you in your observation. Describe what you observe.

2. Predict in the diagram below how the boat's velocity will change with time:

Open the graph window and run the simulation to check your answer.

1. What is the numerical slope of the graph? PSYW

2. What units will the slope be measured in? _________
3. What physical quantity corresponds to the slope of the graph? (Hint: what measurement has the same units?)

3. Repeat the simulation with the values of force and mass given in the table below; complete the table with measured values.

### Slope of Graph

50

40

__________

50

20

__________

50

10

__________

40

20

__________

25

10

__________

20

20

__________

1. Based on the above data values it is apparent that...

... a doubling of the mass (while maintaining a constant force) will result in _________ (2X, 0.5X, 4X, 0.25X, etc.) of the acceleration.

...a quadrupling of the mass (while maintaining a constant force) will result in _________ (2X, 0.5X, 4X, 0.25X, etc.) of the acceleration.

...a doubling of the force (while maintaining a constant mass) will result in _________ (2X, 0.5X, 4X, 0.25X, etc.) of the acceleration.

...a quadrupling of the force (while maintaining a constant mass) will result in _________ (2X, 0.5X, 4X, 0.25X, etc.) of the acceleration.

2. From the table of measurements, formulate an equation relating the force (F), the mass (m), and the acceleration (a).

This is the equation for Newton's second law of motion.

### Conclusion:

Write a paragraph to summarize the major principles which have been studied in this lab. Your summarizing paragraph should relate the purpose of the lab and to the actual knowledge acquired through the activity. Do a bang-up job!