University of Virginia
Physics Department

## Newton's First Law: Observing Inertia

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.10

Objectives

Students will

• observe how objects resist change in motion;
• apply Newton's First Law of Motion to a two-object system.

Motivation for Learning

Discrepant Event - Tablecloth yank

Materials:

• Vase w/ Flowers
• Bottle
• Tablecloth
• Silverware
• Wine Glass
• Plate
• Small Table

Procedure:

Place tablecloth over table with the majority hanging over one side. Set the table and fill the wine glass. Grasping the hanging cloth firmly, jerk quickly downward and away from the table. With practice this will leave the table setting untouched and the cloth off the table.

Discrepant Event - Hanging Mass

Materials:

• Mass
• String
• Ring stand

Procedure
:

Tie a piece of string to the top of the mass and then tie that to a ring stand so that the mass is suspended. Tie another piece of string to the bottom of the mass. If you yank the bottom string quickly, that string should break. If you pull the bottom string and constantly (and fairly slowly) increase the force, the top string should break.

Background Information

Newton's First Law of Motion states that an object at rest will remain at rest unless an external force acts upon it. Similarly, an object in motion will continue at the same velocity in the absence of an external force.

In this experiment, the students will cause a collision between a cart carrying a mass and a piece of cardboard. The forces acting on the cart include the frictional force between its wheels and the floor, the gravitational force which keeps the cart on the floor, and at the point of collision, an external force from the cardboard. Because of this, the cart's velocity will decrease after impact. The forces acting on the mass include the frictional force between the mass and the cart, and the gravitational force. However, the mass will not incur an external force from the cardboard, as they do not come into direct contact. Therefore, the mass will continue with its original motion and move forward across the cart at impact.

When sandpaper is added to the cart surface, it greatly increases the frictional force between the mass and the cart. Upon collision, the mass should still move across the cart, but at a lower velocity because of the additional frictional force.

The concept of this experiment is similar to what happens during a car accident. If the passengers are not wearing seatbelts, they will continue to move forward at the same velocity until they incur an external force (the windshield). However, if seatbelts are worn (analogous to the tape the students will place on the mass in steps 8 and 14), the passengers will experience a force and will not move forward.

### Student Activity

Activity 1: Quarter Catch

Materials

• 1-4 quarters

Procedure

*Practice this first to determine how many quarters you are comfortable using*

2. Stack the quarter(s) on the flat portion of your elbow.
3. Throw your arm forward slowly and try to catch the coins with the same hand (this will probably not be possible).
4. Ask the students for suggestions on how to make this easier.
5. Repeat, this time moving your arm quickly enough to catch the coins.
6. Ask the students why it was more difficult at a slower speed.
7. Explain how this relates to Newton's First Law. If your arm moves quickly enough, the coins will remain motionless for an instant. However, if you move your arm slowly, the coins will fall due to the force of gravity, and you will not have time to catch them.

Activity 2: Rolling Cart and Inertia

Materials

• 2 ring stands
• Cardboard (20 cm x 5 cm)
• Small cart with wheels (can be ordered from Frey Scientific Catalog (1-888-222-1332); item number SO4480)
• 100 g mass
• Metric ruler or meter stick
• Sandpaper

Procedure

1. Place the two ring stands about 20 cm apart on the floor or on a large surface.
2. Tape the cardboard to the ring stands so that it spans the area between them. Make sure the cardboard is close enough to the floor so that the cart cannot fit underneath it.
3. Place the 100 g mass inside the cart. Set the cart on the floor at least 1 meter from the cardboard.

4. Place a piece of tape on the cart underneath the spot where the mass is initially set.
5. Gently push the cart towards the cardboard so that the cart strikes it. Measure the distance that the mass travels from its starting point on the cart and record it on the data table.
6. Replace the mass in its original position. Repeat step 5, but push the cart with a greater force.
7. Measure the distance that the mass travels from its starting point on the cart and record it.
8. Replace the mass in its original position and secure it with a piece of masking tape.
9. Push the cart with the same force as in step 6, and observe what happens to the mass. Record this.
10. Remove the mass from the cart. Tape a piece of sandpaper, cut to fit, onto the cart.
11. Replace the mass in its original position.
12. Gently push the cart into the cardboard and record the distance traveled by the mass.
13. Replace the mass, and push the cart with a greater force into the cardboard. Record the distance.
14. Tape the mass in its original position. Push the cart with the same force as in step 13, and record the observation.

Data Sheet

 Distance traveled by mass Observations First trial (no sandpaper) Gentle push Fast push Mass taped down Second trial (sandpaper) Gentle push Fast push Mass taped down

Questions

1. Describe the forces acting upon the cart when it comes in contact with the cardboard. How does this explain its change in motion?

2. Describe the forces acting upon the mass at the point of collision. How does this explain its motion?

3. Did the addition of sandpaper to the cart affect your results? Why or why not?

5. Do the results from this experiment demonstrate the importance of wearing a seatbelt inside a car?

Students with Special Needs

Some students may have difficulty manipulating some of the objects (especially in the Quarter Catch). These students could work with partners or in small groups.

Click here for further information on laboratories with students with special needs.

Assessment

Data sheet to be completed during the laboratory.