Lab: Unbalanced Forces

Problem: How does the amount of friction between an object and the surface affect motion?

Background: Newton’s first law of motion is about unbalanced forces, basically:

An unbalanced force is what causes an object to speed up, slow down, or change its direction of motion (that is, accelerate).

In this lab, we will investigate two situations, one with and one without friction.

PART 1: FRICTIONAL FORCES “STICKY SNEAKERS”

Procedure:

1. Sneakers are designed to deal with various friction forces, including these:

  • Starting friction (starting from a stopped position)
  • Forward-stopping friction (coming to a stop)
  • Sideways-stopping friction (coming to a stop sideways)

2. Place each sneaker on a balance. Then put masses in each sneaker so that the total mass of the sneaker plus the masses is 1000g. Distribute the masses evenly inside the sneaker.

3. Tape the paper clip to each sneaker and then attach a spring scale to the paper clip.

To measure:

  • Starting friction, attach the paper clip to the back of the sneaker
  • Forward-stopping friction, attach the paper clip to the front of the sneaker
  • Sideways stopping friction, attach the paper clip to the side of the sneaker

4. To measure starting friction, pull the sneaker backward until it starts to move. The force necessary to start the sneaker moving is equal to the friction force. Record the starting friction force in your data table.

5. To measure either type of stopping friction, use the spring scale to pull each sneaker at a slow, constant speed. Record the stopping friction force in your data table.

6. Repeat steps 3-5 for the remaining sneakers.

Observations:Data Table

Shoe / Starting friction (N) / Sideways-stopping friction (N) / Forward-stopping friction (N)
A
B
C

Conclusions:

1. What is the manipulated variable in this experiment?

What is the responding variable?

2. Why is the reading on the spring scale equal to the friction force in each case?

3. Draw a diagram that shows the forces acting on the shoe for each type of motion.

4. Can you identify a relationship between the type of shoe and the type of friction you observed? What do you observe about the shoes that would cause one to have better traction than the other?

PART 2: FRICTIONLESS SURFACE: “HOVER PUCK”

Procedure:

For this part of the lab, we will use a “Hover Puck” to simulate a frictionless surface. The Puck glides on a cushion of air, almost completely canceling the force of friction.

1. First, observe the motion of the puck traveling on a level surface.

2. Measure off one meter, two meters, and three meters, and time how long the puck takes to cover each meter. Take the time measurement as it passes the meter mark. Test each distance twice to ensure good measurements. It is necessary that one person push it EVERY TIME to try to ensure the same amount of force is applied each time.

Distance / Time (first trial) / Time (second trial) / Average Time
1 meter
2 meters
3 meters

3. To find the speed of the puck, divide the Average Time by the Distance. (Just like in math, distance = rate x time, so if we need the rate (or speed), we do distance  time.)

Speed for puck at one meter:

Speed for puck at two meters:

Speed for puck at three meters:

4. Looking at your speed measurements for each trial, were they similar (was the speed constant)? ______Why or why not? ______

What forces are acting on the puck? ______

5. Now observe the effects on the puck when a force is applied over a short time.

Set the puck in motion. After it has traveled a meter or so, tap it with your foot.

DO NOT KICK THE PUCK, just give it a quick bump.

6. Repeat this several times in different directions and with different amounts of force. Tell what happens to the puck in each of the following situations:

a. Right angle motion:

b. Same direction as motion:

c. Opposite direction of motion:

What effect does the direction and size of the force have on the motion of the puck?