In this task, you'll explore three contact forces exerted by surfaces of objects.

Estimated time to complete: 15 minutes

You will need these materials:

safety goggles

cleared space on a smooth, durable table that is at least 3 feet in length

12-inch ruler

ball made of a hard material, such as a baseball, tennis ball, pool ball, lacrosse ball, whiffle ball, or ping-pong ball

large, heavy book or box to use as a barrier

piece of plastic or wood with a broad flat surface, such as a three-ring binder or a cutting board

area rug, carpet, or piece of fabric with nap

Always wear your safety goggles.

Part A

Set the ball and the barrier about two feet apart on the table. Grasp both ends of the ruler. Push the ball using the center of the ruler so the ball rolls, hits the barrier, and rebounds from it. Describe the motion of the ball as it moves.

Part B

Identify the forces exerted on the ball from the time the ruler touches it.

Part C

Question

The force of the ruler on the ball is called an applied force. The force of the barrier on the ball is called a normal force. Both of these forces are exerted by surfaces, and the direction of force is at a right angle to the surface. The difference is that an applied force is exerted by a moving object and a normal force is exerted by a stationary object. Draw two arrows to represent the applied force and the normal force on the ball.

Part D

Now explore friction force. Set the piece of plastic or wood on the table and push it steadily across the tabletop using your fingertip. Note how much opposition to your push you feel. Repeat the steps, but this time push the piece of plastic or wood across a rug, carpet, or piece of fabric. If you're using fabric, be sure to secure the fabric so it doesn't move. How does the opposition to motion on the tabletop compare with that of the rug, carpet, or fabric?

Part E

Friction force is the opposition to the motion of surfaces sliding across each other. Based on your observations, what is one factor that affects the amount of friction?

Part F

Question

Draw an arrow to represent the friction force.

Part G

List several examples of applied force, normal force, and friction that you've observed in your life.

Task 3

Forces Exerted Over a Distance

In this task, you'll explore the forces that can be exerted over a distance.

Estimated time to complete: 20 minutes

You will need these materials:

safety goggles

4 or more identical clothes hangers

any kind of tape

2 bar magnets with labeled or colored ends

2 balloons

2 (2-foot) pieces of thread or string

wool or fur to charge balloon (can use wool yarn, a sweater, or even your hair to create the charge)

2-foot piece of plastic wrap

Always wear your safety goggles. Do not work with balloons if you have an allergy to latex. Do not create static charge on the balloon or work with magnets if you have a pacemaker or another implanted electronic device. Keep the magnets away from all electronic devices, including cell phones and computers.

Note: For steps E and F, choose a place to work where the air is cool and dry. Electric charges won't build up on balloons in humid conditions.

Part A

Take one hanger, and place a hanger on each of its ends as shown. If your hangers aren't notched, wrap tape around the hooks to hold them in place. Does the system of hangers balance? Why or why not?

Part B

Now add a second hanger to one of the sides. What happens, and why does the system behave as it does? What is the name of the force responsible for what you're seeing?

Part C

Question

The gravitational force on each side of your hanger system is different. Use the drawing tool to draw a force arrow on each side of your system. The greater the gravitational force, the longer the arrow.

Part D

Next, we'll examine magnetic force. Bring the ends of your two magnets together. Explore the three possible combinations. In two of the combinations, the two ends are the same. In one combination, the two ends are different. Describe the force you feel in each combination.

Part E

Move on to electric force. Blow up the two balloons and knot them. Then tie a thread onto each balloon. Suspend the two balloons using tape so that they're about six inches apart, and check that they don't move or interact. Rub both balloons with wool or fur. If wool or fur is not available, rub the balloons on your hair. Do they attract or repel (push away) each other?

Part F

Rub one balloon with wool, fur, or hair and the other balloon with a piece of plastic wrap. Again, suspend the two balloons so that they're about six inches apart. Do they attract or repel each other?

Part G

List several examples of gravity, magnetic force, and electric force that you've observed in your life.

Part H

Throughout this activity, you've explored forces. What is one question that you'd like to investigate further about forces?

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