# Changing Force and Motion

Isaac Newton (a 17th century scientist) explained three laws that we still use in science today. These three laws explain why objects move (or don't move) as they do. In the years that followed, these three laws have become known as Newton's three laws of motion. These three laws are important principals that engineers use to design objects and machines. In addition, the principles of these laws help in determining how to change forces or motion.

Perhaps the most important concept to understand is Newton's first law of motion. This law becomes very important in the engineering of many products. As you continue on this page, try to determine how Newton's first law of motion applies in your life.

Newton's first law of motion is often stated as:

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. If the forces acting on an object are balanced, you can describe the motion (or lack of motion) of an object as follows:

You can demonstrate Newton's law by doing the following activity!

Materials:

• Plastic bucket
• Water
• Several friends
• Outdoor location

Safety concerns: Be sure to keep all chemical safety rules. As with all science lab activities, the most important safety rule is to follow all teacher directions.

Procedure:

1. Fill the bucket to the top with water.
2. Get with your friends to make relay group outdoors.
• Begin your relay halfway between home plate and first base.
• The first runner takes the bucket and starts running as quickly as possible to first base.
• At the base turn as sharply as possible and run to the second person waiting halfway to second base.
• Continue with each runner in turn waiting halfway to the next base.
3. If you are the last person in the relay, stop as quickly as you can when you get back to the starting point

Analysis:

1. At which locations in your relay did the water have a tendency to spill?
2. At which points did you notice that the water did not seem to try to spill out?

Newton's first law of motion can explain the behavior of the water during the relay race. The water spills whenever the state of motion of the container changes. The water resists this change in its own state of motion and tends to "keep on doing what it is doing." If the container is moved from rest to a high speed at the starting line, the water remains at rest and spills onto the table. When the container stops near the finish line, the water keeps moving and spills over the container's leading edge. If the container is forced to move in a different direction to make it around a curve, the water keeps moving in the original direction and spills over its edge.

Balanced vs. Unbalanced Forces

When you study Newton's first law, you'll run into the phrase, "balanced and unbalanced forces" quite often. If forces are balanced, what does that really mean? To understand better, look at an object on a table. What are the forces acting on it? There is gravity pulling the object down and the table is pushing the object up. The object is not moving anywhere - its state of motion remains the same because the forces equal each other but point in opposite directions. If you could suddenly pull the table out from under the object, there would be no force to balance the force of gravity (forces become unbalanced) and the object would fall (move in the direction of the force of gravity).

Analysis:

1. If the forces acting on an object are balanced, must the object remain stationary? Explain your answer.
2. How can unbalanced forces acting on a car change the car's motion when the steering wheel is turned?

Review Science safetey rules here.

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