# Practice Applying Newton's First Law

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• 0:04 What Is Newton's First Law?
• 1:41 Sample Problems
• 4:05 Lesson Summary

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Lesson Transcript
Instructor: Michael Blosser

Michael has a Masters in Physics and a Masters in International Development. He has over 5 years of teaching experience, teaching Physics, Math, and English classes.

This lesson introduces the reader to Newton's First Law, gives real world examples, and provides problems to show how we can use it to solve for unknown forces.

## What Is Newton's First Law?

Why is it that stopping suddenly in a car can cause your coffee to spill? Why does the Voyager 1 spacecraft continue to fly farther and farther away from our solar system even though its initial propulsion mechanism has long been exhausted? The answer is Newton's First Law.

Isaac Newton created his revolutionary three laws of motion in the 17th century. Newton's First Law states, 'A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force.' This law is called the law of inertia; that is, every object will remain in their current inertial state (either at rest or in motion) until an external force acts upon that object to change it.

There are numerous real world examples that prove Newton's First Law. The car seat belt is a common one. Long known as an essential safety device in the automobile, the seat belt works by following Newton's First Law. Once a car is in motion at a certain velocity, the car and its passenger are also traveling at that velocity and are likely to stay in motion unless an external force acts upon the car.

If the car were to suddenly brake, the force of the brakes would cause it to rapidly stop. However, the passenger in the car would also be traveling at the car's velocity, and without any external force to stop him or her, the passenger would fly at the same velocity right into the car's windshield. The seat belt brings the passenger from a state of motion to a state of rest in the safest way possible.

## Sample Problems

Let's try solving a sample problem.

• A 10 kg object is sliding across frictionless ice with a constant velocity of 10 m/s. Which of Newton's Force Laws do we use? How much force do we need to apply to this object to keep it in motion?

In this equation an object is in motion, so we immediately know that we will use Newton's First Law. The object is moving in a constant velocity, so we know that it is not accelerating. We know from Newton's Second Law that force = mass * acceleration, so with no acceleration we can determine that there is no net force acting on the object. Finally, using Newton's First Law, we know that objects at rest or in motion tend to stay in motion unless an external force acts upon it. This object is in motion and no external force is acting upon it, therefore we do not need to apply any force to keep the object in motion.

Now let's try another problem.

• What is the tension force of a string holding a 5kg object vertically at rest? Which of Newton's Force Laws do we use?

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