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4.21: Newton's First Law

Difficulty Level: Basic Created by: CK-12
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There’s no doubt from Corey’s face that he loves skateboarding! Corey and his friends visit Newton’s Skate Park every chance they get. They may not know it, but while they’re having fun on their skateboards, they’re actually applying science concepts such as forces and motion.

Starting and Stopping

Did you ever ride a skateboard? Even if you didn’t, you probably know that to start a skateboard rolling over a level surface, you need to push off with one foot against the ground. That’s what Corey’s friend Nina is doing in this picture below .

Do you know how to stop a skateboard once it starts rolling? Look how Nina's friend Laura does it in the Figure above . She steps down on the back of the skateboard so it scrapes on the pavement. This creates friction, which stops the skateboard.

Even if Laura didn’t try to stop the skateboard, it would stop sooner or later. That’s because there’s also friction between the wheels and the pavement. Friction is a force that counters all kinds of motion. It occurs whenever two surfaces come into contact.

Video Break

Laura learned how to use forces to start and stop her skateboard by watching the videos below. Watch the video to see how the forces are applied. You can pick up some skateboarding tips at the same time!

Starting: http://www.youtube.com/watch?v=OpZIVjbMAOU&feature=relmfu

Stopping: http://www.youtube.com/watch?v=6fuOwhx91zM&feature=relmfu

Laws of the Park: Newton’s First Law

If you understand how a skateboard starts and stops, then you already know something about Newton’s first law of motion . This law was developed by English scientist Isaac Newton around 1700. Newton was one of the greatest scientists of all time. He developed three laws of motion and the law of gravity, among many other contributions.

Newton’s first law of motion states that an object at rest will remain at rest and an object in motion will stay in motion unless it is acted on by an unbalanced force. Without an unbalanced force, a moving object will not only keep moving, but its speed and direction will also remain the same. Newton’s first law of motion is often called the law of inertia because inertia is the tendency of an object to resist a change in its motion. If an object is already at rest, inertia will keep it at rest. If an object is already in motion, inertia will keep it moving.

Do You Get It?

Q : How does Nina use Newton’s first law to start her skateboard rolling?

A : The skateboard won’t move unless Nina pushes off from the pavement with one foot. The force she applies when she pushes off is stronger than the force of friction that opposes the skateboard’s motion. As a result, the force on the skateboard is unbalanced, and the skateboard moves forward.

Q : How does Nina use Newton’s first law to stop her skateboard?

A : Once the skateboard starts moving, it would keep moving at the same speed and in the same direction if not for another unbalanced force. That force is friction between the skateboard and the pavement. The force of friction is unbalanced because Nina is no longer pushing with her foot to keep the skateboard moving. That’s why the skateboard stops.

Changing Direction

Corey’s friend Jerod likes to skate on the flat banks at Newton’s Skate Park. That’s Jerod in the picture above . As he reaches the top of a bank, he turns his skateboard to go back down. To change direction, he presses down with his heels on one edge of the skateboard. This causes the skateboard to turn in the opposite direction.

Video Break

Can you turn a skateboard like Jerod? To see how to apply forces to change the direction of a skateboard, watch this video:

http://www.youtube.com/watch?v=iOnlcEk50CM

Do You Get It?

Q : How does Jerod use Newton’s first law of motion to change the direction of his skateboard?

A : Pressing down on just one side of a skateboard creates an unbalanced force. The unbalanced force causes the skateboard to turn toward the other side. In the picture, Jerod is pressing down with his heels, so the skateboard turns toward his toes.

Summary

  • Newton’s first law of motion states that an object at rest will remain at rest and an object in motion will remain in motion unless it is acted on by an unbalanced force.
  • Using unbalanced forces to control the motion of a skateboard demonstrates Newton’s first law of motion.

Vocabulary

  • Newton’s first law of motion : Law stating that an object’s motion will not change unless an unbalanced force acts on the object.

Practice

Do you think you understand Newton’s first law? Go to the URL below to find out. Review Newton’s law and watch what happens to the skateboarder in the animation. Then answer the questions at the bottom of the Web page.

http://teachertech.rice.edu/Participants/louviere/Newton/law1.html

Review

  1. State Newton’s first law of motion.
  2. You don’t need to push off with a foot against the ground to start a skateboard rolling down a bank. Does this violate Newton’s first law of motion? Why or why not?
  3. Nina ran into a rough patch of pavement, but she thought she could ride right over it. Instead, the skateboard stopped suddenly and Nina ended up on the ground (See Figure above . Explain what happened.
  4. Now that you know about Newton’s first law of motion, how might you use it to ride a skateboard more safely?

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Difficulty Level:

Basic

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Aug 15, 2014
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