READ: Newton's First Law
READ: Newton's First Law
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.
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.
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.
What Is Inertia?
Inertia is the tendency of an object to resist a change in its motion. All objects have inertia, whether they are stationary or moving. Inertia explains Newton’s first law of motion, which 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. That’s why Newton’s first law of motion is sometimes called the law of inertia. The inertia of an object depends on its mass. Objects with greater mass also have greater inertia.
Q: You probably don’t realize it, but you experience inertia all the time, and you don’t have to ride a skateboard. For example, think about what happens when you are riding in a car that stops suddenly. Your body moves forward on the seat and strains against the seat belt. Why does this happen?
A: The brakes stop the car but not your body, so your body keeps moving forward because of inertia.
Inertia explains why it’s important to always wear a seat belt.
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 Figure 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.
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.
- Inertia is the tendency of an object to resist a change in its motion. Because of inertia, a resting object will remain at rest, and a moving object will keep moving.
- Objects with greater mass have greater inertia.
- To change the motion of an object, inertia must be overcome by an unbalanced force acting on the object.
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