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# 4.10: Force

Difficulty Level: At Grade Created by: CK-12
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Carson has been riding a scooter for almost as long as he can remember. As you can see, he’s really good at it. He can even do tricks in the air. It takes a lot of practice to be able to control a scooter like this. Carson automatically applies just the right forces to control his scooter.

### Defining Force

Force is defined as a push or pull acting on an object. There are several fundamental forces in the universe, including the force of gravity, electromagnetic force, and weak and strong nuclear forces. When it comes to the motion of everyday objects, however, the forces of interest include mainly gravity, friction, and applied force. Applied force is force that a person or thing applies to an object.

Q: What forces act on Carson’s scooter?

A: Gravity, friction, and applied forces all act on Carson’s scooter. Gravity keeps pulling both Carson and the scooter toward the ground. Friction between the wheels of the scooter and the ground prevent the scooter from sliding but also slow it down. In addition, Carson applies forces to his scooter to control its speed and direction.

### Force and Motion

Forces cause all motions. Everytime the motion of an object changes, it’s because a force has been applied to it. Force can cause a stationary object to start moving or a moving object to change its speed or direction or both. A change in the speed or direction of an object is called acceleration. Look at Carson’s brother Colton in the Figure below. He’s getting his scooter started by pushing off with his foot. The force he applies to the ground with his foot starts the scooter moving in the opposite direction. The harder he pushes against the ground, the faster the scooter will go.

How much an object accelerates when a force is applied to it depends not only on the strength of the force but also on the object’s mass. For example, a heavier scooter would be harder to accelerate. Colton would have to push with more force to start it moving and move it faster. You can explore the how force, mass, and acceleration are related by doing the activity at this URL:

Q: What units do you think are used to measure force?

A: The SI unit for force is the Newton (N). A Newton is the force needed to cause a mass of 1 kilogram to accelerate at 1 m/s2, so a Newton equals 1 kg ∙ m/s2. The Newton was named for the scientist Sir Isaac Newton, who is famous for his laws of motion and gravity.

### Force as a Vector

Force is a vector, or a measure that has both size and direction. For example, Colton pushes on the ground in the opposite direction that the scooter moves, so that’s the direction of the force he is applies. He can give the scooter a strong push or a weak push. That’s the size of the force. Like other vectors, a force can be represented with an arrow. You can see some examples in the Figure below. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force.

Q: How could you use arrows to represent the forces that start Colton’s scooter moving?

A: Colton pushes against the ground behind him (to the right in the Figure above). The ground pushes back with equal force to the left, causing the scooter to move in that direction. Force arrows A and B in example 2 above could represent these forces.

### Summary

• Force is defined as a push or pull acting on an object. Forces include gravity, friction, and applied force.
• Force causes changes in the speed or direction of motion. These changes are called acceleration.
• The SI unit for force is the Newton (N).
• Force is a vector because it has both size and direction. Like other vectors, it can be represented by an arrow.

### Vocabulary

• force: Push or pull acting on an object.
• Newton (N): SI unit of force, equal to the amount of force that causes a mass of 1 kilogram to accelerate at 1 m/s2 (kg • m/s2).

### Practice

Apply different types of forces to materials in the interactive animated lab at the following URL. Then answer the questions below.

1. What is compression, and how does it affect materials?
2. What is tension? What is a real-life example?
3. Describe shear as a force.
4. What is torsion?

### Review

1. What is force?
2. Relate force and motion.
3. What forces control the motion of everyday objects?
4. Identify and define the SI unit for force.
5. Draw a diagram to represent a foot kicking a resting soccer ball. Use arrows to represent the force applied to the ball and to show how the ball moves after it is kicked. To see an animation of this force and motion, go to the URL below.

### Vocabulary Language: English

force

force

Push or pull acting on an object.
Newton

Newton

(Abbreviation: N) SI unit of force, equal to the amount of force that causes a mass of 1 kilogram to accelerate at 1 m/s2 (kg • m/s2).

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7 , 8
Date Created:
Nov 01, 2012