LESSON: Introduction to Forces
LESSON: Introduction to Forces
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. Every time 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 to the left. 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:
http://www.harcourtschool.com/activity/newton/
Q: What units 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: Can you think of a situation that might be represented by the forces in example 1?
A: One possibility is a toddler pulling on a wagon, being "helped" by an adult pulling on the same wagon. The toddler's pull would be force A, and the adult's pull would be force B. Both would contribute to the acceleration of the wagon.
Q: Can you think of a situation that might be represented by the forces in example 2?
A: One possibility is two sumo wrestlers pushing against each other at the start of a match. They're pushing against each other with equal-sized forces, so it's almost like the forces "cancel out" and they will not move.
Watch this video on 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).
Review (questions you should be able to answer)
- What is force?
- Relate force and motion.
- What forces control the motion of everyday objects?
- Identify and define the SI unit for force.
After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete 