READ: Motion and Force


The wings of this hummingbird are moving so fast that they’re just a blur of motion. You can probably think of many other examples of things in motion. 

In science, motion is defined as a change in position. An object’s position is its location. Besides the wings of the hummingbird in the opening image, you can see other examples of motion in the Figure below. In each case, the position of something is changing.

Examples of moving objects

Force


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.  

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 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.

Scooter rider pushing off the ground

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. 

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.

Summary


  • Motion is defined as a change of position.
  • 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).

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Last modified: Wednesday, 21 December 2016, 12:30 PM