Teacher’s Lesson Plan

Gravity and Air Resistance

The teacher will have to adjust this material to the level of difficulty of the children.  Some of the concepts in this lesson might seem too easy for high school kids, while other concepts might be too difficult for elementary aged children to understand. The teacher should ask the students what they already know about what happens to things when they fall before explaining everything. 

When you drop an object, what does it do?  It falls.  Objects do not fall because they are supposed to; they fall because there are forces that act on an object as it falls.  A force is essentially a push or a pull, or more technically speaking a physical influence that changes the position, or acts in a way so that it can change the position of an object.  The weight of an object is one of these forces.  Weight is caused by gravity and the objects mass, in fact; it is the product of the two.

Weight = mass * gravity

When we step on a scale, we are finding out how much we weigh in pounds (lbs.).  What we are actually finding is the force that we exert on the ground when we are not moving.  In the metric scale, to find out the weight of an object, you would multiply the mass (in kg) buy the acceleration due to gravity (9.8 m/s²).  Your result is in the units of Newton’s. 

Gravity is what causes things to fall toward the center of the earth.  It also causes things to accelerate as they fall, this means that they fall slowly at first and then pick up speed the more they drop.  Without gravity, things would be floating in the air, they would not fall, and there would be nothing to keep them on the ground. 

The other primary force that acts on an object as it falls is called air resistance, or sometimes it is called drag.  When an object falls, the air exerts an upward force on the object.  Air resistance occurs because as things fall, they push through the molecules and atoms that make up air.  If air resistance did not exist, then gravity would cause everything to fall at the same rate.  So, if you dropped two items at the same time, no matter what they were, they would hit the ground at the same time also.  They would fall in a similar manner as items in a vacuum.   

As the speed of an object increases as it falls, the force of the air resistance increases also.  When the air resistance force equals the weight of the object, the object stops accelerating and falls at a constant speed called the terminal velocity. Light objects with large surface areas reach their terminal velocity very quickly.  This can be demonstrated by dropping one of the small feathers, or a piece of paper. 

Questions and Answers

• How do Parachutes work?
• Gravity causes the parachute to fall, however the air resistance under the chute is so great, because the air molecules get stuck, that the force of the air resistance counterbalances the weight of the object attached to the parachute.  Thus, the parachute and object that it is attached to fall at a steady speed. 

• Why does the little feather fall slower than the larger feather? Most students are surprised that the smaller one falls much slower than the larger one.
• The little feather is much lighter than the larger feather, and has a larger surface area in proportion to its weight.  Therefore, it will reach its terminal velocity very quickly.  The larger feather is much heavier than the smaller feather and has a smaller surface area in proportion to its weight.  Basically, air doesn’t get trapped under the larger feather as well as it does the smaller feather, thus the larger feather experiences less air resistance.