Principles illustrated
This is a simple demonstration that can foster discussion of many physics topics.
Begin by standing on a desk, holding a book and an open piece of paper. Ask "which will hit the ground first if I drop them?" Typically, students will answer the book. Ask why, the answer you will probably get is because "the book is heavier." Then, crumple up the paper (usually to remarks such as "cheater") and drop them. Some follow up questions might be...
- Does crumpling the paper add mass to it?
- When the paper is not crumpled does its mass make it fall faster?
- What makes the paper fall more slowly when it is not crumpled? (Note here you are leading students thought toward air resistance)
- What force makes things fall toward the ground?
This demonstration can be followed up with discussions on Terminal Velocity to talk about balanced and unbalanced forces.
This can also be followed up with a discussion of projectile motion with the classic question:
"If a bullet is shot forward parallel to the ground without anything in its way, and at the exact same moment another bullet is dropped from the same height, which bullet will hit the ground first?" (Both bullets hit the ground at the same time since gravity is the only force pulling them down)
Projectile Motion Demonstration
- Set-up two pennies and a ruler on a table as in the picture below.
- Hit the ruler as shown, watch and listen to which penny falls first. (both should hit the ground very close to the same time)
Standards
8th Grade
2b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.
2d. Students know how to identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.
2g. Students know the role of gravity in forming and maintaining the shapes of planets, stars and the solar system.
Physics
1f. Students know applying a force to an object perpendicular to the direction of its motion causes the object to change direction but not speed (e.g., Earth's gravitational force causes a satellite in a circular orbit to change direction but not speed).
Questioning Script
Prior knowledge & experience:
- Feathers fall slower than heavier objects.
- Parachutists fall slower when the parachute is open.
- Heavy objects push with greater force on your hand.
- You can feel a force on your hand if you place it out the window of a moving car.
Root question:
- Which falls more quickly in free fall, a feather or a hammer? (See answer here.)
- Does mass change the acceleration of the object if gravity is the only force acting on it?
- If you shoot a bullet parallel to the ground and at the exact same moment, from the exact same height, which will hit the ground first? (Assume the bullet does not hit anything)
- What makes a feather fall slower on earth than a hammer?
Target response:
- Both objects fall at the same speed.
- Mass does not affect the speed of falling objects, assuming there is only gravity acting on it.
- Both bullets will strike the ground at the same time. The horizontal force applied does not affect the downward motion of the bullets -- only gravity and friction (air resistance), which is the same for both bullets.
- Air resistance makes a feather fall slower.
Common Misconceptions:
- Objects with a greater mass will fall faster (with a greater acceleration?)
- An objects forward motion will change the rate at which objects fall.
References & Links:
Free Fall vs Air Resistance:
http://www.glenbrook.k12.il.us/gbssci/phys/Class/newtlaws/u2l3e.html
Applet on Projectile Motion:
http://galileo.phys.virginia.edu/classes/109N/more_stuff/Applets/ProjectileMotion/jarapplet.html
http://www.walter-fendt.de/ph11e/projectile.htm