Mass and Weight

Mass and Weight

Description

Two balls of different mass but similar size are dropped from a table top. They strike the floor at the same time. When the same balls are accelerated using air from a hair dryer, the less massive ball accelerates more rapidly than the more massive one.

Set

Mass is an intrinsic property of matter and is directly proportional to the quantity of the sample taken. For a given sample, neglecting relativity, the mass is constant in any location in the universe.
Weight is a measure of the force of attraction of one body for another and is a function of the amount of mass of the two bodies. Since the Earth is so massive, weight on the surface of the Earth results from the attraction between the ball and the center of the earth.
Weight is a force which varies with location in the universe.

Procedure

Place a ping pong ball and a golf ball relatively close to one another on a level table. Turn on the blow dryer so that the air stream is directed toward both balls. Observe the relative motion of the balls.
Drop both balls (by pushing them over the table edge using a small board) from an equal height and observe the time it takes for them to hit the floor.

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

DoChem 014 Mass and Weight

Answer these questions before watching the movies. The balls used in the movie are a golf ball and a ping pong ball.

Describe a difference between mass and weight.
Predict the effect of an equal force on each ball before using the blow dryer.
Predict the outcome of dropping the balls. Drop the balls.

Watch the movies.

Describe and explain the actual result.
Mass is usually determined with a balance in the laboratory. Describe another method of determining mass which is suggested by the first experiment.

Teachers Guide

Purpose

To demonstrate a distinction between mass and weight.

Materials

1 ping pong ball
1 golf ball
1 small blow dryer
Level table surface; small board

Lab Hints

Make sure that the surface upon which the balls roll is level.
In place of a blow dryer, a bicycle pump or the release of air from balloons may be substituted as the propelling force. The teacher may wish to apply a force on the balls by blowing on them.
Any two balls of the same size but different density may be used.

Time

Teacher preparation: 10 minutes

Presentation Time: 10 minutes

Hazards

A shock from the hair dryer is possible.

Precautions

Take normal precautions when using an electrical apparatus. Use a ground fault interrupter.

Disposal

Save all materials for future reuse.

Presentation

Ask the students to describe a difference between mass and weight.
Have the students predict the effect of an equal force on each ball before using the blow dryer.
Ask the students to predict the outcome of dropping the balls. Drop the balls.

Sample Data

The ping pong ball moves faster than the golf ball when pushed by the air blower. The ping pong ball and golf ball both hit the ground at the same time when dropped together.

Applications

A bathroom scale uses the force of gravity on an object (i.e., its weight) to compress a spring.
The centigram balance and most balances measure mass. The results would not change in different gravity fields.
Thompson's experiments leading to the mass of the electron, and the mass spectrometer, are examples of the determination of the mass of particles using electromagnetic forces and their resulting accelerations.

Background

Newton's 2nd Law, F = ma summarizes the effect of applying a force upon an object. F is the force applied, m is the mass of the object, and a is the acceleration.
In the first part of the demonstration, the blow dryer provides the force on the balls. An equal force is applied to both balls; therefore any difference in acceleration will be due to a difference in mass. The golf ball is more massive and accelerates more slowly than the ping pong ball. (This illustrates that mass may be measured without using gravity.)
Dropping the balls from equal heights is a reproduction of Galileo's experiment. Neglecting air friction, the two balls strike the floor at the same time. The accelerations are equal for the two dropped balls. Since the force on a ball is directly proportional to its mass, each ball experiences a different force; that is, a different weight. w = mg = GmM/R²; and g = GM/R² = acceleration due to gravity. In these equations: w = weight; g = acceleration due to gravity; G = universal gravitational constant; m = mass of object; M = mass of earth; and R = the distance between the centers of mass of the object and the earth, the radius of the earth for objects at the surface.

Makeup Ans.

See Set and Sample Data.

Key Words

mass
weight
acceleration
acceleration due to gravity
Newton's 2nd law
Newton's second law
gravity field
gravitational field
force