4.25. Mass vs Weight http://www.ck12.org
its weight. For example, if mass doubles, weight doubles as well. You can learn more about weight, mass, and
acceleration at this URL:
http://www.nasa.gov/mov/192448main_018_force_equals_mass_time.mov
Calculating Weight
All objects on Earth accelerate downward due to gravity at a rate of 9.8 m/s^2. Therefore, if you know the mass of an
object, you can calculate its weight using this equation:
F = m×9.8 m/s^2
Q: Sam has a mass of 50 kilograms. What is his weight in Newtons?
A: You can calculate Sam’s weight in Newtons by substituting his mass in kilograms into the weight formula:
F = m×9.8 m/s^2 = 50 kg×9.8 m/s^2 = 490 kg *bull; m/s^2 , or 490 N
You’re probably more familiar with weight in pounds (lb) than in Newtons. One Newton equals 0.225 pounds. In
other words, there are 0.225 pounds per Newton. You can use this ratio to convert Newtons to pounds.
Q: What is Sam’s weight in pounds?
A: In pounds, Sam’s weight is 490 N x 0.225 lb/N = 110 lb.
Summary
- Weight is a measure of the force of gravity pulling down on an object. It depends on the object’s mass and the
acceleration due to gravity, which is 9.8 m/s^2 on Earth. - The formula for calculating weight is F = m×9.8 m/s^2 , where F is the object’s weight in Newtons (N) and m
is the object’s mass in kilograms. The Newton is the SI unit for weight, and 1 Newton equals 0.225 pounds.
Vocabulary
- weight: Measure of the force of gravity pulling on an object.
Explore More
At the following URL, solve the first four problems, and then check your answers. If any of your answers are
incorrect, listen to the audio guided solutions.
http://www.physicsclassroom.com/calcpad/newtlaws/problems.cfm
Review
- What is weight? What is the SI unit for weight?
- Explain how to calculate an object’s weight on Earth from its mass.
- Sam’s older sister Nina, who is pictured in theFigure4.49, has a mass of 52 kilograms. How much does Nina
weigh in Newtons? How much does she weigh in pounds? - The moon has less gravity than Earth. If you were on the moon, how would your weight be different than it is
on Earth?