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CORC 1331 MAIN PAGE

PHYSICS TOPICS

Speed
Velocity
Acceleration
Constant Acceleration
Free Fall
Forces and Newton's Law
Kinetic Energy
Potential Energy

LIST OF EXAMPLES

CLASSIC PROBLEMS

Free Fall
Forces
Conservation of Energy
Conservation of Momentum

PHYSICS LINKS

Thinkquest Physics Library
Physics Zone
Multimedia Physics Studio

Classic Problem in Force

Here is an example of a typical question in the area of forces that many students have difficulty solving.

Problem:
What is the weight of a 150 lb person on
a) the earth?
b) the moon?

Girl standing on the earth and moon

Solution:

a) Calculate the weight of the person on the earth.

First of all you should understand the concept of weight. Weight is the gravitational force of an object when it is on the earth. It's very important not to confuse weight with mass (as many students do). Mass indicates the quality of matter (it's not a gravitational force!). Weight is directly proportional to mass: the greater the mass the greater the force. The following formula is used to calculate the weight of an object:

m-the mass of an object
g- the gravitational acceleration

In order to solve part a) we first must convert the weight in pounds to kilograms.

Then write down the formula for weight and plug in the numbers.

CHECK YOURSELF! Think you know the answer? Enter it in the box below and press "Check!" to see if it's correct. Don't worry -- this is not a test, and if your answer is wrong, we'll tell you the solution!

Question: What is the weight of the person on the earth?

Type your answer here: N


b) Calculate the weight of the person on the moon.

The mass is going to be the same, but the gravitational acceleration is going to be different because every planet has a different constant acceleration. The gravitational acceleration of the moon, as is stated in your textbook, is 1.7 m/s2--in order to calculate the weight of any object on the moon we should use that number:

CHECK YOURSELF! Think you know the answer? Enter it in the box below and press "Check!" to see if it's correct. Don't worry -- this is not a test, and if your answer is wrong, we'll tell you the solution!

Question: What is the weight of the person on the moon?

Type your answer here: N


At this point, the problem has been solved. If we compare the results from part a) and part b) then we see that even though the mass of a 150 lb person on both the earth and the moon is the same, the person's weight changes due to the different gravitational accelerations on the two planets.