

Speed Velocity Acceleration Constant Acceleration Free Fall Forces and Newton's Law Kinetic Energy Potential Energy Free Fall Forces Conservation of Energy Conservation of Momentum PHYSICS LINKS Thinkquest Physics LibraryPhysics 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: 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: mthe mass of an object 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!
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/s^{2}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!
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. 