where F is the net force to be applied to an object to be displaced by a distance L.
Note: this formula holds if the applied force is constant throughout the displacement and if it is in the same direction of it.
Suppose you wish to move an apple from the ground to 1 meter off the ground. The force you need to apply is the one to win gravity. The gravitational force is given by:
where M is the mass of the object you want to
lift and "g" is the gravitational acceleration on the surface of the Earth - we can take
it to be 10 meters/sec/sec.
If M=0.1 Kg (mass of an apple), we get: F_gravity=0.1 Kg x 10 m/sec/sec = 1 Newton, where
Newton is the unit fo force when mass is in Kg and acceleration is in meter/sec/sec. Thus,
the force to lift an apple of mass=0.1 Kg by a meter is:
where Joule is the unit of work, when force is in Newton and displacement is in meters.
Unfortunately, there is more than one unit to express energy or work. (See Appendix on Units ! ). In this module, we will encounter the unit BTU which stands for British Thermal Unit . It is the energy to raise the temperature of 1 pound of water by one deg. F near the freezing point of water (32 deg. F). It is approximately the energy liberated in lighting a match. It corresponds to 1,055 Joule. For our purpose, we will take
For more information and exercises check the textbook (Chapter 3, pg. 63-65) and/or the glossary.
