(Lucier,V, Madeira,C, & McNamara,T, 2009 Nelson Education Limited)
http://www.physicsclassroom.com/class/newtlaws/Lesson-2/Types-of-Forces
http://study.com/academy/lesson/gravitational-force-definition-equation-examples.html
http://www.livescience.com/49254-weak-force.html
http://www.computerhope.com/jargon/s/screw.htm
http://quatr.us/physics/machines/wedge.htm
http://www.mikids.com/Smachines.htm
http://study.com/academy/lesson/gravitational-force-definition-equation-examples.html
http://www.livescience.com/49254-weak-force.html
http://www.computerhope.com/jargon/s/screw.htm
http://quatr.us/physics/machines/wedge.htm
http://www.mikids.com/Smachines.htm