Machines – pulleys
http://www.thephysicsaviary.com/Physics/Programs/Labs/PulleyLab/index.html
Forces and friction
http://physics.bu.edu/~duffy/HTML5/force_motion_1D_friction.html
Investigate F = ma with/without friction
http://www.physicsclassroom.com/Physics-Interactives/Newtons-Laws/Force/Force-Exercise-1
Change masses, friction, measure time
http://www.physicsclassroom.com/Physics-Interactives/Newtons-Laws/Atwoods-Machine
Forces, Newton’s Laws, friction
https://phet.colorado.edu/en/simulation/forces-and-motion-basics
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