1. Dynamics: Cause of Motion

2. Experience & Experiment
Experience shows that:
a greater force for a given mass causes greater acceleration
a greater mass for a gives force gives lesser acceleration
Experiment shows that:
acceleration is directly proportional to force
acceleration is inversely proportional to mass

3. Newton’s 2nd Law, ΣF=ma
The sum of all forces on an object equals mass times acceleration
An object accelerates at a rate of 1.5m/s2 under a sum force of 53N. What is its mass?
How much upward force does the ground apply to someone with a mass of 75kg to counter balance the pull of gravity? Note that g = -9.8m/s2 and that Fnet = ma and that a = g. That is, W = mg

4. Newton’s Third Law
If object A exerts a force FAB on an object B, then object B exerts a force FBA on object A such that FAB = -FBA.
If an object is not accelerating, then FAB = -FBA and the sum of forces (vectors) equals zero.
An object can accelerate only if the vector sum of forces acting on it does not equal zero.
If Newton’s third law is correct, how is it possible to for anyone to win a tug-of-war?