Newton’s Laws of Motion 1. If the sum of all external forces on an object is zero, then its speed and direction will not change. Inertia 2. If a nonzero net force is applied to an object its motion will change F= ma 3. In an interaction between two objects, the forces that each exerts on the other are equal in magnitude and opposite in direction. 30

Forces l Non-Contact ---- Gravity (F = G m M /r 2 ) è G = 6.7x m 3 / (kg s 2 ) l Example: (two people) è F = (6.7x ) (60) (60)/ (.5) 2 = è Weight of 100g = 31

Example Weight of Person l Calculate the gravitational force on a 60 kg person near the surface of the earth. l Earth: Mass = 6x10 24 kg, radius = 6.4 x10 6 m. F = G M m / r 2 = (6.7x m 3 / (kg s 2 )) (6x10 24 kg) (60 kg)/ (6.4x10 6 ) 2 m 2 = kg m/s 2 (Newtons) l Most common gravitational calculation involves object close to surface of the earth è F = (G M / r 2 ) * m è F = * m = 33

Newton’s 2 nd Law l F net = m a What if Gravity is the only force?

Normal Force l Perpendicular to Surface l Repulsive l Reactive l Contact

Example: (person in elevator) M = 60kg Constant velocity

Example: (person in elevator) M = 60kg Accelerating up at 2 m/s 2

Example: (person in elevator) M = 60kg Accelerating down at 3m/s 2

Tension l Similar to normal force (Reactive) l Can only pull (can’t push on rope) l Constant throughout rope

Example: Box in Elevator 10kg a= 2 m/s 2

Springs l Similar to tension, normal l Reactive l Changes length according to tension l F = -k Δx 10kg k=50 N/m