Newton’s Laws of MotionNewton’s Laws of Motion1. 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

ForcesForces

Non-Contact ---- Gravity (F = G m M /r2 ) G = 6.7x10-11 m3 / (kg s2)

Example: (two people)

F = (6.7x10-11 ) (60) (60)/ (.5)2 =

Weight of 100g =

31

Example Weight of Person Example Weight of Person Calculate the gravitational force on a 60 kg

person near the surface of the earth. Earth: Mass = 6x1024 kg, radius = 6.4 x106 m.

F = G M m / r2

= (6.7x10-11 m3 / (kg s2)) (6x1024 kg) (60 kg)/ (6.4x106 )2 m2

= kg m/s2 (Newtons)

Most common gravitational calculation involves object close to surface of the earth F = (G M / r2) * m F = * m =

33

Newton’s 2Newton’s 2ndnd Law Law Fnet = m a

What if Gravity is the only force?

Normal ForceNormal Force Perpendicular to Surface Repulsive Reactive Contact

Example: (person in elevator)Example: (person in elevator)

M = 60kg

•Constant velocity

Example: (person in elevator)Example: (person in elevator)

M = 60kg

•Accelerating up at 2 m/s2

Example: (person in elevator)Example: (person in elevator)

M = 60kg

•Accelerating down at 3m/s2

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

Example: Box in ElevatorExample: Box in Elevator

•10kg

a= 2 m/s2

SpringsSprings Similar to tension, normal Reactive Changes length according to tension F = -k Δx

10kg

k=50 N/m