Chapter 4 Laws of Motion and Forces Goals: Newtons Laws, Inertia and mass, Mass vs Weight, Free Body diagrams, Fg, Fn, Fy, Fx, Ff, coefficients of friction

What causes a change in velocity? Unbalanced force Vector Acceleration Sum of forces and the direction Types of forces include contact forces and field forces Force diagram uses arrows to represent the vector where length represent the magmitude Free Body diagram arrows represent force directions and magnitudes x and y coordinates SR-1 pg 124

Newton’s First Law of Motion An object will remain at rest or in constant straight motion until an unbalanced force acts on the object Unbalanced force – net force Ʃ F Unbalanced forces cause an acceleration- change in direction or speed Change in Velocity Inertia – resistance to an acceleration Mass – measure of inertia and of matter PP – A pg 124

Newton’s Second Law A Force is directly related to mass and directly related to an acceleration F = m × a the unit for force F = m x a = kg m/s2 = N PP – B pg 128 PP – C pg 132

Newton’s Third Law Newton’s third law – for every action there is an equal and opposite reaction Forces occur in pairs Why? Inertia even field forces occur in pairs ex: accelerate towards Earth and Earth accelerates Tides

Weight Weight- force of attraction toward Earth depend upon mass and distance from Earth weight = mass x acc due to gravity = kg x 9.81 m/s2 = N

Normal Force Normal force = force on a surface perpendicular to the surface Fn 3 situations object laying flat Fn = Fg Fg = m g object on a sloping surface Fn = Fy Fy =Fg cos Ɵ Fx = Fg sin Ɵ object laying flat being pulled at an angle Fn = Fg – Fy because Fn and Fy have to equal Fg Fy = Fapplied sin Ɵ Fx = Fapp cos Ɵ

Friction Friction – force that opposes motion when surfaces are in contact Reduce friction by separating surfaces Coefficient depends on the surfaces and is a ratio of Ff and the Fn µ = Ff/Fn Fsf > Fkf µs = Fsf/Fn µ = Fkf/Fn PP – D pg 139 PP - E #1 pg 141

Think about Forces…………………… Contact and field forces Normal force Component forces Fx and Fy Forces of friction Fs and Fk There are forces everywhere! all over the Universe/Multiverse Forces are FUN and a lot of work