PHYSICS Newtons Laws Overview Review of Newtons Laws

PHYSICS Newton’s Laws Overview

Review of Newton’s Laws of Motion st 1 nd 2 rd 3 Objects in motion stay in motion* and objects at rest stay at rest if there is zero net force (balanced) ΣF = m·a (the forces will be unbalanced) Every force has an equal and opposite force

Inertia n Depends on mass ¡ More mass more resistance ¡ Less mass less resistance

Demo: NFL Hits

Equilibrium n Equilibrium: Net force is zero (ΣF = 0) ¡ ΣFx = 0 ¡ ΣFy = 0 FN FEngine FAir Fg

Equilibrium n n ΣF = 0 Newton’s First Law applies An object in equilibrium can be: ¡ in motion (straight line/constant speed) ¡ at rest FN Ff Fair Fengine Fg

Terminal Velocity n n Once the forces of air resistance and gravity become balanced equilibrium is reached No more acceleration

Newton’s Second Law n

Newton’s Second Law n Equations: ¡ ¡ ¡ F = ma a = F/m m=F/a F = net force m = Mass a= Acceleration

Use one of the equations you just wrote down…

Acceleration n

Weight vs Mass n Weight Force Fg Fg = m ·g Mass: Amount of matter (does not change) Weight: Pull of gravity (changes)

Weight Force (Fg) g = 9. 8 m/s 2 g = 1. 6 m/s 2 g = 26 m/s 2 m = 50 kg Fg = 490 N ( 110 lb) m = 50 kg Fg = 80 N ( 18 lb) m = 50 kg Fg = 1300 N ( 292 lb)

In-Class Problem #1 A 2000 kg car has a push force of 5000 N from its engine. If it experiences a friction force of 3000 N determine it’s (a) acceleration, (b) weight and (c) the normal force acting on it. a = 1 m/s 2 Fg = 19, 600 N FN = 19, 600 N

Review of Newton’s Laws of Motion ΣF = 0 ΣF ≠ 0 First Law Second Law a = 0 m/s 2 Accelerates at rest in motion* stays at rest stays in motion* depends on net force depends inversely on mass * Straight line/constant speed

Friction Force that resists motion due to imperfections in surfaces FRICTION MOTION

Two Types 1. Static (rest): Keeps object from moving 2. Kinetic (moving): Slows moving object

Friction Force Equation n μs (static) μk (kinetic)

Coefficient of Friction Table

In-Class Problem #2 A 30 kg desk is at rest on the floor. It takes 200 N of force to start it in motion. Determine the static coefficient of friction between the desk and the floor. μs = 0. 68

In-Class Problem #3 Once the desk in the previous problem is set in motion the 200 N force continues to be applied. Determine the acceleration of the desk if the coefficient of kinetic friction is 0. 52. a = 1. 57 m/s 2