Or Trust in the Force Luke/Ani Forces Or Trust in the Force Luke/Ani Standards: 1b Students know that when forces are balanced no acceleration occurs; thus an object continues to move at a constant speed or stays at rest (Newton’s 1st Law).

Forces in General A Force is a push or pull, an interaction between two bodies Force is a vector quantity Adds vectorally (tip to tail) Can be decomposed into perpendicular components that are independent of one another Force is measured in Newtons (Kg m/s2) 1 Newton ~ ¼ pound Question: Define force. How many objects are involved in a force? Is force a vector or scalar quantity? What units are used to measure force?

Force Types Contact forces are forces that require physical contact between the two objects in the interaction. E.g. Springs, muscular pushes and pulls, friction, normal force, air resistance, ropes, chains Non-Contact or action at a distance forces do NOT require physical contact between two objects in interaction E.g. Gravity, Electric and Magnetic Forces, Strong and Weak Nuclear Forces Forces on object are those things touching it and gravity Questions: Distinguish between a contact and non-contact force. Give examples of each.

Common Forces Force Definition Direction Magnitude Equation Weight Gravitational force of planet on object Towards center of planet Fweight = mg where g is the gravitational field strength (-9.8 N/kg near Earth) Spring Force Force of spring on object In direction opposite spring’s displacement Fspring = -kx where k measures spring stiffness Normal Force Perpendicular force of contact between two surfaces Perpendicular to surfaces contact Never larger than net force in opposite direction Frictional Force Horizontal force of contact between two surfaces Parallel to surface contact in direction that opposes motion Ffriction =mFnormal where mu depends on stickiness of two surfaces Air Resistance Force between air molecules and body moving through air Opposite direction of body’s motion Fair resistance = Cv2 where C depends on shape and mass of object Common Forces Define the following forces, their direction and the magnitude equations they use.

Check Question What is the weight of 2 kg? 2 N 1 N 20 N 0.2 N

Net Force Sum of all forces in a linear direction (eg horizontal and vertical). If net force is NOT zero then object will accelerate in that direction If net force is equal to zero then object will move with a constant velocity in that direction (eg stay stationary or retain initial velocity) Define net force. What type of motion occurs when the net force is zero? What type of motion occurs when the net force is NOT zero? Are all objects with zero net force stationary?

Examples of Net Force Stationary box on floor Mover leaning on stationary box on floor Mover pushing box on floor at constant velocity Mover pushing box on floor at constant acceleration Object falling w/ air resistance

Ropes, Chains, Strings and Equilibrium Problems Ropes, strings, and chains exert a contact force on other objects called a tension. Equilibrium Tension Problem has an object suspended by strings, chains and ropes. Since no acceleration occurs, net force = 0. Forces in horizontal direction balance Forces in vertical direction balance As rope becomes more horizontal the tension in the rope must increase Questions: What are forces due to the pull of a rope, string or chain called? If an object is suspended by some arrangement of chains, strings and ropes what can be said of the horizontal and vertical net force? As any of the chains, ropes or strings are moved so they are more horizontal, what happens to the tension of that connector?