Force (Chapter 3) Sep 29 Write everything in the yellow font

 Acceleration is the change in velocity per unit time Acceleration = v final - v initial /time  Any change in speed and/or direction causes an acceleration  Example Units: m/s/s (m/s 2 ), km/h/s, mph/s

1. A car accelerates from rest to 35mi/h in 18s. What is its acceleration? 2. A car accelerates from 22m/s to 35m/s in 12s. What is its acceleration? 3. A car slows down from 60km/h to 35km/h in 11s. What is its acceleration?

 Objects dropped will free fall to earth.  acceleration due to gravity is 9.8 m/s 2  For every second that passes an object will gain 9.8 m/s in speed.  All objects fall at the same acceleration when air resistance is minimal  See fig 3.4, p. 36

 As we have discussed, all object must be acted upon to change their state. When a force is applied, the object experiences acceleration.  This means that the net force is not zero during accelerations!!  Acceleration is directly proportional to net force, and accelerations are always in the direction of the net force.  See fig 3.5, p. 37

 Mass is a measure of inertia. The greater the mass of an object the greater its inertia. More force is required for any given acceleration.  Mass ≠ weight  weight is dependent upon gravity, mass is not  Acceleration is inversely proportional to mass.  fig. 3.11, p. 40

 Mass and weight are not the same thing!  Mass is the amount of matter, weight is how much gravity pulls on that mass, so it is a force weight = mass(kg) x 9.8 m/s 2 mass must be in kg

 Force equals mass times acceleration.  Units: Force = Newtons (kg∙m/s 2 ) Mass = kilograms Acceleration = m/s 2 So, what is weight actually a measure of?  Force triangle

4. Consider a 1000 kg car pulled by a cable with 2000 N of force. What will be the acceleration of the car? 5. What force is needed to accelerate a 85kg bowling ball 13m/s/s? 6. A 25N force is applied to a toy that then accelerates 5m/s/s. What is the mass of the toy? 7. What is the weight of the toy?

 When two objects are in contact with each other friction is present. Friction is always in the opposite direction of motion.  If the object is not accelerating, the force causing the motion and the force of friction are equal.  When objects fall through the air we call the friction air drag.

 Friction is dependent upon the types of materials and the incline of the surface.  When you pull or push an object across the floor with a constant force of 75 N the friction force is also 75 N, but in the opposing direction. (∑F = 0)

 When a free falling object’s air drag and the weight force of the object are equal then acceleration stops and the maximum speed has been reached. (All forces cancel!!)  Lighter, less uniform objects reach terminal velocity sooner than heavier, more uniform objects. Feathers vs. rocks, golf ball vs. Styrofoam ball, etc.  See fig 3.18, p. 47