Unit 2: Motion and Force in One Dimension  6.1 Mass, Weight and Gravity  6.2 Friction  6.3 Equilibrium of Forces and Hooke’s Law Chapter 6: Forces and Equilibrium

Chapter 6 Objectives  Calculate the weight of an object using the strength of gravity (g) and mass.  Describe the difference between mass and weight.  Describe at least three processes that cause friction.  Calculate the force of friction on an object when given the coefficient of friction and normal force.  Calculate the acceleration of an object including the effect of friction.  Draw a free-body diagram and solve one-dimensional equilibrium force problems.  Calculate the force or deformation of a spring when given the spring constant and either of the other two variables.

Chapter 6 Vocabulary Terms  mass  weight  weightless  g-force  friction  static friction  sliding friction  rolling friction  viscous friction  air friction  normal force  extension  net force  free-body diagram  lubricant  equilibrium  ball bearing  dimension  spring  Hooke’s law  compression  spring constant  deformation  restoring force  coefficient of friction  engineering  design cycle  subscript  prototype  coefficient of static friction

Forces/Equilibrium Do Now:  How does this setup work?

6.1 Mass, Weight, and Gravity  Mass is a measure of matter.  Mass is constant.  Weight is a force.  Weight is not constant.

6.1 Mass, Weight, and Gravity  The weight depends on the strength of gravity.  The mass always stays the same. Vs.

6.1 Weight F w = mg Gravity (9.8 m/sec 2 ) Mass (kg) Weight force (N)

6.1 Free fall and weightlessness  If an elevator is accelerating down at 9.8 m/sec 2.  The scale feels no force because it is falling away from your feet at the same rate you are falling.  As a result, you are weightless.

6.1 Calculate weight  How much would a person who weighs 490 N (110 lbs) on Earth weigh on Jupiter?  The value of g at the top of Jupiter’s atmosphere is 23 N/kg.  (Since Jupiter may not actually have a surface, “on” means at the top of the atmosphere.)

6.1 Calculate force  A 10-kilogram ball is supported at the end of a rope. How much force (tension) is in the rope?

6.1 Mass, Weight, and Gravity Key Question: What is speed and how is it measured? *Students read Section 6.1 BEFORE Investigation 6.1

6.2 Friction  Friction results from relative motion between objects.  Frictional forces are forces that resist or oppose motion.

6.2 Types of Friction  Static friction  Sliding friction  Rolling friction

6.2 Types of Friction  Air friction (drag)  Viscous friction

6.2 Friction F f =  F n Normal force (N) Coefficient of friction Friction force (N)

How strong can friction be?

6.2 Calculate force of friction  A 10 N force pushes down on a box that weighs 100 N.  The coefficient of sliding friction is  Determine the force needed to move it.

6.2 Sliding Friction F f =  s F n Normal force (N) Coefficient of sliding friction Friction force (N)

Table of friction coefficients

Why is ice slippery?

6.2 Calculate using friction  A steel pot with a weight of 50 N sits on a steel countertop.  How much force does it take to start the pot sliding? (coefficient of static friction is 0.75)

6.2 Calculate using friction  A steel pot weight of 50 N on steel countertop.  Once you start it moving, what would its acceleration be if you exert the same force. (coefficient of static friction is ~0.75) (coefficient of kinetic friction is ~0.60)

6.2 Calculate using friction  The engine applies a forward force of 1,000 newtons to a 500-kg car.  Find the acceleration of the car if the coefficient of rolling friction is 0.07.

6.2 Friction Key Question: How can we describe and model friction? *Students read Section 6.2 AFTER Investigation 6.2

6.3 Equilibrium  When the net force acting on an object is zero, the forces on the object are balanced.  We call this condition equilibrium.

6.3 Equilibrium Newton’s second law simply requires that for an object to be in equilibrium, the net force, or the sum of the forces, has to be zero.

6.3 Equilibrium Many problems have more than one force applied to an object in more than one place.

6.3 Calculate net force  Calculate the acceleration of the 200. kg box

6.3 Calculate force using equilibrium  Find the force in the other chain given the picture to the left

Application: The design of structures