CPO Science Foundations of Physics Unit 2, Chapter 6

Chapter 9

Unit 2: Motion and Force in One Dimension Chapter 6: Forces and Equilibrium

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6.1 Mass, Weight and Gravity

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6.2 Friction

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6.3 Equilibrium of Forces and Hooke’s Law

Chapter 6 Objectives 1.

2.

3.

4.

5.

6.

7.

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.

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mass



weight Chapter 6 Vocabulary Terms

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normal force

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compression

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weightless

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extension

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net force

 

spring constant deformation

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g-force

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friction

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static friction

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sliding friction

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rolling friction

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viscous friction

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air friction

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free-body diagram

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lubricant

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equilibrium

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ball bearing

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dimension

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spring

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Hooke’s law

      

restoring force coefficient of friction engineering design cycle subscript prototype coefficient of static friction

6.1 Mass, Weight, and Gravity

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Mass is a measure of matter.

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Mass is constant.

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Weight is a force.

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Weight is not constant.

6.1 Mass, Weight, and Gravity

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The weight of an object depends on the strength of gravity wherever the object is.



The mass the same.

always stays

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

6.1 Free fall and weightlessness

 An elevator is accelerating downward 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

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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.

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(Since Jupiter may not actually have a surface, “on” means at the top of the atmosphere.)

6.1 Calculate force

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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

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Friction results from relative motion between objects.

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Frictional forces are forces that resist or oppose motion.

6.2 Types of Friction

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Static friction

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Sliding friction

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Rolling friction

6.2 Types of Friction

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Air friction

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Viscous friction

6.2 Friction Normal force (N) Friction force (N) F f =

m

F n Coefficient of friction

6.2 Calculate force of friction



A 10 N force pushes down on a box that weighs 100 N.



As the box is pushed horizontally, the coefficient of sliding friction is 0.25.



Determine the force of friction resisting the motion.

6.2 Sliding Friction Friction force (N) F f =

m

s F n Normal force (N) Coefficient of sliding friction

Table of friction coefficients

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?

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 and Hooke's Law

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When the net force acting on an object is zero, the forces on the object are balanced.

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We call this condition equilibrium .

6.3 Equilibrium and Hooke's Law 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 and Hooke's Law Many problems have more than one force applied to an object in more than one place.

6.3 Calculate net force

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Four people are pulling on the same 200 kg box with the forces shown.



Calculate the acceleration of the box.

6.3 Calculate force using equilibrium

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Two chains are used to lift a small boat. One of the chains has a force of 600 newtons.

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Find the force in the other chain if the mass of the boat is 150 kilograms.

6.3 Equilibrium and Hooke's Law

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The most common type of spring is a coil of metal or plastic that creates a force when it is extended (stretched) or compressed (squeezed).

6.3 Equilibrium and Hooke's Law

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The force from a spring has two important characteristics:

—

The force always acts in a direction that tries to return the spring to its unstretched shape.

—

The strength of the force is proportional to the amount of extension or compression in the spring.

6.3 Hooke's Law Force (N) F = - k x Deformation (m) Spring constant N/m

6.3 Calculate force



A spring with k = 250 N/m is extended by one centimeter.



How much force does the spring exert?

6.3 Equilibrium and Hooke's Law

 The restoring force from a wall is always exactly equal and opposite to the force you apply, because it is caused by the deformation resulting from the force you apply.

6.3 Calculate using equilibrium

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The spring constant for a piece of solid wood is 1 ×10 8 N/m.



Use Hooke’s law to calculate the deformation when a force of 500 N (112 lbs) is applied.

6.3 Equilibrium of Forces and Hooke's Law Key Question:

How do you predict the force on a spring?

*Students read Section 6.3 AFTER Investigation 6.3

Application: The design of structures