Unit 2, Chapter 6

CPO ScienceFoundations of Physics

Chapter 9Chapter 9

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 Objectives1. Calculate the weight of an object using the

strength of gravity (g) and mass.2. Describe the difference between mass and weight.3. Describe at least three processes that cause

friction.4. Calculate the force of friction on an object when

given the coefficient of friction and normal force.5. Calculate the acceleration of an object including

the effect of friction.6. Draw a free-body diagram and solve one-

dimensional equilibrium force problems.7. 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

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

The mass always stays the same.

6.1 Weight

Fw = mg

Gravity (9.8 m/sec2)

Mass (kg)

Weight force (N)

6.1 Free fall and weightlessness An elevator is accelerating downward at 9.8

m/sec2. 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

Viscous friction

6.2 Friction

Ff = Fn

Normal force (N)

Coefficient of friction

Friction force (N)

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

Ff = sFn

Normal force (N)

Coefficient of sliding friction

Friction force (N)

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

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 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

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

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

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

6.3 Equilibrium and Hooke's Law

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 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

F = - k x

Spring constant N/m

Force (N)Deformation (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

The spring constant for a piece of solid wood is 1×108 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