ENGINEERING MECHANICS BASICS

7/30/2019 ENGINEERING MECHANICS BASICS

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ENGINEERING

MECHANICS


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Motion and Force inOne Dimension

Mass, Weight and Gravity

Friction

Equilibrium of Forces and Hookes Law

Forces and Equilibrium


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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.


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Vocabulary Terms mass weight

weightless

g-force

friction

static friction

sliding friction

rolling friction

viscous friction

air friction

normal force

extension

net force

free-bodydiagram

lubricant

equilibrium

ball bearing

dimension

spring

Hookes law

compression

spring constant

deformation

restoring force

coefficient offriction

engineering

design cycle

subscript

prototype

coefficient ofstatic friction


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Mass, Weight, and Gravity

Mass is a measure of

matter.

Mass is constant.

Weight is a force.

Weight is not constant.


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Mass, Weight, and Gravity

The weight of an object

depends on the strengthof gravity wherever the

object is.

The mass always stays

the same.


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Weight

Fw = mgGravity (9.8 m/sec2)

Mass (kg)

Weight force (N)


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Free fall and weightlessness An elevator is accelerating downward at 9.8 m/sec2.

The scale feels no forcebecause it is falling away from

your feet at the same rate you are falling. As a result, you are weightless.


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Calculate weight

How much would a person

who weighs 490 N (110 lbs)

on Earth weigh on Jupiter? The value ofg at the top of

Jupiters atmosphere is 23

N/kg.

(Since Jupiter may notactually have a surface, on

means at the top of the

atmosphere.)


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Calculate force

A 10-kilogram ball is supported atthe end of a rope. How much force

(tension) is in the rope?


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Mass, Weight, and GravityKey Question:

What is speed and how is it measured?

*Students read Section 6.1 BEFORE Investigation 6.1


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Friction

Friction results from relative motionbetween objects.

Frictional forces are forces that resist oroppose motion.


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Types of Friction

Static friction

Sliding friction

Rolling friction


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Types of Friction

Air friction

Viscous friction


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Friction

Ff = FnNormal force (N)

Coefficient of friction

Friction force (N)


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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.


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Sliding Friction

Ff = sFnNormal force (N)

Coefficient ofsliding friction

Friction force (N)


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Table of friction coefficients


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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?


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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 thecoefficient of rolling friction is 0.07.


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FrictionKey Question:

How can we describe and model friction?

*Students read Section 6.2 AFTER Investigation 6.2


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Equilibrium and Hooke's Law

When the net force

acting on an object is

zero, the forces on theobject are balanced.

We call this condition

equilibrium.


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Newtons 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.


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Many problems have more than one force applied to an object inmore than one place.


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Calculate net force

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

Calculate the acceleration of the box.


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Calculate force using equilibrium

Two chains are used to lift a small

boat. One of the chains has a forceof 600 newtons.

Find the force in the other chain if

the mass of the boat is 150

kilograms.


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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).


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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.


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Hooke's Law

F = - k x

Spring constant N/m

Force (N)Deformation (m)


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Calculate force

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

centimeter.

How much force does the spring exert?


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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.


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Calculate using equilibrium

The spring constant for a piece of solid wood is 1108

N/m.

Use Hookes law to calculate the deformation when a

force of 500 N (112 lbs) is applied.


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Equilibrium of Forces and Hooke's Law

Key Question:

How do you predict

the force on a

spring?


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Application: The design of structures

ENGINEERING MECHANICS BASICS

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