Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

PHYS1171 - Forces and MotionLecture 6 - Tension

Dr. Tim McIntyre

The University of Queensland

Semester 1, 2013

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Review - force diagrams

Draw separate force diagrams for the two blocks shown in eachdiagram when touching. Identify any action-reaction pairs thatexist in your diagrams.

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Lecture 6 - Tension

Today’s lecture covers

Tension in a string

Objects connected by strings in staticsituations

Preparation

Read Five Minute Physics -Lecture 6 - Tension

Reference

Biological Physics (2010), Section 2.3 (review)

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Reading Quiz (assessed)

Which of the following is false?

1 Tension is a ”stretching” force on a cable

2 The tension force in a cable only acts alongthe direction of the cable

3 The tension force is transmitted unchangedalong the length of the cable

4 A pulley changes the direction and magnitudeof the force in the cable

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Tension

A force applied to a string results in a tension in the string

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Tension

For a massless (very light) string, the tension is constant alongthe string

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Pulley

A pulley changes the direction of the tension, but not itsmagnitude

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Question...

You tie a rope to a tree and pull on the end with a force of 100N. The tension in the rope is

1 0 N

2 50 N

3 100 N

4 200 N

5 400 N

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Question...

Two tug of war teams each pull with a force of 100 N on eachend of a rope. The tension in the rope is

1 0 N

2 50 N

3 100 N

4 200 N

5 400 N

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Question...

Two people attempt to test the breaking strength of a steelcable. The best approach to break it would be to . . .

1 Have one person pull at each end

2 Tie the rope to a tree and both pullfrom the same end

3 Either will do - both of the aboveare equivalent

4 Bite through it with their teeth

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Example - Russell Traction System

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Example - Russell Traction System

A Russell traction system as shown in the accompanyingdiagram is used to keep an injured leg under tension. If themass of the leg to be supported is 3.0 kg, what weight shouldbe used so that only a horizontal force is applied along the leg?How big is this force?

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Example - Russell Traction System

A Russell traction system as shown in the accompanying diagram is used to keep an injured leg under

tension. If the mass of the leg to be supported is 3.0 kg, what weight should be used so that only a

horizontal force is applied along the leg? How big is this force?

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Example - Russell Traction System

A Russell traction system as shown in the accompanying diagram is used to keep an injured leg under

tension. If the mass of the leg to be supported is 3.0 kg, what weight should be used so that only a

horizontal force is applied along the leg? How big is this force?

PHYS1171 Lecture 6

Lecture 6

PHYS1171

Introduction

Reading

Theory

Questions

Example

Summary

Summing Up

After this lecture you should . . .

Understand the concept of tension in a string

Be capable of solving problems involving static objectsconnected by strings

Supplementary Questions Biological Physics (2010)

None

Next lecture

Read Five Minute Physics -Lecture 7 - Introduction to Kinematics

PHYS1171 Lecture 6

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1#Tension#is#a#”stretching”#force#on#a#cable#

2#The#tension#force#in#a#cable#only#acts#along#the#direc?on#of#the#cable#

3#The#tension#force#is#transmiAed#unchanged#along#the#length#of#the#cable#

4#A#pulley#changes#the#direc?on#and#magnitude#of#the#force#in#the#cable#

Which#of#the#following#is#false?#!!

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1#0N# 2#50N## 3#100N## 4#200N## 5#400N#

You#?e#a#rope#to#a#tree#and#pull#on#the#end#with#a#force#of#100#N.#The#tension#in#the#rope#is!!

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1#0N# 2#50N## 3#100N## 4#200N## 5#400N#

Two#tug#of#war#teams#each#pull#with#a#force#of#100#N#on#each#end#of#a#rope.#The#tension#in#the#rope#is!!

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1#Have#one#person#pull#at#each#end# 2#Tie#the#rope#to#a#tree#and#both#pull#from#the#same#end#

3#Either#will#do#P#both#of#the#above#are#equivalent#

4#Bite#through#it#with#their#teeth#

Two#people#aAempt#to#test#the#breaking#strength#of#a#steel#cable.#The#best#approach#to#break#it#would#be#to!!

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