General Physics 1 (Phys 110 : Mechanics)

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General Physics 1

(Phys 110 : Mechanics)

CHAPTER 6 Force and Motion - II

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Phys 110

Friction: a force parallel to the surface and opposes the direction of motion.

Static Frictional force:

body is still stationary.

has changeable magnitude = magnitude of the net force component along same axis.

magnitude of its maximum value is:

Kinetic Frictional force:

body is moving.

has a constant magnitude = where

Chapter 6 : Force and Motion 2

Revision :

Lesson 2 of 2 Slide 1

𝒇_{𝒔,𝒎𝒂𝒙} = 𝝁_𝒔𝑭_𝑵

𝒇_𝒌 = 𝝁_𝒌𝑭_𝑵 𝒇_𝒌 < 𝒇_{𝒔,𝒎𝒂𝒙} coefficient of static/kinetic friction

Normal force

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Phys 110

1. To explain the centripetal force in uniform circular motion and its direction.

2. To calculate the centripetal force.

3. To identify the nature of the centripetal force in different uniform circular motions.

Objectives of this lesson :

Lesson 2 of 2 Slide 2

Force and Motion:

Centripetal Force

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When a body moves in a circle at constant speed v, it is said to be in uniform circular motion.

The body has a centripetal acceleration of constant magnitude given by

The direction of the centripetal acceleration is towards the center of the circle.

Phys

110 Chapter 6 : Force and Motion 2 Lesson 2 of 2 Slide 3

6-5 : Uniform circular motion :

a = v²

R : R is the radius of the circle

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Phys

According to Newton’s second law; The cause of the centripetal acceleration is a force called The Centripetal Force

The magnitude of centripetal force is given by :

The centripetal force is directed toward the center of the circle Centripetal Force

F = ma = m v

²

r

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Phys

F^g

T

As a car makes a turn, the force of friction acting upon the turned

wheels of the car provides centripetal

force required for circular motion

As a bucket of water is tied to a string and spun in a circle,

the tension force acting upon the bucket provides the centripetal force

required for circular motion As the moon orbits the Earth,

the force of gravity acting upon the moon

provides the centripetal force required for circular motion Centripetal Force

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Phys 110

Centripetal force is not a new type of force , but rather a description of the direction of the net force on an object that moves in a circular motion.

Any force -from those we studied- can be a centripetal force.

A centripetal force accelerates a body by changing the direction of the body’s velocity without changing the body's speed.

Chapter 6 : Force and Motion 2 Lesson 2 of 2 Slide 6

Centripetal Force

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Phys 110

The magnitude of centripetal force is

Because the speed v here is constant, the magnitudes of the acceleration and the force are also constant.

The directions of the centripetal acceleration and force vary

continuously so they always point toward the center of the circle.

Chapter 6 : Force and Motion 2 Lesson 2 of 2 Slide 7

F = m v

²

r = ma

Centripetal Force

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Phys

Centripetal force

Answer: 𝑎 is directed towards the center of the circular path:

(a) 𝑎 downward, 𝐹 _𝑁 upward.

(b) 𝑎 and 𝐹 _𝑁 upward.

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Phys

Sample Problem (6-6) :

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Motion in 2D and 3D:

Centripetal force.

Phys 110

Lesson 2of 2 Slide 10 (last)

Summary:

Any Questions?

Next lesson we will start Chapter 7 and will cover:

Section (7-1).

Section (7-2).

Section (7-3).

Sample Problem (7-1).

Section (7-4).