Chapter 3: Force, Work and Energy - WordPress.com Power Chapter 3: Force, Work and Energy Introduction Navapadol Kittiamornkul, Ph. D. According to previous chapter, we described about

Chapter 3: Force and Force Equilibrium

Navapadol Kittiamornkul, Ph. D.

Chapter 3:

Force, Work and Energy


3.1 Mass and Weight3.2 Newton's Law of Gravitation3.3 Force and Newton's 3 Laws of Motion3.4 Types of force3.5 Work3.6 Energy3.7 Conservation of Energy3.8 Power

Chapter 3: Force, Work and Energy

Introduction


According to previous chapter, we described about motionsuch as displacement, distance, velocity, speed, acceleration.

In this chapter, we will discuss about force which is cause of motions. This study called ‘Dynamics”


Introduction

Something that causes a object to move, changes itsspeed or direction, or distorts its shape.

Force

“Force has its Magnitude and Direction. So it is Vector quantity.”


Introduction

Force can be divided into 2 types

1. Contact Force 2. Non-contact Force

3.1 Mass and Weight


The quantities that define weight of object in physics are mass and weight.

Definition of Mass

“Mass Quantity of inertia that resists motion.”

Ball and metal ball have thesame size. Which object canmove easier?

“Object that has lower mass can move easier than object that has greater mass.”


3.1 Mass and Weight

Normally, we can define that“mass is quantity of matter composed in object.”

Therefore, mass can indicate only quantity of object Scalar quantity in kilogram unit.

Earth Moon Saturn

m

m

m

Mass of object always has constant value anywhere in the universe, because mass depends on number of its atoms and molecules.


3.1 Mass and Weight

“Weight Earth gravity reacts to object.”

Definition of Weight

The “g” value of the earth is 9.8 m/s2 approximately at sea level.

According to equation, unit of weight is kg.m/s2 called Newton (N)

“Weight is vector quantity that defines quantity of gravitation force reacted to object.”

“Direction of weight always directs to the center of the earth.”


3.1 Mass and Weight

Earth Moon Saturn

m

m

m

Object weight is not always constant like mass. It depends on gravity that object is located.

gE = 9.8 m/s2 gM = 1.62 m/s2 gS = 11.2 m/s2

For 1 kg of mass

WE = 9.8 N WS = 11.2 NWM = 1.62 N


3.1 Mass and Weight

Actually, g-value of earth is not constant.

The g value at Cambridge is 9.80398 m/s2

The g value at Lampang is 9.78591 m/s2

The g-value around equator is lower thanthe g-value around the earth pole


3.1 Mass and Weight

Ex A man has 490 N of his weight on the earth. What is his weight when he measure on the moon?

Hint: gravity of the moon is 1.62 m/s2

(Answer: 81 N)


3.2 Newton's Law of Gravitation

Newton's Law of Universal Gravitation

“An Object attracts every other object in the universe using a force that is directly proportional to the product of their masses but also inversely proportional to the square of the distance between them.”

m

Md

FG,M

FG,m

G-value obtained by experiment is 6.67 x 10-11 N.m2/kg2

M and m are mass of each object (kg).d is displacement between objects.



Ex. A nurse standing at the earth surface has 45 kg of her mass. If the earth has 5.97x1024 kg of its mass and 6,378 km of its radius, What is the gravity between a nurse and the earth? (Answer: 4.41x102 N)



Ex. Mr. A and Mr. B have the same mass which are 60 kg. the distance between them is 1 m. Find gravity between Mr. A and Mr. B.

(Answer: 4.4x10-7 N)



m m

d

M

Object falls above the earth surface



The earth radius at equator is 6378136 m

The earth radius at earth pole is 6356751 m

g-value of the earth is not constant.

The g value at Cambridge is 9.80398 m/s2

The g value at Lampang is 9.78591 m/s2

“If object is more far away from the earth surface, the gravity decreases continuously.”



Gravity and Nursing Science

Gravity reacts to body liquid such as blood.

Patient body sets after surgery.

Varicose Veins


3.3 Force and Newton's 3 Laws of Motion

Force Any interaction that will change the motion of an object. In other words, a force can cause an object with mass to change its velocity.

It can be described by intuitive concepts such as a push or a pull.

It can make object changed their shape.

“Force has both magnitude and direction, making it a vector quantity.”



Newton's 3 Laws of Motion

It describes the relation between force and motion.

1st Law of Newton

“Object is still or moveing linearly with constant speed, when the resultant force equals to zero.”

Sometime, we call “Law of inertia”.

Inertia A property of object to keep its motion condition.

“The more mass, The more inertia.”



Examples of Inertia



2nd Law of Newton

“Object moves linearly with acceleration in the same direction of resultant force , when the resultant force is not equal to zero.”

“Force and acceleration are vector quantity”

Unit of force is kilogram x acceleration kg.m/s2 N (Newton)


High acceleration Low acceleration


“Acceleration depends on mass.”



System Object or group of objects that we need to study.

Internal Force Force occurs in system.

External Force Force occur out of system.



Force F reacts to mass m1 and m2. Therefore, forces between m1

and m2 occur as shown in Figure.F

“If we consider both m1 and m2 as system, F is external force.”

F12 and F21 are internal forces in this system.

“If we consider m1 as system, F and F21 are external force.”

F12 is internal forces in this system.

“If we consider m2 as system, F and F12 are external force.”

F21 is internal forces in this system.



Find force that reacts to 0.5 gram object. The object move with 2 m/s2 of acceleration. (Answer: 1 N)



Ex. John pushes a 20 kg box. If he uses 5 N to push. Find the acceleration. (Answer: 0.25 N)



Ex. Joe pulls 2 boxes shown by figure. If he pulls with 6 N, Find acceleration of each box. (No friction) (Answer: 0.25 N)

system

msystem = 2 kg + 1 kg = 3 kg



3rd Law of Newton

“A force is a push or a pull that acts upon an object as a results of its interaction with another object. ”

Action force = Reaction force


3.4 Types of force

Normal force Force reacts to object surface perpendicularly.

Normally

Sometime, N is not equal to W


3.4 Types of force

Friction force Force reacts the object movement. It depends on normal force. It has coefficient of friction. The direction is opposite the object movement.

Examples of friction


3.4 Types of force

Type of Friction 1. Static friction 2. Kinetic friction

Static Friction Kinetic Friction

Object is still Object is Moving

Static coefficient of friction Kinetic coefficient of friction


3.4 Types of force


3.4 Types of force

Ex. A boy pushes metal box on wooden floor with 10 N. This metal box has 10 kg. Find friction of box and wooden floor. (µs = 0.3 and µk = 0.5)


3.4 Types of force

Tension force Force occurs in rope, cable, string, etc. It occurs in the same line of rope. The direction is out of the system.


3.4 Types of force

There are more about types of force

Gravitational force

Electromagnetic force

Nuclear force


3.5 Work

Work In physics, work relates to force and motion directly.

Work = Force x Displacement in force direction.

A. B. C.


3.5 Work

How can we calculate work in this situation?


3.5 Work


3.5 Work

Unit of work is N.m Joule, J.

Work is scalar quantity (No direction), but It can be -, + or 0depending on F direction.

NEGATIVE POSITIVEZERO

F opposites to d. F perpendiculars to d. F has the same direction to d.


3.5 Work

NEGATIVEPOSITIVE

Work of woman pushes barbell. Work of gravity reacts to barbell.

Show the situation that work is zero.


3.5 Work

Ex. If we push 1 N object moving for 1 m., Find work of pushing force.

1 N

1 m

(Answer: 1 J)


3.6 Energy

Energy Ability to do work

Water energy uses to generate electricity.

Bowling ball energy can impact pins.

Spring energy can shoot a bullet.


3.6 Energy

Relation between Work and Energy

Work reacts to system

Energy transfers to system

System energy increase

No work reacts to system

Energy transfers out of the system

System energy decrease

We can say that “work is energy transfer.”


3.6 Energy

Kinetic Energy (K.E.) Energy of motion

Every moving object (v ≠ 0) always has kinetic energy.

Kinetic energy can be calculated by

Unit of K.E. is Joule, J.


3.6 Energy

Ex. A 0.5 kg rock is thrown with 4 m/s of its speed. Find the kinetic energy of this rock. (Answer: 4 J)


3.6 Energy

Potential Energy (P.E.) Energy depends on position and shape.

P.E. depending on position.

Gravitational potential

energy

P.E. depending on shape.

Elastic potential energy

Unit of P.E. is Joule, J.


3.6 Energy

Gravitational potential energy

Reference point

We can choose referent point anywhere we want.

P.E. of gravity

P.E. of force

***P.E. grav depends on height from referent point


3.6 Energy

At referent point P.E. grav = 0 J

P.E. grav is negative value

P.E. grav is positive value

P.E. can be +, -or 0 depending on referent point.

Referent point

**Consider potential energy of gravity


3.6 Energy

Ex. Hold a 0.4 kg book far from table with 50 cm. If the height of table is 50 cm, find P.E. when

A. Use table as referent pointB. Use floor as referent point

(Answer: A. 2 JB. 5.2 J )


3.6 Energy

P.E.elastic depends on displacement from referent point.

We can find elastic constant (k) from this equation.


3.6 Energy

Ex. A spring balance can read 0 – 50 N. It can extend for 0.2 m when it read 50 N. If 3 kg mass is measured, find P.E. of this spring.

(Answer: 1.8 J)


3.7 Conservation of Energy

In physics, the law of conservation of energy states that the total energy of an isolated system remains constant.

Energy can neither be created nor destroyed; rather, it transforms from one form to another.

Conservation of Energy

We can draw an equation of the conservation of energy using the following equation.



Consider when object falls.

Referent point



How to calculate conservation of energy

Initial Final



Ex. According to figure, 20 kg snow board starts sliding. Find the velocity of this board at the referent point, if the height of this mountain is 100 m.

Referent point

(Answer: 44.3 m/s)


3.8 Power

Power is quantity of work in time unit or rate of energy using.

Work

Time

Unit of power is joule/second (J/s) Watt (W)

Some favorite unit of power is horse power 1 hp = 746 W

END OF CHAPTER 2


END OF CHAPTER 3

Force, Work and Energy

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Chapter 3: Force, Work and Energy - WordPress.com Power Chapter 3: Force, Work and Energy Introduction Navapadol Kittiamornkul, Ph. D. According to previous chapter, we described about