CHAPTER 4CHAPTER 4FORCES IN 1-DFORCES IN 1-D

FORCEFORCEForceForce is anything which is anything which

causes a body to start causes a body to start moving when it is at rest, or moving when it is at rest, or stop when it is moving, or stop when it is moving, or deflect once it is moving.deflect once it is moving.

ForceForce is a push or pull on an is a push or pull on an object.object.

KINDS OF FORCESKINDS OF FORCESGravitational Force:Gravitational Force: is an attractive is an attractive

force that exists between all objects. force that exists between all objects. It is a weak force.It is a weak force.

Electromagnetic Force:Electromagnetic Force: consists of consists of electric & magnetic forces. These electric & magnetic forces. These forces give materials their strength, forces give materials their strength, their ability to bend, squeeze, stretch, their ability to bend, squeeze, stretch, or shatter.or shatter.

KINDS OF FORCESKINDS OF FORCESNuclear Force:Nuclear Force: hold particles hold particles

in the nucleus together. It is in the nucleus together. It is the strongest kind of force.the strongest kind of force.

Weak Force:Weak Force: is a form of is a form of electromagnetic force and is electromagnetic force and is involved in the radioactive involved in the radioactive decay of some nuclei.decay of some nuclei.

TYPES OF FORCESTYPES OF FORCESWeight (W):Weight (W): A field force due A field force due

to gravitational attraction to gravitational attraction between two objects. (Ex. between two objects. (Ex. You and Earth)You and Earth)

Normal Force (FNormal Force (FNN):): The The contact force exerted by a contact force exerted by a surface on an object. (Ex. surface on an object. (Ex. You and the floor)You and the floor)

TYPES OF FORCESTYPES OF FORCESFriction (FFriction (Fff):): A contact force A contact force

that acts to oppose sliding that acts to oppose sliding motion between surfaces. motion between surfaces. (Ex. Sliding a box across (Ex. Sliding a box across carpet)carpet)

Tension (T):Tension (T): The pull exerted The pull exerted by a string, rope or cable by a string, rope or cable when attracted to a body and when attracted to a body and pulled taut. (Ex. A climber pulled taut. (Ex. A climber hanging from a rope)hanging from a rope)

TYPES OF FORCESTYPES OF FORCESSpring Force (FSpring Force (FSS):): A restoring A restoring

force; the push or pull a force; the push or pull a spring exerts on an object.spring exerts on an object.

Thrust (FThrust (Ftrusttrust):): A general term A general term

for the forces that move for the forces that move objects such as rockets, objects such as rockets, planes, cars and people.planes, cars and people.

Newton’s First LawNewton’s First Law states that an states that an object at rest remains at rest, object at rest remains at rest, and an object in motion and an object in motion continues moving in a straight continues moving in a straight line at constant speed, unless line at constant speed, unless acted upon by an unbalanced acted upon by an unbalanced force.force.

Inertia Inertia is the tendency of an is the tendency of an object to remain at rest or to object to remain at rest or to keep moving in a straight line at keep moving in a straight line at constant speed.constant speed.

Newton’s Second LawNewton’s Second Law states states that a net force acting on an that a net force acting on an object causes the object to object causes the object to accelerate in the direction of accelerate in the direction of the force. A larger mass the force. A larger mass requires a greater force than requires a greater force than a smaller mass would a smaller mass would require to achieve the same require to achieve the same acceleration.acceleration.

If an object has a net force If an object has a net force exerted on it, it will exerted on it, it will accelerate. Force and accelerate. Force and acceleration both have acceleration both have direction and magnitude. direction and magnitude. The acceleration is in the The acceleration is in the same direction as the force same direction as the force causing it. (Ex. Picking up a causing it. (Ex. Picking up a stack of books)stack of books)

One Newton: is the amount of force needed to accelerate an object with a mass of 1 kg at an acceleration of 1 m/s2.

1 N = 1 kg x 1 m / s2

F = m x aF = Force (N)m = mass (kg)a = acceleration (m / s2)

EXAMPLE 1EXAMPLE 1

What is the net force required to What is the net force required to accelerate a 1500 kg race car at accelerate a 1500 kg race car at 3 m/s 3 m/s22..

NamF

sma

kgm

450031500

/3

15002

EXAMPLE 2EXAMPLE 2An artillery shell has a mass of 55 An artillery shell has a mass of 55 kg. The shell is fired from a gun, kg. The shell is fired from a gun, leaving the barrel with a velocity of leaving the barrel with a velocity of 770 m/s. The gun barrel is 1.5 m 770 m/s. The gun barrel is 1.5 m long. Assume the force, and thus long. Assume the force, and thus the acceleration of the shell is the acceleration of the shell is constant while the shell is in the constant while the shell is in the gun barrel. What is the force on the gun barrel. What is the force on the shell while it is in the gun barrel?shell while it is in the gun barrel?

NamF

sma

kgm

75

25

101.110255

/102

55

smv

smv

Dml

kgm

f

i

/770

/0

5.1

55

smD

vva

vvaD

fi

fi

/1022

2

5

22

22

FALLING OBJECTSFALLING OBJECTS

Near the Earth’s surface gravity Near the Earth’s surface gravity causes all falling objects to causes all falling objects to accelerate at accelerate at 9.8 m/s9.8 m/s22..

WeightWeight of an object W, is the of an object W, is the force of gravity acting on its force of gravity acting on its mass.mass.

W = m x g

W = m x gW = m x g

WW = weight = weight (N)(N)

mm = mass = mass (kg)(kg)

gg = acceleration due to gravity = acceleration due to gravity

(9.8 m/s(9.8 m/s22))

EXAMPLE 3EXAMPLE 3 Find the weight of a 2.26 kg bag Find the weight of a 2.26 kg bag

of sugar. (g=9.8 m/sof sugar. (g=9.8 m/s22).).

The direction of the weight is The direction of the weight is downward.downward.

NgmW

smg

kgm

1.228.926.2

/8.9

26.22

EELLEEVVAATTOORR

PPRROOBBLLEEMMSS

A person is standing on a scale in the elevator. What does the scale read?

STEPS TO SUCCESSSTEPS TO SUCCESS

1. Draw Free-Body Diagram1. Draw Free-Body Diagram

2. Consider what’s happening2. Consider what’s happening

3. Follow the GUESS method3. Follow the GUESS method

4. Remember the scale reads normal 4. Remember the scale reads normal forceforce

5. Put a box around your final answer5. Put a box around your final answer

WHAT’S HAPPENINGWHAT’S HAPPENING

A) Elevator is at restA) Elevator is at rest

B) Elevator is moving upB) Elevator is moving up

C) Elevator is moving downC) Elevator is moving down

D) Elevator is in free fall D) Elevator is in free fall

A) Elevator is at restA) Elevator is at rest

Elevator acceleration is zero (a=0)The two opposing forces are the normal force and weight of the object. N – W = 0 N = W (zero indicates no accel)

N

WScale reads W of object

B) Elevator is moving upB) Elevator is moving up

Elevator has accelerationThe person on the scale is supported by the scale so they are balanced, however we must take the acceleration of the elevator into consideration

N

W

a

REMEMBERREMEMBER

N

W

a

The elevator has to exert a force to move the object, the scale reading is affected by the elevator’s acceleration N – W = ma N = ma + W(accel is positive here)

C) Elevator is moving downC) Elevator is moving down

Elevator has accelerationThe person on the scale is supported by the scale so they are balanced, however we must take the acceleration of the elevator into consideration

N

W

a

AGAINAGAIN

N

W

a

The elevator has to exert a force to move the object, the scale reading is affected by the elevator’s acceleration N – W = - ma N = W – ma (accel is negative here)

D) Elevator is in free fallD) Elevator is in free fall

Example of when the support cable breaks Elevator’s acceleration is equal to gravity in this caseN – W = -mgN = W – mg (where W = mg N = mg – mg N = 0

N

W

a

Air Resistance:is the force air exerts on amoving object. This force actsin the opposite direction to that of the objects motion.

Air resistanceForce of Gravity

AIR RESISTANCEAIR RESISTANCE Air resistance pushes up as gravity Air resistance pushes up as gravity

pulls down.pulls down. The amount of air resistance depends The amount of air resistance depends

on the size, speed, shape, and on the size, speed, shape, and density of the object.density of the object.

The larger the surface area the greater The larger the surface area the greater the amount of air resistance on it.the amount of air resistance on it.

Newton’s Third Law of Motion states that when one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude but opposite in direction.

STATIC FRICTION is the force that opposes the start of motion.

KINETIC FRICTION is the force between surfaces in relative motion.

Kinetic Friction < Static Friction

The static friction of an object is greater than its kinetic friction

FRICTIONFRICTION

1 BODY SYSTEMS IN 1-D1 BODY SYSTEMS IN 1-D

W

NFPushing down on an object with a force (F)Pushing down on an object with a force (F)

W

N

F

In the y- direction:N – W – F = 0object is not movingso forces balance

1 BODY SYSTEMS IN 1-D1 BODY SYSTEMS IN 1-D

W

TConsider an object hanging from a Consider an object hanging from a chain.chain.

W

T

In the y- direction:T – W = 0 T = Wobject is not movingso forces balance

2 BODY SYSTEMS IN 1-D2 BODY SYSTEMS IN 1-DConsider two objects hanging over a Consider two objects hanging over a pulley.pulley.

m1

T

m2

T

a

m1

T

m2

T

a T

a

m1

m1g

Object 1

T

m2

m2g

a

Object 2

T

m2

m2g

T

m1a

m1

m1g

m1a = m1g – T

m2a = T – m2g

m2a