Forces INewtons Laws

Kinematics

• The study of how objects move

Why do objects move?

Dynamics

• The study of why objects move

Newton’s Laws and Forces

• What is force?

• What are they?

Force

• A push or a pull

• Symbol is F

• Unit is N (Newton)

• One newton is the force necessary to cause a one kilogram mass to accelerate at the rate of 1m/s2

• 1N=1 kg m/s2

FORCEA force is any influence that can change the velocity of a body. Forces can act either through the physical contact of two objects (contact forces: push or pull) or at a distance (field forces: magnetic force, gravitational force).

Contact Forces Action-at-a-Distance Forces

Frictional Force Gravitational Force

Tensional Force Electrical Force

Normal Force Magnetic Force

Air Resistance Force

Applied Force

Spring Force

What do we mean by balanced and unbalanced forces?

The forces on the book are unbalanced

Balanced forces are EQUAL and OPPOSITE in direction

Unbalanced forces have a greater force in one direction

If all the forces are balanced, we say the object is in EQUILIBRIUM

A net force is the vector sum of the forces acting on an object

What are the net forces?FNET = Σ forces ≠ 0 N

These are free-body diagrams

Free Body Diagram Practice1.A book is at rest on a tabletop. Diagram the forces acting on the book.2.A girl is suspended motionless from the ceiling by two ropes. Diagram the forces acting on the girl. 3.An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it is falling.4.A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. 5.A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. 6.A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. Diagram the vertical forces acting on the backpack. 7.A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver.8.A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration. Neglect air resistance. Diagram the forces acting upon the sled

Newton Three Laws of Motion

• His laws explain why objects move (or don’t move) as they do

Newton's First Law of Motion (Law of Inertia) or [Law of Balanced Forces]

Newton's First Law of Motion says: an object at rest stays at rest and an object in motion stays in motion unless the object is acted on by an unbalanced force.

“Things keep doing what they are doing!”

Constant Velocity

It can be Zero or it can be greater than zero…..It just can’t change!!!!!!

What is required to change the constant velocity?

• An unbalanced force

Diagram from the Physics Classroomhttp://www.physicsclassroom.com/mmedia/newtlaws/efff.html

Diagram from the Physics Classroomhttp://www.physicsclassroom.com/mmedia/newtlaws/efff.html

Other Examples to Consider:

• Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.

• The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.

Inertia and Mass

• Inertia: the resistance an object has to a change in its state of motion

• The more mass the more inertia

• As mass increases the more an object resists change in its motion

• Mass (kg) is the measurement of Inertia

FREE BODY DIAGRAM:Identify the missing force marked with a (?) in the diagram below. The car is moving at a constant velocity of 25 m/s eastward.

Car20N

15N

Air Friction

Ground Friction

Ground Friction

?

Force pushing car

Newton’s First Law

Remember an object in constant motion has balanced forces working on it. This is the first law described

mathmaticallyΣF = 0

•Where Σ (sigma) is sum

•F is force

Hands On

• Using Newton scale:

• drag mass across table at constant velocity. What was the force?

.

Newton's Second

Law of Motion

(Law of Acceleration)

[Law of Unbalanced

Forces]

SECOND LAW OF MOTION According to Newton's Second Law of Motion, the net force acting on a body equals the product of the mass and the acceleration of the body. The direction of the force is the same as that of the acceleration. In equation form:

F = ma 

F = force applied to an object [N = Newtons = kg ∙m/s2]m = object’s mass [kg]a = accelerationa = acceleration [m/s[m/s22]]

What is the relationship between Force, Mass, and Acceleration?

According to Newton's second law of motion, the acceleration experienced by an object increases in a direct variation with?

Force

According to Newton's second law of motion, the acceleration experienced by an object increases in an inverse variation with?

Mass

FREE BODY DIAGRAM:Identify the missing force marked with a (?) in the diagram below. The car is accelerating eastward with a net force of 50 N.

Car20N

15N

Air Friction

Ground Friction

Ground Friction

?

Force pushing car

2nd Law: F = m·aEX A

• A 2000 N net force acts horizontally to the right on a 50 kg object. Draw the free body diagram. Calculate the acceleration.

amF akgN 502000

2/40 sma

Are weight and mass the same thing?

WEIGHT (of a body or object) aka Force of Weight

The gravitational force with which the Earth attracts the body. Causes it to accelerated downward with the acceleration of gravity g.

Vector quantity Measured in Newtons

Varies with its location near the Earth (or other astronomical body)

ForceWeight = mass x gravity

FW = m x gNewton = kg x m/s2

1 N = 1 kgm/s2

Mass

A scalar quantity

Same everywhere in the universe. It doesn’t change Measured in kg

Hands On

• Using Newton scale:

• drag mass across table at a changing velocity. What happens to the force?

• Suspend the mass. What is the force? What does this represent?

EX B: What is the weight of a person whose mass at sea level is 72 kg? Draw the free body diagram. g = 9.80 m/s2

m = 72 kgF = mg

Fw = mg

Fw = (72 kg)(9.80 m/s2)

Fw = 706 kg m/s2

Fw = 706 N

Ex C: What is the mass of a box that weighs 625N? Answer to the nearest tenth. Draw the free body

diagram.

F = ma

m = F/m

m = 625N/9.8 m/s2

m = 63.8 kg

Constant Force LabYou will have a quiz over thisYou do not have to convert grams nor cmWhen graphing you must come up with a reasonable scale. The y intervals must be equal, the x intervals must be equal

What is the third law of motion

• For every action force there is an equal and opposite reaction force

• Two different objects and two different forces (equal but opposite)

• Forces always occur in pairs• Also known as The Law of Force Pairs• Are the effects of the force the same?• NO• They are not always acting on the same amount of mass• Examples:• Bug hitting windshield, recoil of shot gun, pushing off of on a raft

Third Law Mathmatically

F = F

Which can be written as:

ma= ma

Forces occur is pairs…

What forces cause the swimmer to move forward?

• Her push on the wall?

Or

• The wall’s push on her?

THIRD LAW OF MOTION According to Newton's third law of motion, when one body exerts a force on another body, the second body exerts on the first an equal force in opposite direction.

The Third Law of Motion applies to two different forces on two different objects: "The action force one object exerts on the other, and the equal but opposite reaction force the second object exerts on the first."

Ex D

• A 2500 kg car hits a 0.001 kg bug with a force of 500N. According to the third law of motion, the bug hits the car with how many Newton's of force? Why isn’t the car damaged? Calculate the acceleration of the bug due to the force. Calculate the acceleration of the car due to the force.

A 2500 kg car hits a 0.001 kg bug with a force of 500N. According to the third law of motion, the bug hits the car with how many Newton's of

force? Why isn’t the car damaged? Calculate the acceleration of the bug due to the force. Calculate the acceleration of the car due to the force.

acceleration of the bug due to impact:

a = F/m

a = 500N/.001 kg = 500,000 m/s2 !

acceleration of the car due to impact :

a = F/m

a = 500N/2500kg = 0.2 m/s2

Third Law of Motion

• The girl pushes the boy with 10N of force. How much does the boy push back with?

Review F=ma calculations

Net Force (Fnet )• Fnet in simple situations (only 1 force implied): Fnet

= ma

• Fnet and acceleration are always in the same direction

• How is net force on an object determined with multiple?

• It is the sum of all forces

• Fnet = ΣF

• Remember force is a vector, therefore it has direction (+,-, N,S etc)

Net force (F) = maThe second law of motion is the key to understanding the behavior of moving bodies since it links cause (force) and effect (acceleration) in a definite way.

Example EA 2000 N net force (horizontally to the right) acts on

a 50 kg object. Draw the free body diagram. Calculate the acceleration. If the object started from rest, how far would it move in 5 seconds?

amF akgN 502000

2/40 sma

Example EA 2000 N net force (horizontally to the right) acts on

a 50 kg object. Draw the free body diagram. Calculate the acceleration. If the object started from rest, how far would it move in 5 seconds?

d = vit + .5at2

d = (.5)(40 m/s2)(5sec)2

d = 500m

Could we determine the velocity at 5 seconds? How?

a = (vf – vi)/t

vf = vi + at

vf = 0 m/s + (40 m/s2)(5sec) = 200 m/s

vf

Ex F A 1000 kg car goes from 10 to 20 m/s in 5 s. What force is acting on it?

m = 1000 kg vi = 10 m/s

vf = 20 m/s

t = 5 s

F = ma

a = (vf – vi)/t a = (20m/s-10 m/s) / 5sa = 2 m/s2

F = (1000 kg) (2 m/s2) = 2000 N

Ex G A 60-g tennis ball approaches a racket at 15 m/s, is in contact with the racket for 0.005 s, and then rebounds at 20 m/s. Find the average force exerted by the racket.

m = 0.06 kg vi = 15 m/s

t = 0.005 svf = - 20 m/s

a = (vf – vi)/t a = (-20m/s-15 m/s) / 0.005sa = -7000 m/s2

F = maF = (0.06 kg) (-7000 m/s2) = -420 N

Ex H The brakes of a 1000-kg car exert 3000 N.a. How long will it take the car to come to a stop from a velocity of 30 m/s?

m = 1000 kgF = -3000 Nvi = 30 m/s

vf = 0 m/sm

Fa

1000

3000 = - 3 m/s2

a

vvt if

3

300

= 10 s

b. How far will the car travel during this time?

d = vit+ .5at2

= 30(10)+ .5 (-3)(10)2

= 150 m

Ex I A net horizontal force of 4000 N is applied to a car at rest whose weight is 10,000 N. What will the car's speed be after 8 s?

FA = 4000 N

Fw = 10,000 N

t = 8s g

w

a

Fm

8.9

10000 = 1020.4 kg

m

Fa

4.1020

4000 = 3.92 m/s2

vf = vi + at

= 0 +3.92(8) = 3.14 m/s

Concept Review questions on following 5 slides

Which diagram represents a book sitting on a table?

A B C

What vector indicates force due to gravity?

plane

B

CA

D

Draw a free body diagram of a car accelerating to the left with a net

force of 400N

My bag of Halloween candy weighs 3 N. What is it’s mass?

I have a brand new car. It can go from 0mph to 50mph in 5

seconds!!!!! What is its rate of acceleration?

• An object is thrown in the air with an initial velocity of 15m/s. How high above the ground is it after 2.5 secs?

• d = vit + .5at2

• (-15m/s) (2.5 sec) + [(0.5)(9.8m/s2)(2.5sec)2]

• -6.88 m (the negative indicates height)

• Luke Autbeloe drops an approximately 5.0 kg fat cat (weight = 50.0 N) off the roof of his house into the swimming pool below. Upon encountering the pool, the cat encounters a 50.0 N upward resistance force (assumed to be constant). Use this description to answer the following questions.

Which one of the following dot diagrams best describes the motion of the falling cat from the time

that they are dropped to the time that they hit the bottom of the pool? The arrows on the diagram

represent the point at which the cat hits the water.

Support your answer with sound reasoning.

• Tape A is correct.• The cat first accelerates with a negative

(downward) acceleration until it hits the water. Upon hitting the water, the cat experience a balance of forces (50 N downwards due to gravity and 50 N upwards due to the water). Thus, the cat will finish its motion moving with a constant velocity. Diagram A depicts both the initial downward acceleration and the final constant velocity.

• Several of Luke's friends were watching the motion of the falling cat. Being "physics types", they began discussing the motion and made the following comments. Indicate whether each of the comments are correct or incorrect?

• Once the cat hits the water, the forces are balanced and the cat will stop.

• Upon hitting the water, the cat will accelerate upwards because the water applies an upward force.

• Upon hitting the water, the cat will bounce upwards due to the upwards force.

Fun with Forces#1 16 N↑ 4N→

#7 130 lb → if net is 85lb right ; must have 85lb greater than the 120lb left. Therefore need a total of 205lb right. Already have 75lb right, so need an additional 130lb right (205lb-75lb)