Newton’s First Law

Chapter 4

TimelineNote: Aristotle is not on this timeline. He was born in the 4th century BCE (384-322 BCE), about 2000 years before Copernicus died

Aristotle (384-322 BCE)• Aristotle's Physics was written in the fourth century BCE. For

more than two thousand years this book served as the basis of natural philosophy up to the sixteenth century, the time of Galileo.

• Aristotle had two types of motion: natural motion and violent motion.

• Natural motion is motion arising from the nature of an object. – This motion does NOT require an external cause to occur. – All early object are composed of four elements: earth, water, air,

and/or fireMotion of things is determined by their natural tendencies to move towards their proper place (only straight up or down):

• Earthly things towards the center of the Earth. • Things composed of air float up into the air.

• Violent motion is contrary to the nature of the object.– This motion does require a FORCE to cause motion.

• e.g. a stone thrown into air moves in a violent motion.

Aristotle on the Heavens

• Aristotle distinguished between the motions of earthly and heavenly bodies

• In the heavens there was heavenly (celestial) motion. • All objects in the heavens move in perfect circles

with the Earth at the center, motionless.• Aristotle believed that the universe was Earth

centered (geocentric).• Humanity believed Aristotle for 2000 years.

Copernicus (1473-1543)• Nicolaus Copernicus was a Polish

astronomer, mathematician, and physicists

• Major achievements include:– He studied the heavens and data on the

motion of the heavens and came to the conclusion that Aristotle’s geocentric model was not correct

– He wrote the famous book De revolutionibus about his heliocentric (sun centered) model of the universe which was not published until he was on his deathbed because he feared persecution.

Galileo(1564 – 1642)• Galileo Galilei was an Italian physicist,

mathematician, astronomer, and philosopher

• Major achievements include:– first systematic studies of uniformly accelerated

motion– Through these studies he came up with his

Principle of Inertia:"A body moving on a level surface will continue in the same direction at constant speed unless disturbed."

– improvements to the telescope which allowed him to observe Jupiter’s moon leading to him to support the Copernican view of the universe and disproved Aristotle’s view

Inertia and Mass

• The resistance of an object to acceleration is called inertia OR Inertia is the resistance an object has to a change in its state of motion.

• A more massive object is more difficult to accelerate than a less massive object.

• Mass resists acceleration – the more mass, the more resistance.

• Mass is the quantity of matter in an object and is also the measure of the inertia of an object.

Newton

• English mathematician and scientist who lived from 1643 to 1727

• Newton added physics interpretations to the mathematical descriptions of astronomy by Copernicus, Galileo and Kepler

• Major achievements include:– Invented calculus as a necessary tool to

solve mathematical problems related to motion

– Discovered the three laws of motion– Discovered the universal law of mutual

gravitation

Aristotle, Copernicus, Galileo and Newton Questions1) How would Aristotle explain a rock falling to Earth?2) How would Aristotle explain a hot air balloon floating in the sky?3) What is the difference between violent motion and natural motion?4) Is an arrow shot from a bow, natural or violent motion? Explain.5) Make a list of those who believed in a geo centric view of the

universe and those who believed in a heliocentric model of the universe.

6) Did Aristotle believe and object required a force to continue moving? Did Galileo?

7) Who is credited with coming up with the concept of inertia?8) Who is credited with inventing calculus?9) What did Galileo observed that convinced him Copernicus was

correct?10) What did Galileo’s famous experiment at the leaning tower of Pisa

prove?

Newton’s First Law – The Law of Inertia• An object at rest tends to stay at rest and an

object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Force• A force is a push or pull acting upon an object as a

result of its interaction with another object.• The unit for force is Newton, N.• Forces may be placed into two broad categories based

on whether the force resulted from the contact or non-contact of the two interacting objects

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

Gravity, Normal Force, Friction and Tension• Fgrav or W: The force of gravity is the force with which the earth,

moon, or other massive body attracts an object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity which is directed "downward" towards the center of the earth.

• Fnorm or FN:The normal force is the support force exerted upon an object which is in contact with another stable object. The normal force is always perpendicular (90 degrees) to the surface.

• Ffrict or f: The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it. The friction force opposes the motion of the object.

• Ftens or T: Tension is the force which is transmitted through a string, rope, or wire when it is pulled tight by forces acting at each end. The tensional force is directed along the wire and pulls equally on the objects on either end of the wire.

Balanced and Unbalanced Forces

Vectors and Scalars• In studying motion and forces in 2D it is helpful separate quantities into

those that depend on direction and those that do not• Vectors are quantities which are fully described by both a magnitude and

a direction.• Scalars are quantities which are fully described by a magnitude alone.Quantity Vector Scalar Comments

Distance X Refers to "how much ground an object has covered" during its motion.

Displacement X refers to "how far out of place an object is"

Speed X how fast something is moving

Velocity X how fast and in what direction

Acceleration X

Temperature X

Time X

Force X

Vectors• Physical quantities that have both magnitude and direction are

conveniently represented by vectors.• Vectors are represented graphically by arrows indicating their direction

and having lengths proportional to their magnitudes (size).• All vectors have a tail and head.• The resultant is the vector sum of two or more vectors.

5 m East

10 m East

10 m West

5 is the magnitude of this vector, East is the direction

These vector is twice as long because their magnitude is 10, instead of 5

Equivalent sets of vectors?• Examine the following sets of vectors and

decide if they are equivalent or not. Justify your answer in each case.

5 m East10 m East

10 m East

10 m West

5 m East

5 m South

5 m East

5 m East

a)

b)

c) d)

Vector Addition• If vectors are in the same direction, simply add the numbers to

get the resultant• If vectors are in the opposite direction, subtract to get resultant• If vectors are at a right angle, use Pythagorean theorem

Adding Velocity Vectors1) Find the resultant velocity of an airplane flying at

100 km/hr with a 25 km/hr tail wind. 100 km/hr + 25 km/hr = 125 km/hr

2) Find the resultant velocity of an airplane flying at 100 km/hr with a 25 km/hr head wind. 100 km/hr - 25 km/hr = 75 km/hr

Resultant

25 km/hr 100 km/hr

75 km/hr

Adding Velocity Vectors• The resulting flight path is a result of both velocity vectors

and can be found using a2 + b2 = c2 or a scale diagram. • 1002 + 252 = c2 c = 103.1 km/hr• http://www.physicsclassroom.com/mmedia/vectors/

plane.html RESULTANT

25 km/hr crosswind 100 km/hr direction

Vectors Question

• Which angle of attack will bring a kayaker to the other bank at a point directly opposite her entry point?

Free Body Diagram• Free-body diagrams are diagrams used to show the relative

magnitude and direction of all forces acting upon an object in a given situation. A free-body diagram is a special example of the vector diagrams discussed previously.

• It is important to remember that only forces, not motion are included in free body diagrams and arrows should show size and direction of force.

• Examples of free-body diagrams:

An object resting on a table OR an object sliding across a frictionless surface

An object hanging from two ropes

An object in free fall

An object falling with air resistance

An object accelerating across a table where friction is smaller than the applied force – forces are unbalanced

An object moving at constant velocity across a table where friction is equal to the applied force –forces are balanced

Newton’s First Law in Action• If a car were to abruptly stop and the seat belts were

not being worn, then the passengers in motion would continue in motion. http://www.physicsclassroom.com/mmedia/newtlaws/cci.html

• If a truck were to abruptly stop and the straps holding the ladder were no longer functioning, then the ladder in motion would continue in motion. http://www.physicsclassroom.com/mmedia/newtlaws/il.html

Can you think of a few more examples which further illustrate applications of Newton's first law?

• blood rushes from your head to your feet when riding on a descending elevator which suddenly stops.

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

• to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted.

• headrests are placed in cars to prevent whiplash injuries during rear-end collisions.

• while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard.

Newton’s First Law Questions1) Imagine a place in the cosmos far from all gravitational and frictional

influences. Suppose an astronaut in that place throws a rock. Describe what happens to the rock.

2) An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction?

3) Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his jello with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. With whom do you agree? Why?

4) If you were in a weightless environment in space, would it require a force to set an object in motion?

5) Mr. Wegley spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia? Explain.

Sources

• Conceptual Physics by Paul Hewitt• www.physicsclassroom.com