PS Notes ForceS

8/8/2019 PS Notes ForceS

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Newton’s Laws of Motion

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Force Notes

Newton’s Laws

So far, we have discussed displacement, velocity, and acceleration (all ways to describe

motion). In this unit we will discuss forces, which explain why objects move.

In the 17th century, Sir Isaac Newton discovered animportant relationship in physics – the relationship

between force, mass, and acceleration.

His 3 laws (Newton’s Laws of Motion) explain why

objects move (or don’t move) as they do.

Force – A push or a pull.

Ex: gravity, magnetic, electrical, or a physical push/pull. Whenever there is an interaction between

two objects, there is a force upon each of the objects.

Force is a vector quantity (it has direction). So, wecan represent forces with arrows.

Types of Forces

All forces (interactions) between objects can be placed into two broad categories:

• contact forces (push, pull, friction, etc)• forces resulting from action-at-a-distance (gravity, magnetism)

Type of Force Description of Force

Force by a person or another object (A push

or pull)

(also known asWeight)

Fgrav

Force of the earth attracting another object towardsitself. Objects on Earth experience a force of gravity,

which pulls "downward" towards the center of the

earth.

5NUnit for force is

Newtons (N)5N5N

Multiple forces mayact upon the same

object5N1N




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Fnorm

The support force exerted upon an object which is incontact with another stable object. Often balances out

gravity.

Ffrict

Force exerted by a surface as an object moves across it

or tries to move across it.

Fair

Special frictional force which acts upon objects as theytravel through the air.

Ftens

Force transmitted through a string, rope, or wire when

it is pulled tight by forces acting from both ends.

Fspring

Force exerted by a compressed or stretched spring

upon any object which is attached to it.

Free Body Diagram – Drawing that shows all forces acting on an object.(Object may be represented as a “dot”

Draw the following free-body diagrams in your notes:

1. book is at rest on a table top.2. A girl is suspended motionless from the ceiling by two ropes.

3. An egg is free-falling from a nest in a tree. Neglect air resistance.

4. A flying squirrel is gliding from a tree to the ground at constant velocity.Consider air resistance.

5. A rightward force is applied to a book in order to move it across a desk with a

rightward acceleration. Consider frictional forces.

6. A rightward force is applied to a book in order to move it across a desk atconstant velocity.

Name forces based upon what is cau

the force.

Fgrav = Force of Gravity

F p = Force of Push

Fnorm = Normal Force (opposes gravit

F pull

Fg




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Determining Net Forces

Add forces just like all other vectors.- Same direction: add

- Opposite direction: subtract

Q: What is the net force (Fnet) acting on each of these blocks?

6N

4N

3N

Free-body diagrams for four situations are shown below. The net force is known for each situation.

However, the magnitudes of a few of the individual forces are not known. Analyze each situationindividually and determine the magnitude of the unknown forces.




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Newton’s First Law: 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.

This is often called the law of inertia. Inertia is the tendency of an object to resistchange in its state of motion (velocity).

Balanced Forces _________ Unbalanced Forces

Fnet = 0 Fnet ≠ 0

Velocity remains the same Velocity changes (objectaccelerates)

Example: When driving in a car, how do you feel (which way do you lean) when the

car…

Action of Car Passenger Observations Why?

Goes straight, at constantspeed

Turns left

Speeds up

Slows down

Q: So why does a ball rolling along the ground eventually come to a stop? Shouldn’t it

keep going because of inertia?

Try this: Push your textbook across the surface of your desk.

What happens when you stop pushing? Why?

Q: Imagine a place in the cosmos far from all gravitational and frictional influences.Suppose that you visit that place (just suppose) and throw a rock. The rock will

a. gradually stop.

b. continue in motion in the same direction at constant speed.

Q: A 4.0-kg object is moving across a friction-free surface with a

constant velocity of 2 m/s. Which one of the following horizontalforces is necessary to maintain this state of motion?

a. 0N b. 0.5N c. 2.0 N d. 8.0 N




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Is a net force displayed in the following situations?

Description of MotionNet Force: Yes or

No?

Mass and Inertia

What is mass?

- Mass is a measure of the inertia of the object.- Related to the amount of matter in an object

- Measured in grams, kilograms

Mass is different from weight. Weight is _______________________________________

____________________________________________________ (measured in Newtons).

Mass and weight are related because the earth pulls more strongly on objects with more

mass, but the numbers are not the same.

Practice This:

Q: What is the weight (in Newtons) of a 7.0-kilogram bowling ball on Earth’s surface?

Objects with greater mass have greater inertia. (Picture the friction created when you push a

Styrofoam brick and a cement brick across the table with equal force.)

Weight (Fgrav) = mass x g

where g = 9.8 m/s2 on earth




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Q: What is the weight of a 7.0-kg bowling ball on the surface of the moon (where g=2.6)?

Q: What is the mass of a 7.0-kg bowling ball on the surface of the moon?

Q: Does a 2-kg iron brick have twice as much inertia as a 1-kg iron brick? Twice asmuch mass? Twice as much weight? Twice as much volume?

Q: Does a 2-kg iron brick have twice as much inertia as a 1-kg wooden brick? Twice as

much mass? Twice as much weight? Twice as much volume?

Newton’s Second Law

Unbalanced forces acting on an object result in acceleration. Newton’s second law tells

us the relationship between the force and the resulting acceleration.

Law 2: The acceleration of an object is directly proportional to the net force

acting on the object, is in the direction of the net force, and is inversely proportional to

the mass.

**Remember, net force is the sum of all forces acting on an object

Use the equation a = Fnet/m to fill in the chart below.

Net Force Mass Acceleration

Acceleration = =

Net Force

Mass

Fnet

m




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(N)(kg)

(m/s/s)

1. 10 2 ?

2. 20 2 ?

3. 20 4 ?

4. ? 2 5

5. 10 ? 10

Q: What (2) changes result in the acceleration being doubled?

Q: In a physics lab, Kate and Rob use a hanging mass and pulley system to exert a 2.45 N

rightward force on a 0.500-kg cart to accelerate it across a low-friction track. If the total resistance force to the motion of the cart is 0.72 N, then what is the cart's

acceleration?

Q: An applied force of 50 N is used to accelerate an

object to the right across a frictional surface. The object

encounters 10 N of friction. Use the diagram to determinethe normal force, the net force, the mass, and the

acceleration of the object. (Neglect air resistance.)

Q: An applied force of 20 N is used to accelerate an object to the right across a frictional

surface. The object encounters 10 N of friction. Use the diagram to determine the normal

force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.)




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When acceleration is zero – Equilibrium

Balanced Forces

Object is either at rest (static) or moving at a constant speed: Dynamic EquilibriumFill in the chart below:

Test your understanding:Two students are discussing the object shown at the right. Anna Litical

suggests to Noah Formula that the object could be moving. Can you think of a scenario in which this object is moving vertically? Horizontally?

When acceleration is zero – Equilibrium

Balanced Forces

Object is either at rest (static) or moving at a constant speed: Dynamic Equilibrium

Fill in the chart below:

Q: A jumbo jet cruises at constant velocity of 1000 km/h when the thrusting force of itsengines is at a constant 100,000 N. What is the acceleration of the jet? What is the force

of air resistance on the jet?

Acceleration = __________________ Fair = __________________

Movement Static or Dynamic

Equilibrium?A hockey puck slides at

constant speed

A pen sits motionless on a

desk

Two friends push with

equal force on each other’s

hands.

Two equally strong teams

play tug-of-war. The ropedoes not move

Movement Static or Dynamic

Equilibrium?

A hockey puck slides at

constant speed

A pen sits motionless on adesk

Two friends push withequal force on each other’s

hands.Two equally strong teams

play tug-of-war. The rope

does not move

Summarize: When is zero

acceleration NOT zero

velocity?

Summarize: When is zeroacceleration NOT zero velocity?




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Friction: Force exerted by a surface as an object moves across it or makes an effort to

move across it.

• Opposes motion

• Depends on characteristics of 2 surfaces in contact

2 Types of Friction

1. Sliding (________________) Friction: results when an object slides across asurface (ex – pushing a box across the floor. The floor surface exerts a friction

force upon the box.)

2. _____________________ Friction: the surfaces of two objects are at rest relative

to one another. A force exists on one of the objects to set it into motion.

Ex: you push on a heavy box and it does not move. The static friction force balances

the force you exert on the box and the box remains at rest.

The harder you push an object, the greater the static friction force becomes. (Your push

must = friction if the object is not moving). This is true until the object finally beginsmoving, then the friction is kinetic.

1. A rightward force is applied to a 6-kg object to move it across a rough surface at

constant velocity. The object encounters 15 N of frictional force. Use the diagramto determine the gravitational force, normal force, net force, and applied force.

(Neglect air resistance.)

2. In a Physics lab, Ernesto and Amanda

apply a 34.5 N rightward force to a 4.52-kg cart to accelerate it across a horizontal

surface at a rate of 1.28 m/s/s. Determine the friction force acting upon the cart.




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Terminal velocity (Nonfree-Fall): The velocity at which acceleration terminates for

falling objects.

Air resistance acts in the direction opposite the object’s motion. (For falling objects, air resistance pushes __________)

2 Factors influence air resistance:

1. Frontal area of object How much surface area is in contact with the air. For

a feather, large frontal area and little weight slow down movement.

2. Speed of falling object The greater the speed, the more air molecules per

second the object must push through.

So, air “drag” depends on size and speed of a falling object.

An object will only accelerate when the downward force is larger. Since increased speed

= increased drag, the upward force gets bigger and bigger. Eventually, it is the samemagnitude as Fgrav.

Time

Q: Describe the changing forces acting on a skydiver from the moment they leave the

plane until they reach terminal velocity.

Terminal velocity for a skydiver ~ 200 km/h

Terminal velocity for a feather ~ few cm/s

Recall that all objects on Earth accelerate at ________________whenfalling because of gravity. This is true for heavy and light objects, as long

as they are not too influenced by air resistance (ex – a feather).

Note: a rock and a feather will fall at the same speed in a vacuum. Why?

Recall, acceleration depends on the______________________

that is acting upon an object.

1. Gravity pulls down

2. Air resistance (drag) pushes up

1. Fg > FairObject

accelerates

2. Fg > FairAccelerates more

slowly

Fg = FairNet Force = 0, constant speed

Terminal Velocity Reached




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Q: A man and a woman are parachuting together. The man weighs twice as much as the

woman and they have the same sized chutes. Who reaches the ground first? Why?

Q: A skydiver jumps from a high-flying helicopter. As she falls faster and faster through

the air, does her acceleration increase, decrease, or remain the same?

Newton’s 3rd Law of Motion

Force can be defined as a push or pull. More specifically, a force is an

interaction between 2 things. All forces exist in pairs.

Ex: When your book sits on a desk, it doesn’t move. Why? Are the forces balanced or

unbalanced?

Draw a free body diagram of the book. What forces are equal and opposite?

Q: On a cold, rainy day, your car battery is dead and you must push the car to get itstarted. Why can’t you push the car by remaining comfortably inside and pushing on the

dashboard?

Q: We know the Earth pulls on the Moon. Does the moon also pull on the Earth?

Law 3: Whenever one object exerts a force on a second object, thesecond object exerts and equal and opposite force on the first.




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Q: You hit a baseball with a bat. If we call the force exerted on the ball by the bat theaction force, identify the reaction force.

Q: When a rifle fires a bullet, a force pair exists between the bullet and the rifle. The

force exerted on the bullet is equal to the force exerted on the rifle, which is why the rifle“kicks back” when fired. Why doesn’t the rifle recoil with the same speed as the bullet?

Touching your finger to

your nose

Walking

Holding hand out the

window of a moving car

Two bumper cars

collide

A bird flaps its wing

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