FORCES AND THE LAWS OF MOTION Porter2 · PDF fileNewton's First Law • Newton's first law states: An object at ... force the object is at rest or constant ... to keep the object...

FORCES AND

THE LAWS OF

MOTION

FORCE

• A force is the cause of an acceleration, or the

change in an object’s velocity (speed or

direction). Forces are usually thought of as a

push or a pull.

• The SI unit of force is the Newton (N).

• The Newton is defined as the amount of force

that, when acting on a 1 kg mass, produces an

acceleration of 1 m/s² . Therefore, 1 N = 1

kg· m/s².

FORCES CAN ACT THROUGH

CONTACT OR AS A FIELD

• Contact Forces are forces that arise from the physical contact of two objects. (Push or Pull)

• Field Forces are forces that can exist between objects, even in the absence of physical contact between the objects. Examples: gravity, electromagnetic, weak/strong nuclear

Physics 151: Lecture 5, Pg 10

WEIGHT• The weight of an object is measure

of the gravitational force on the object. It is the result of the interaction of an object’s mass with the gravitational field of another object, such as Earth.

• 1 lb = 4.448 N

• 1 N = 0.225 lb

NEWTON’S FIRST LAW• Newton’s First Law: An object at rest

remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force.

• The tendency of an object to maintain its state of motion is called inertia.

• Newton’s first law is often referred to as the Law of Inertia.

Newton's First Law

• Newton's first law states: An object at rest will remain at

rest, an object in uniform motion will stay in motion -UNLESS acted upon by an outside force

• This is why you should always wear a seat belt!

Outside Force

NEWTON’S FIRST LAW

MASS IS A MEASUREMENT OF

INERTIA• Imagine a basketball and a golf ball at rest side

by side on the ground. Newton’s first law states

that both remain at rest as long as no net external

force acts on them. Now imagine supplying a net

force by striking each ball with a golf club. If the

two are struck with equal force, the golf ball will

accelerate much more than the basketball. The

basketball experiences a smaller acceleration

because it has more inertia than the golf ball.

ACCELERATION IS

DETERMINED BY NET

EXTERNAL FORCE• The net external force is the vector sum of all

the forces acting on an object. It is sometimes

referred to as the resultant force.

• When there is no net force the object is at rest

or constant motion. When there is a net force

there is acceleration in the direction of the the

resultant force.

• The ΣΣΣΣ symbol means the sum of. The net

external force must be found on the x and the y

axis.

EQUILIBRIUM

• Objects that are either at rest or

moving with constant velocity are said

to be in equilibrium.

• Newton’s first law states one condition

that must be true for equilibrium:the

net external force acting on a body in

equilibrium must be equal to zero.

• ΣΣΣΣ Fx = 0 and ΣΣΣΣ Fy = 0

EQUILIBRIUM

EQUILIBRIUM

NON-EQUILIBRIUM

Force of Push

Types of ForcesContact Forces Action-at-a-Distance

Forces

Frictional Force

Tensional Force

Electrical Force

Normal ForceMagnetic Force

Air Resistance Force Applied Force

Gravitational Force

Spring Force

FieldForces

TYPES OF FORCES

• Weight: the magnitude of the force of gravity acting on an object (always a downward force) Fw = mag

• Normal Force: a force exerted by one object on another in a direction perpendicular to the surface of contact.Youmust have a surface to have a normal force.

• Tension: The pull exerted by a string, rope, or cable when attached to a body and pulled taut.

TYPES OF FORCES• Spring: A restoring force, that is, the push

or pull a spring exerts on an object

• Thrust: A general term for the forces that move objects such as rockets, planes, cars, and people

• Push/Pull forces: Physical contact force

• Friction: the resistive force that opposes the relative motion of two contacting surfaces that are at rest with respect to one another.

FORCE DIAGRAMS• A force is a vector. It has magnitude and

direction.

• A force diagram is a diagram of the objects involved in a situation and the forces exerted on the objects.

• A free-body diagram helps analyze a situation.

• In a force diagram, vector arrows represent all the forces acting in a situation. A free-body diagram shows only the forces acting on the object of interest.

FREE-BODY DIAGRAMS

UNBALANCED FORCES

Physics, Friday, November3

Turn in your illustrations in the

basket!

Grab a whiteboard and expo!

FORCE PROBLEMS

ANSWERS

• 0 N

• 5 N; LEFT

FORCE PROBLEMS

ANSWERS

• 0 N

• 15 N; UP

FREE-BODY DIAGRAM

PRACTICE• A book is at rest on a table top. Diagram the forces acting on the book.

• A girl is suspended motionless from a bar which hangs from the ceiling by two ropes.

ANSWERS

FREE-BODY DIAGRAM

PRACTICE• 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.

• A car is coasting to the right and slowing down. Diagram the forces acting upon the

car.

ANSWERS

QUESTIONS 1 AND 2

• Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will:

• a) gradually stop.

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

• 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?

ANSWERS 1 AND 2

• B

• 0 N

QUESTIONS 3 AND 4

• 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?

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

ANSWERS TO 3 AND 4

• Tosh

• YES!!! An object has mass even in space – Newton’s laws still apply

CONCEPTUAL QUESTION 5

• Ben is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

Exit Ticket1. Explain the relationship between Weight and Mass

2. What is the net force

on this object:

3. What is the net force on this object:

CONCEPTUAL QUESTION 6

• Several physics teachers are taking some time off to play a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim which putters must use to guide their ball towards the hole. Mrs. Davis guides her golf ball around the metal rim. When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?

NEWTON’S SECOND LAW

• Newton’s Second Law: The acceleration of an

object is directly proportional to the net external

force acting on the object and inversely

proportional to the object’s mass.

• ΣΣΣΣ F = ma (The sum of forces on a particular axis

equals mass times acceleration)

• A small force on an object causes a small

acceleration, but a larger force causes a larger

acceleration.

PRACTICE PROBLEM

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

ANSWER

• FN: 80 N

(not moving on y-axis so FN = Fw)

• MASS: 8.15 kg (Fw/ag=mass)

• Fnet: 40 N; right

• a: 4.91 m/s² ; right (Fnet/m)

NEWTON’S THIRD LAW

• Newton’s Third Law: If two objects interact, the

magnitude of the force exerted on object 1 by

object 2 is equal to the magnitude of the force

simultaneously exerted on object 2 by object 1,

and these two forces are opposite in direction.

• For every action there is an equal, but opposite

reaction.

NEWTON’S THIRD LAW

• Forces always exist in pairs.

• Action-reaction pair: a pair of

simultaneous equal, but opposite

forces resulting from the

interaction of two objects.

• Field forces also exist in pairs.

http://phyz.org/hewittdrewit/neth.html

INTRO TO NEWTON’S

THIRD LAW

NEWTON’S 3RD LAW

NEWTON’S 3RD LAW

http://www.flippingphysics.com/third-law.html

http://www.flippingphysics.com/third-law-

misconception.html

ACTION-REACTION PAIRS

Newton’s Third Law Pairs

What are the Newton Third Law

Pairs you identified yesterday?

• Station 1: standing from the ground

• Station 2: two spring scales

• Station 3: two lab chairs

• Station 4: Wheels of the car

• Station 5: tennis ball

QUESTION 7

•While driving, Anna observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?

ANSWER 7

• The same force

QUESTION 8• A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is ...

a.greater than the acceleration of the bullet.

b.smaller than the acceleration of the bullet.

c.the same size as the acceleration of the bullet.

ANSWER 8

• B; The forces are equal, but the mass of the bullet is smaller than the gun and the acceleration is inversely proportional to the mass

FRICTION

• Friction occurs because the surface of any object is rough.

• Even surfaces that appear smooth are really covered with microscopic hills and valleys.

http://www.mrteverett.com/physics/newton's%20first%20law%20of

%20motion.asp

FRICTION

• Friction opposes the applied force

• Kinetic friction is the resistive force that opposes the relative motion of two contacting surfaces that are moving past one another.

• Static friction is the resistive force that keeps a stationary object at rest

• Kinetic friction is less than static friction.

FRICTION

• The force of friction is proportional to the normal force.

• The coefficient of friction (µk or s)is the ratio of the force of friction to the normal force acting between two objects.

• FF = µ FN

• µ = FF / FN

• Friction depends on the surfaces in contact.

FRICTION

TYPES OF KINETIC FRICTION• Sliding Friction:

type of friction that

occurs when objects

slide past each

other

• Rolling Friction:

type of friction

when a rounded

object rolls over a

flat surface.

• Rolling < Slidinghttp://www.school-for-champions.com/science/friction.htm

REDUCING UNWANTED FRICTION

• Use low-friction materials

• Lubricants: substances

that are applied to surfaces

to lower the friction

between them.

• Ex) oil, wax, greasehttp://en.wikipedia.org/wiki/Cookware_and_bakeware

BALL BEARINGS

• A bearing can reduce friction by virtue of its

shape, by its material, or by introducing and

containing a fluid between surfaces.

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INCREASING HELPFUL FRICTION• Helpful friction is

increased by making surfaces rougher.

• Friction is also greater if the force pushing the surfaces together is increased.

Putting sand on icy roads. http://quizlet.com/8107954/chapter-10-motion-flash-cards/Paper weights

increase force and therefore increase friction.

Shoe treads increase friction by increasing roughness.

EXIT TICKET

• Explain why the force normal and the force

of gravity on a book sitting on a table are

NOT a Newton’s third law force pair

(action-reaction pair).