Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Lecture 8

Chapter 6

Newton LawsFriction forces

Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsI

Physics I

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Today we are going to discuss:

Chapter 6:

Newton 1st and 2nd Laws Kinetic/Static Friction: Section 6.4

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Newton’s laws

In 1687 Newton published his three laws in his Principia Mathematica.

These intuitive laws are remarkable intellectual achievements and work spectacular for everyday physics

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Newton’s 1st Law (Law of Inertia)

In the absence of force, objects continue in their state of rest or of uniform velocity in a straight linei.e. objects want to keep on doing what they are already doing

It helps to find inertial reference frames

Why do we need N.1st law?

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Inertial reference frame

An inertial reference frame is one in which Newton’s first law is valid.

Inertial reference frame– A reference frame at rest – Or one that moves with a constant velocity

This excludes rotating and accelerating frames (non-inertial reference frames), where Newton’s first law does not hold.

How can we tell if we are in an inertial reference frame? - By checking if Newton’s first law holds!

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

A physics student cruises at a constant

velocity in an airplane.A ball placed on the

floor stays at rest relative to the airplane.

Inertial Reference Frame (Example)

There are no horizontal forces on the ball, so when .

Newton’s first law is satisfied, so this airplane is an inertial reference frame.

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Newton’s first law is violated, therefore this airplane is not an inertial reference frame.

Non-Inertial Reference Frames (Example)A physics student is

standing up in an airplane during takeoff.

A ball placed on the floor rolls toward the back of the plane.

There are no horizontal forces on the ball, and yet the ball accelerates in the plane’s reference frame.

In general, accelerating reference frames are not inertial reference frames.

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Newton’s Second Law of MotionNewton’s second law is the relation between acceleration and force. Acceleration is proportional to force and inversely proportional to mass.

amF

2smkgN

New unit of force Newton

• It takes a force to change either the direction or the speed of an object.

• More force means more acceleration;

• the same force exerted on a more massive object will yield less acceleration.

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Mass

Mass is the measure of inertia of an object, sometimes understood as the quantity of matter in the object. In the SI system, mass is measured in kilograms.

Mass is not weight.

Mass is a property of an object. Weight is the force exerted on that object by gravity.

If you go to the Moon (gravitational acceleration is about 1/6 g),you will weigh much less. Your mass, however, will be the same.

amF Proportionality defines mass of object.

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

What average net force is required to bring a 1000-kg car to rest from a speed of 20 m/s within a distance of 100 m? Assume acceleration is constant

Force to stop a car.Example

When the fly hit the truck, it exerted a force on the truck (only for a fraction of a second). So, in this time period, the truck accelerated (backward) up to some speed. After the fly was squashed, it no longer exerted a force, and the truck simply continued moving at constant speed.

ConcepTest Truck on Frozen LakeA very large truck sits on a frozen lake. Assume there is no friction between the tires and the ice. A fly suddenly smashes against the front window. What will happen to the truck later after the collision?

A) it is too heavy, so it just sits there

B) it moves backward at constant speed

C) it accelerates backward

D) it moves forward at constant speed

E) it accelerates forward

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

FrictionStatic Kinetic

Kinetic/Static Friction: Section 6.4

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

A new contact force…friction

Friction is always present when two solid surfaces slide along each other.

The microscopic details are not yet fully understood.

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Static friction is the contact force that keeps an object “stuck” on a surface, and prevents relative motion.

The static friction force is directed tangent to the surface.

Static friction points opposite the direction in which the object would move if there were no static friction.

Friction Force

When an object slides along a surface, the surface can exert a contact force which opposes the motion.

This is called kinetic friction.

The kinetic friction force is directed tangent to the surface, and opposite to the velocity of the object relative to the surface.

Kinetic friction tends to slow down the sliding motion of an object in contact with a surface.

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Friction. Static-vs-Kinetic

m

mg

m

NFA

mg

N

mg

m

NFA

fsfs

no motionstatic

no motionstatic

Static. Limit.Motion is about to start

Fx max0 sA fF

As Ff

0

ASs FNf

frf

AF

NS

45°

m

NFA

fkSliding.

Friction (kinetic) is constantNf Kk

As Ff

Kinetic friction

NKk Ff Static friction

frictionstaticoftcoefficienS frictionkinetic

oftcoefficienK

NS

Nf Ss

If a horizontal force is applied on an object , the object does not move.It means a second force, Static Friction, must be opposing the applied force.

An object remains at rest as long as fs < fs max.

The object just begins to slip when fs = fs max.

End of the lecture

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Coefficients of FrictionNote that, in general, μs > μk.

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Summary

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Thank you

The static friction force has a maximum of sN = 50 N. The tension in the rope is only 30 N. So the pulling force is not big enough to overcome friction.

ConcepTest Will It move?A) moves to the left

B) moves to the right

C) the box does not move

D) moves down

E) moves up

A box of weight 100 N is at rest on a floor where s = 0.5. A rope is attached to the box and pulled horizontally with tension T = 30 N. Which way does the box move?

T=30Nm

Follow-up: What happens if the tension is 55 N?

NSfr FF NmgS 501005.0

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

mg

N

Fx max

Fy mayGiven: μs, with frictionFind: θ

A block lies on an inclined plane with μs. The angle of the incline is slowly increased. At what angle does the block start to move?

fs0cos mgNFy

cosmgN

Nf Ss cosmgS

0sin sx fmgF

mgsin smgcos 0mgsin smgcos

sincos

tan s

0

About to start moving

By measuring angle, when an object gets lose, we can find static friction coefficient

Example How to measure μS?

Department of Physics and Applied PhysicsPHYS.1410 Lecture 8 Danylov

Demonstration

Two Interleaved BooksSimply lay the pages of two phone books on top of each other one by one before

attempting to pull them apart.

FA Fperp

Fpull

It increases friction

It pulls books apart

Ffr

NxFfr

http://www.youtube.com/watch?v=AHq82D78Igg

Nf Ss