04 lecture outline part i

Copyright 2012 Pearson Education Inc.

PowerPoint Lectures forUniversity Physics, Thirteenth Edition Hugh D. Young and Roger A. Freedman

Chapter 4

Newtons Laws of Motion

Part I


Goals for Chapter 4

To understand the meaning of force in physics

To view force as a vector and learn how to combine forces

To understand the behavior of a body on which the forces balance:

Newtons First Law of Motion: if there is no NET force on an object it will remain in the same state of motion


Goals for Chapter 4

To learn the relationship between mass, acceleration, and force:

Newtons Second Law of Motion: F = ma

a = F/m

To relate mass (quantity of matter) and weight(force on that matter from gravity)


Goals for Chapter 4

To see the effect of action-reaction pairs:

Newtons Third Law of MotionForce on object a from object b

is EQUAL in magnitude (and opposite in direction) to

Force on object b from object a


What are some properties of a force?

A force is an interaction between two objects, or between an object and its environment

A force is a VECTOR quantity, with magnitude and direction.


There are four common types of forces

#1: The normal force:

When an object pushes on a surface, the surface pushes back on the object perpendicular to the surface.

This is a contact force.



#2: Friction force:

This force occurs when a surface resists sliding of an object and is parallel to the surface.

Friction is a contact force.



#3: Tension force:

A pulling force exerted on an object by a rope or cord.

This is a contact force.



#4: Weight:

The pull of gravity on an object.

This is a long-range force, not a contact force, and is also a field force.


What are the magnitudes of common forces?


Drawing force vectors

Use a vector arrow to indicate the magnitude and direction of the force.


Superposition of forces

Several forces acting at a point on an object have the same effect as their vector sum acting at the same point.


Decomposing a force into its component vectors Choose coordinate system with perpendicular x and y axes.

Fx and Fy are components of force along axes.

Use trigonometry to find force components.


Notation for the vector sum

Vector sum of all forces on an object is resultant of forces

The net force.

1 2 3 r r r r r

LR=F +F +F + = F


Superposition of forces

Force vectors are added using components: Rx = F1x + F2x + F3x + Ry = F1y + F2y + F3y +


Newtons First Law

An object at rest tends to stay at rest, 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.


Newtons First Law

Mathematically,

A body acted on by zero net force moves with constant velocity

and zero acceleration.


Newtons First Law II

In part (a) net force acts, causing acceleration.

In part (b) net force = 0 resulting in no acceleration.


When is Newtons first law valid?

You are on roller skates in a stopped BART car

The car starts to accelerate forwards.

What happens to you?

If no net force acts on you, you maintain a constant velocity (0!)


When is Newtons first law valid?

You are on roller skates in a moving BART car

The car starts to slow -accelerate backwards.

What happens to you?

If no net force acts on you, you maintain a constant velocity


When is Newtons first law valid? If no net force acts on you,

you maintains a constant velocity (a vector!)

But as seen in the non-inertial frame of the accelerating vehicle, it appears that you are being pushed to the outside!

Newtons first law is valid only in non-accelerating inertial frames of reference.


Newtons Second Law If the net force on an object is zero, the object will not accelerate.


Newtons Second Law If the net force on an object is not zero, it causes the object to accelerate.


An object undergoing uniform circular motion

An object in uniform circular motion is accelerated toward the center of the circle.

So net force on object must point toward the center of the circle.


Force and acceleration Magnitude of acceleration

of an object is directly proportional to the net force S on the object.

| | = F/m

rFra


Mass and acceleration

Magnitude of acceleration of an object is inversely proportional to objects mass if net force remains fixed.

| | = F/mra


Newtons second law of motion

The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to the mass of the object.

The SI unit for force is the newton (N).

1 N = 1 kgm/s2

m=r rF a


Using Newtons Second Law Ex. 4.4 Worker pushes box of mass 40 kg, with constant

force of 20N. What is acceleration?


Using Newtons Second Law Ex. 4.4


Using Newtons Second Law IIExample 4.5 Shove bottle of mass 0.45 kg at initial speed of 2.8

m/s a distance of 1 m before it stops. What is the magnitude and direction of force on bottle?


Using Newtons Second Law IIExample 4.5 We know:

Displacement in x = +1.0 m

Initial x velocity = +2.8 m/s

Final x velocity = 0 m/s

THREE THINGS!


Using Newtons Second Law IIExample 4.5

vf2 = vi2 + 2aDx

So

a = (vf2 - vi2)/2Dx

a = - 3.9 m/s2

NOTE a points in the direction of f !

+x

04 lecture outline part i

Documents

pearson education

contact force

components of force

force components

net force

friction force

tension force

field force