Physics 151 Week 8 Day 1 Topics System Schema and Force Diagrams Newton’s First Law of Motion Gravitational Force and g Working with tilted coordinate.

Physics 151 Week 8 Day 1

Topics System Schema and Force Diagrams Newton’s First Law of Motion Gravitational Force and g Working with tilted coordinate systems Applying Newton’s 1st Law of Motion

Galileo (1564-1642)

Developed the idea of experimental science

Re-examined natural motion of objects and how objects move

Made astronomical observations that challenged Earth-centered solar system model.

Newton (1642-1727)

Newton's work based on experiments of how objects interact.

His laws of motion and law of gravity described how all objects interact with each other.

Summary

Slide 4-39

Replace “no force” with “no net force”

Clicker Question 22. Which of the following is NOT one of the steps used to identify

the forces acting on an object?

A. Name and label each force the object exerts on the environment.

B. Name and label each contact force acting on the object.C. Draw a picture of the situation.D. Identify “the system” and “the environment.”E. Name and label each long-range force acting on the

object.

Slide 4-9

Answer2. Which of the following is NOT on of the steps used to identify

the forces acting on an object?

A. Name and label each force the object exerts on the environment.

B. Name and label each contact force acting on the object.C. Draw a picture of the situation.D. Identify “the system” and “the environment.”E. Name and label each long-range force acting on the

object.

Slide 4-10

Newton's First Law of Motion

DEMO - Cart at rest - Pushing the cart on track

DEMO - Hoverpuck at rest - Hoverpuck in motion

How much force is need to make an object move with constant velocity?

How does an object move with a constant net force?

What Causes Motion?

In the absence of any forces acting on it, an object will continue moving forever. Motion needs no “cause.”

Slide 4-8

Newton's First Law of Motion

Every object continues in a state of rest or a state of motion with a constant speed in a straight line unless acted on by an unbalanced force.

or

Velocity = constant if and only if (IFF) Fnet = 0

Seat Belts: An Application of Newton’s First Law

Slide 4-16

Newton's First Law of Motion Demonstrations

DEMO - Smash the HAND

DEMO - Tablecloth

DEMO - Pull the String

• Free-body diagram is another name for a force diagram

• Weight force is another name for gravitational force• All objects exert gravitational forces on one another

• These forces are too small to be noticed unless one object is at least the size of the Sandia Mountains

• Pay attention to the lines indicating motion. • If there are no lines indicating motion and there is nothing

in the wording that indicates the object is moving, do not assume the object is moving

• Assume Drag forces are negligible unless otherwise stated or implied

• For friction, consider what the system/object would do without friction. Friction will respond to minimize the motion between the surfaces in contact.

• See if Fnet = 0 N without friction

Quick Points

An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. • Is FT greater than, equal to, or less than Fg? Or is there not

enough information to tell?

A => FT > Fg

B => FT = Fg

C => FT < Fg

Example Problem

Slide 4-30

Gravitational Force and gMake measurements to make a data table of Fg and mass

Graph this relationship

What is the significance of the slope?

Slide 4-26

Gravitational Force & Coordinate Systems

Slide 4-19

A.

B.

C.

D.

E. Can’t tell what Fg,y is

Fg, y =−mg

Fg, y =−mgsinθ

Fg, y =−mgcosθ

Fg, y =mgcosθ

Example Problem: What holds it up?Use a free body diagram to determine what type of force holds up a refrigerator magnet.

What type of force holds up a refrigerator magnet?

A. Magnetic ForceB. Gravitational ForceC. Normal ForceD. Friction ForceE. Some other force

What is the magnitude of this force?

Slide 4-26

Fnet = 0 Problems

Determine tension in each cable. (Hint: There is more than one way to define the system.)

5 kg

Fnet = 0 Problems

Determine tension in each cable. (Hint: There is more than one way to define the system.)

Fnet = 0 Problems

The object hung from the cable has a weight of 25 N.

● Write the equation for the sum of the forces in the y-direction.

● What is the tension in the cable?

● What if the angles are smaller?● What if the angles are not symmetrical?

Fnet = 0 Problems

The cable at left exerts a -30 N force.a. Write the equation for the sum of the forces in the

x-direction. What is the value of T2?

b. Write the equation for the sum of the forces in the y-direction. What is the force of gravity acting on the ball?

General Force Model

Newton 0th Law of Motion

Newton 1st Law of Motion

Newton’s 3rd Law of Motion

Diagrams: Other Equations:System Schema Fg = mgForce Diagrams