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 Replace “no force” with “no net force” Slide 4-39

Clicker Question 2 Which of the following is NOT one of the steps used to identify the forces acting on an object? Name and label each force the object exerts on the environment. Name and label each contact force acting on the object. Draw a picture of the situation. Identify “the system” and “the environment.” Name and label each long-range force acting on the object. Answer: A Slide 4-9

Answer Which of the following is NOT on of the steps used to identify the forces acting on an object? Name and label each force the object exerts on the environment. Name and label each contact force acting on the object. Draw a picture of the situation. Identify “the system” and “the environment.” Name and label each long-range force acting on the object. Answer: A 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

Quick Points 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

Example Problem 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 Slide 4-30

Gravitational Force and g Make measurements to make a data table of Fg and mass Graph this relationship What is the significance of the slope? Slide 4-26

Can’t tell what Fg,y is Gravitational Force & Coordinate Systems Slide 4-19

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? Magnetic Force Gravitational Force Normal Force Friction Force 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. 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 = mg Force Diagrams 21