Physics 11 Advanced Mr. Jean March 19th, 2012

The plan: Video clip of the day Review of Newton’s first two laws of motion. Check solutions to weight questions. Unit Test Returned on Tuesday if everyone has written Newton’s 3rd Law Free Body Diagrams

Newton’s Second Law: “to move an object with mass you need a force.” the acceleration of an object is directly proportional to the net force on it and inversely proportional to its mass. Force = (mass)(acceleration) F = m a

International Units (SI) Quantity Symbol SI Unit Force Mass Acceleration F m a N (Newtons) Kg (Kilograms) m/s2 (metres per second squared)

F = m a

A net force acting on an object causes it to accelerate. The larger the mass of an object, the smaller the acceleration. Thus a massive object has more inertia than a less massive object.

Unit of force - a force that causes a mass of one kg to accelerate at a rate of one meter per second squared is one newton (N). F = ma = (1.00kg)(1.00m/s2 ) = 1.00N 1N = 1kgm/s2

Common misconception: WEIGHT DOES NOT EQUAL MASS!

Weight: An objects weight is Fg is the product of its mass m, and the acceleration due to gravity, g. Fg = mg Fg = force of gravity in newtons (N) m = mass in kilograms (kg) g = acceleration due to gravity (m/s2)

Check Solutions: P. 137 Questions 1-4

Demonstration: Weight & Newton Meter

Newton’s Third Law: “For every action there is an equal and opposite reaction.” when one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude but opposite in direction.

Free Body Diagrams: VERY IMPORTANT: Free-body diagrams are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation. These diagrams will be used throughout our study of physics.

Free Body Diagrams (FBD): There will be cases in which the number of forces depicted by a free-body diagram will be one, two, or three. Diagrams are to depict all the forces that exist for that object in the given situation.

Examples of FBD:

Key pieces to a FBD 1) Start with the object of interest. Place a rectangle at the center of your page to represent this object. 2) Lay out all forces being experienced by the object. Draw neat straight lines representing the forces. 3) List variables on the right hand side so people reading your diagrams know what you are talking about.

Let’s create a force diagram for the picture of Wile E. Coyote.

Force diagrams to try: http://www.physicsclassroom.com/class/newtlaws/u2l2c.cfm

Example Questions 1: What net force is required to accelerate a 1500.00 kg race car at 3.00 m/s2 [E] on a frictionless surface?

Example Questions 2:

Forces are vectors: Forces are vectors. To understand the effects of forces in two directions, we assign signs. If we sum up the forces acting on an object, we can find the net force acting on an object.

Normal Force: The normal force is the component, perpendicular to the surface of contact. Example: the surface of a floor is preventing the object from penetrating the surface. The floor supplies the normal force.

Surface Friction: The magnitude of the force of surface friction is the product of the coefficient of friction and the magnitude of the normal force. The direction of the force of friction is always opposite to the direction of the motion.