Forces and Newtons Laws Part 3 Everyday Forces

Forces and Newton’s Laws Part 3: Everyday Forces of Weight, Normal, and Friction

Review • What do we already know? • Write out Newton’s 1 st, 2 nd and 3 rd Laws • 1 st Law: An object at rest stays at rest and an object in motion stays in motion unless a force is applied. • 2 nd Law: The net force on an object is equal to its mass times its acceleration. Fnet = ma • 3 rd Law: Forces have equal magnitudes and opposite directions.

Everyday Forces • Everyday forces are those that we can observe in our surroundings. • They happen everyday - whether we observe them or not! • There are three that we will learn about: Weight, Normal Force & Friction.

Weight • Weight is the magnitude of the gravitational force exerted on an object. • Weight is how much gravity pulls on our mass. • Weight is an object’s mass times the acceleration of gravity. • Equation: W = m • g (Unit is Newtons) • If you go to the moon will your mass or weight be different?

Example #1 - Weight • An object has a mass of 55 kg. Calculate the objects weight: – On Earth - g = 9. 81 m/s 2 – On Venus - g = 8. 87 m/s 2 – On Mercury - g = 3. 59 m/s 2 • Where does the object weigh the most? The least?

Normal Force • Normal Force, FN, is one that acts on an object, perpendicular to the surface the object is on. • The object must be on a surface. • Newton’s 3 rd Law pair to the Weight.

Why the “Normal Force”… • And not the Perpendicular Force? • One definition of “Normal” is perpendicular. • The FN is ALWAYS PERPENDICULAR to the SURFACE.

Example #2 - Normal Force • A book with weight 100 N is resting on a table. • What is the Normal Force exerted on the book by the table? • Hint: The Normal Force and Weight are 3 rd Law pairs

Friction • Friction is a force that opposes an applied force • Oppose: to actively resist

Friction continued… • Friction exists when an object is moving - called Kinetic Friction • Kinetic - resulting from motion • Friction attempts to keep an object from moving when a force is applied called Static Friction • Static - lacking motion

Example #3 - Friction • Rub your hands together. What happens? • This heat is a result of Friction opposes your hands motion, and this causes energy to be changed into heat. • Watch Miss Probst attempt to move the block. Is she applying a force? Why is it not moving?

Activity - Part 1 • You will identify objects in the classroom that exhibit the everyday forces. • Follow along in your activity sheet. Be sure to find an object for each quesiton!

Free Body Diagrams: What are they? • A free body diagram is a simple sketch that shows the forces acting on an object • Free body diagrams do not show the forces the object exerts on something else!

How to use Free Body Diagrams? • Before you can draw a free body diagram you must: – Identify all forces acting on the object – Know what direction those forces act • Free body diagrams help us to visualize the forces acting on an object. • This is important because we will need to “Sum the forces” for Newton’s 2 nd Law problems!

Drawing Free Body Diagrams • Steps for drawing free body diagrams: 1. Identify all the forces acting on your object 2. Draw a simple sketch of your object 3. Forces are drawn from the center of the object (objects center of mass) 4. Forces are drawn in the direction they act 5. Forces are labeled with correct magnitude if you know it

Example #4 - Free Body Diagram • You have a calculator sitting on the table in front of you. Draw a free body diagram to illustrate the forces on the calculator. • Follow the steps in your note taking guide to solve!

Example #4 - Free Body Diagram Continued • Now you begin to push the calculator across the table to a friend. Draw the free body diagram to illustrate the forces on the calculator. • Follow the steps in your note taking guide to help!

Activity - Part 2 • For each of the objects you found in Activity Part 1 you will draw the Free Body Diagram. • Follow the steps in your note taking guide to help.