Review  

Weight Apparent weight is the force an object experiences as a result of all the contact forces acting on it, giving it an acceleration

Normal Force For an object resting on a table, the normal force is to the objects weight EQUAL Normal force weight

Questions for today Is it possible for the apparent weight of an object to be more than the actual weight? If so, when would this occur? Is it possible for the apparent weight to be less than the actual weight? If so, when would this occur? If either of these situations are possible, would there be an acceleration? Why or why not?

Hints To answer these questions you will need to be able to identify 1. All forces acting on an object 2. If there is an acceleration on the object

Elevator RIde On an upward elevator ride: When do you feel heavier than normal? When do you feel lighter than normal?

Elevator Ride On a downward ride: When do you feel heavier than normal? When do you feel lighter than normal?

Identify the places in the ride when the spring scale reads a normal value for the mass. Describe the motion for the mass. Are the forces balanced or unbalanced? Identify the places where the spring scale reads heavier than the mass. Which direction is Fnet? Which direction is the acceleration? Draw a free body diagram and label the forces acting on the mass. Identify those places in the ride when the spring scale reads a lighter value. Which direction is Fnet? Which direction is the acceleration? Draw a free body diagram and label the forces acting on the mass.

Example Your mass is 75.0 kg and you are standing on a bathroom scale in an elevator. Starting from rest, the elevator accelerates upward at 2.0 m/s2 for 2.0 s and then continues at a constant speed. Is the scale reading during acceleration greater than, equal to, or less than the scale reading when the elevator is at rest? (Hint: 1. Draw a diagram of what is happening. 2. Determine the force of weight when the elevator is motionless. 3. Determine the net force of the elevator by summing the forces in action.)

Everyday Forces

Bell Work What is the difference between mass and weight? On Planet X a 50 kg barbell can be lifted by exerting a force of 280 N. What is the acceleration of gravity on Planet X? If the same barbell were lifted on Earth, what minimal force is needed? In order for an object to accelerate, what must be true of the net force? State Newton‘s first law.

Terms to Know Weight Terminal velocity Drag Air friction

Weight  

Example Mass = 59.7 kg Force on moon: 95.5 N On Earth, a scale shows that you weigh 585 N. What is your mass? What would the scale read on the moon, where gravity is 1.67 m/s2? Mass = 59.7 kg Force on moon: 95.5 N

What is Drag? Drag is the force exerted by a fluid on the object moving through the fluid Air is a fluid Drag is dependent on the motion of the object, the properties of the object, and the properties of the fluid. Drag is a resistance to motion, so it acts opposite of motion (air friction)

What effects drag? Does surface area affect drag? Yes . . . Drag will substantially effect light objects with a large surface area What effect does a parachute have on a skydiver?

What effects drag? Does mass affect drag? Complete the coffee filter activity What can you conclude about the drag force on heavier objects as they fall? Heavier objects are less effected by the drag force http://www.youtube.com/watch?v=kNED5Rz qxOo

Terminal Velocity Terminal Velocity is the constant velocity that is reached when the drag force equals the force of gravity Draw a force diagram showing the terminal velocity for the coffee filters

Drag and Aerodynamics Aerodynamics is the study of forces and the resulting motion of objects through the air. Drag also effects objects not in free fall Air exerts force in all directions http://www.nsf.gov/news/special_reports/so s/drag.jsp

Force interactions We have already said that forces act in pairs What pairs have we seen today? The idea that forces act in pairs is summarized in Newton’s 3rd Law For every action, there is an equal and opposite reaction.

Identifying Force Pairs Identify the system of forces . . . What objects are acting on each other? The interaction pair act in opposite directions with equal magnitude

Tug of War Tension is a specific name for the force exerted by a string or rope What is true of the TENSION in the rope if the two teams are not moving? What other force pairs can be identified? When a team wins a game of tug of war, what has happened to the net force?

Horse and Wagon Farmer Brown hitches Old Dobbin to his wagon one day, then says, "OK, Old Dobbin, let's go!" Old Dobbin turns to Farmer Brown and says, "Do you remember back in high school, when we took Physics together?" "Yes, I do. We were lab partners in that class, and we had a lot of fun." says Farmer Brown. "Ah, yes! Those were the good old days, all right!", says Old Dobbin, "You do remember Newton's Three Laws, of course, which tell how all objects move?" "Yes, I do! I remember that Newton's Laws of Motion are the cornerstone of mechanics. Now, let's get this wagon moving!" "Do you remember how Newton's Third Law says that every action force has an equal and opposite reaction force?", says Old Dobbin, ignoring Farmer Brown's impatience. "Yes, I do." says Farmer Brown, sensing trouble. "Newton's Third Law says that if I pull on the wagon, the wagon exerts an equal and opposite force on me. Don't you agree?", asks Old Dobbin. "Yes... but..." "If these two forces are equal and opposite, they will cancel, so that the net force is zero, right?", argues Dobbin. "Well, I suppose so," stammers Farmer Brown. "The net force is always the important thing. If the net force is zero, then Newton's Second Law (and Newton's First Law, too) says that the acceleration of the wagon must be zero." "Yes, I remember Newton's Second Law very well, Old Dobbin.", says Farmer Brown, hopefully. "This physics discussion is certainly interesting, but let's get going!" "But that's the point!", objects Old Dobbin, "If the wagon's pull is always equal and opposite of my pull, then the net force will always be zero, so the wagon can never move! Since it is at rest, it must always remain at rest! Get over here and unhitch me, since I have just proven that Newton's Laws say that it is impossible for a horse to pull a wagon!" At this point, Farmer Brown throws up his hands in dismay and turns to you. "Please help me!" he says, "I really should have paid more attention in physics class! I know that Newton's Laws are correct, and I know that horses really can pull wagons. There has to be an error in Old Dobbin's argument, but what is it? How can I convince Old Dobbin that if he pulls on the wagon, it will move?" So, what is your reply?