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Physics 102-002 Announcements Clickers – should have by now WebAssign – must have Account Codes by tomorrow (Tues, Feb 6) Exam #1 Wednesday –Covers Chapters 1-4
Class Schedule 1/17Chapter 1Introduction, About Science 1/22Chapter 2Newton’s First Law 1/24Chapter 3Linear Motion 1/29Chapter 4Newton’s Second Law 1/31Midterm Exam 1 2/5Chapter 5Newton’s Third Law 2/7Chapter 6Momentum
Chapter 5 Newton’s Third Law Forces and Interactions Newton’s Third Law –Defining your system –Action and reaction on different masses Summary of Newton’s Three Laws Vectors
Force and Interactions Force: A push or pull exerted on an object Examples: Hand against a wall. Book on a table. Prize fighter and tissue paper Earth and you. Interacting objects exert forces on each other. Force of skater B on skater A F BA Force of skater A on skater B F AB You can’t exert a force on something unless the other objects exerts and equal and opposite force on you!
Question 1 While driving down the road, an unfortunate bug strikes the windshield of a bus. Quite obviously, this is a case of Newton's third law of motion. The bug hit the bus and the windshield hit the bus. Which of the two forces is greater: a.the force on the bug? b.the force on the bus?
Question 2 A gun recoils when it is fired. The recoil is the result of action- reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... a.greater than the acceleration of the bullet. b.smaller than the acceleration of the bullet. c.the same size as the acceleration of the bullet.
Newton’s Third Law ON impact, the interaction forces between the blue ball and the yellow ball STOP the blue ball and MOVE the yellow ball. To every action there is an equal and opposite reaction.
Defining the “System” System: Gives the boundaries beyond which forces are “external” Example in the book: The orange and the apple. Here, the orange is the system: an external force is acting on the orange, causing it to accelerate. The apple provides the external force to accelerate the system to the right. Notice that the force exerted by the apple on the orange is still equal and opposite to the force of the orange on the apple. If the system is the apple AND the orange: an external force is still acting on the system to accelerate it to the right. The force is provided by the friction between the apple’s “feet” and the ground.
Action and Reaction on Different Masses The action and reaction forces are equal and opposite, no matter whether the masses are the same or not! The earth pulls “downward” on the rock. The falling rock pulls “upward” on the earth (with the exact same amount of force). The force of attraction between the 2 “planets” is equal and opposite regardless of the mass difference. (Both planets attract each other) The force exerted on the cannonball is the same as the force exerted on the cannon. But the cannonball accerates more because of its much lower inertia (or mass). Cannonball:Cannon: (Smaller mass, Larger acceleration) (Larger mass, Smaller acceleration)
Newton’s 3 Laws: Summary Newton’s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion at constant speed and in the same direction unless acted on by an unbalanced force. Newton’s Second Law F = ma Newton’s Third Law To every action there is an equal and opposite reaction.
Vectors Vectors have magnitude and direction. They add or subtract depending on their directions. Parallel vectors are pretty simple: 50 N = 100 N + 50 N = + 0 N What if the vectors are NOT parallel: Example: What if I walked 16 km East and 12 km North The result is a NET movement of 20 km Northeast 16 km East 12 km North 20 km Northeast Component Vectors Resultant Vector
Vectors continued Resultants of vectors applet http://www.walter-fendt.de/ph14e/resultant.htm Vector components applet http://id.mind.net/~zona/mstm/physics/mechanics/vectors/ components/vectorComponents.html All vectors (force, velocity, acceleration, etc) add the same way. The 30N and 40N forces add to get a resultant force of 50N. The DIRECTION of the resultant 50N force is given by the diagonal of the “parallelagram” (The Parallelagram Rule) Nellie illustrates the parallelagram rule. Note that the 2 tensions add to more than Nellie’s weight! Support Force Interactive Figure
Vectors continued Velocity vectors add: The crosswind of 60 km/h adds to the planes velocity of 80 km/h to blow the plane offcourse at 100 km/h. Some example resultant velocities. When you throw a rock, it’s velocity has a horizontal and vertical component
Question 3 The 3 boats shown are crossing a river. Which of the boats will follow the shortest path to the opposite shore? Velocity of the boat Velocity of the river