Presentation on theme: "- Homework 1.2 due on Thursday, Jan. 28. - Web page for class is: -Bring i-clicker to class -You are."— Presentation transcript:
- Homework 1.2 due on Thursday, Jan Web page for class is: -Bring i-clicker to class -You are allowed 30 missed points in the i-clicker total score (~ 160 points) -Last day to add class: Jan. 27 -Homework solutions are posted on web page (will be password protected) Announcements:
PHY110 TUTOR SESSIONS Tutor: Jillian Bjerke & Maggie Baldwin Session 1: Mo, 4-6 pm (Jill) Session 2: We, 4-6 pm(Jill) Session 3: Th, 5-7 pm (Maggie) All tutorial session will be in Olin 101 (class room). The tutor sessions in semesters past were very successful and received high marks from many students. All students are encouraged to take advantage of this opportunity. There are also private tutors available, contact Judy Swicegood in the Physics office (Olin 100)
Chapter 1: The laws of motion, Part I First two chapters: Introduce the “language of physics” Subsequent chapters: Explore objects and underlying physical concepts - Reading assignment for today: Chapter Reading assignment for next class:Chapter Homework 1.3 (Calli Nguyen): (due Tuesday, Feb. 2, in class): Exercises: 20, 23, 24, 27, 34, 35, 39 Problem: 8, 9, 10, 13, 14, 15, 16, 22
Chapter 1.3 Newton III, energy,work, ramps - Tug of war - Lifting stuff - Carrying stuff - Using a ramp - Newton’s third law: every action has an equal and opposite reaction - Net force - Work and energy - Kinetic energy - Gravitational potential energy - Ramps Demos and Objects Concepts
i-clicker question-1: An apple is sitting on your desk. Which statements are true. A.Only the force of gravity acts on the apple. B.At least one more force acts on the apple. C.Not enough information. D.It is not possible that more than one force acts on an object. E.A & D.
Type of Force On ball: Weight (gravity): down Support force: up –Prevents something from penetrating a surface –Points directly away from that surface Ball resting on table: What kind of forces can we see? The net force on the apple is zero
Physics Concept Net Force –The sum of all forces on an object. –Determines object’s acceleration. Tug-of-war
Newton’s Third Law For every force that one object exerts on a second object, there is an equal but oppositely directed force that the second object exerts on the first object. F 12 = -F 21
If you push on a friend (on ice, no friction), how will the force you exert on your friend compare to the force your friend exerts on you? A. You push harder B.Your friend pushes harder C.The forces are equal in magnitude i-clicker question-2:
Forces Present: Ball resting on table (revisited): What kind of forces can we see? 1.On earth due to gravity from the ball 2.On ball due to gravity from the earth (weight) 3.On ball due to support from table 4.On table due to support from ball Pair Since the ball doesn’t accelerate, 2 and 3 must cancel perfectly
Two Crucial Notes: While the forces two objects exert on one another must be equal and opposite, the net force on each object can be anything. Each force within an equal-but-opposite pair is exerted on a different object, so they don’t cancel directly.
If the force of the cart on the donkey is the same (but oppositely directed) as the force of the donkey on the cart, why does it move?
Why does the Donkey Move? F=ma so a donkey = F (on donkey)/ m donkey Force of cart on donkey Force of the ground on donkey Net Force on donkey = F ground on donkey +F cart on donkey Force of gravity +F gravity
Why does the Cart AND Donkey Move? F=ma so a cart+donkey = F (on cart+donkey)/ m cart+donkey Force of the ground on donkey Net Force on donkey = F ground on donkey +F ground on cart Force of gravity +F gravity Force of the ground on cart
i-clicker question-3: The diagram shows a top view of three people pulling on a donkey (disk) of mass 100 kg. Ignoring other forces (friction, etc), what is the acceleration of the donkey? A.0.5 m/s 2 up; 1.0 m/s 2 left B.1.0 m/s 2 up; 2.0 m/s 2 left C.0 m/s 2 up; 1.0 m/s 2 left D.50.0 m/s 2 up; 100 m/s 2 left E.0 m/s 2 up; 0 left 100 N 50 N 100 N
Sum of Floor Forces Force Due to Gravity = m g F t = F f + F g So F t = m a = 0 and a = 0!
Force due to gravity = weight Component into ramp Force from ramp Net Force = m a Net force on piano
small F medium F large F Component into ramp Force from ramp Weight = mg Net Force Net downwards force on piano depends on angle
2000N! Challenge: Get the piano up to the second floor! Ramp or Straight Lift? RAMP Straight Lift
Observations About Ramps Lifting an object straight up is often difficult Pushing the object up a ramp is usually easier The ease depends on the ramp’s steepness Shallow ramps require only gentle pushes You seem to get something for nothing How does distance figure in to the picture?
Physical Quantities: Energy and Work Energy –A conserved quantity –The capacity to do work Work –The mechanical means of transferring energy. –work = force · distance (where force and distance are in the same direction) Unit of energy & work: 1 N·m = 1J (Joule) = cal
Energy and Work Energy: the capacity to make things happen Work: is the transference of energy Forms of Energy Kinetic Energy – Energy of motion Potential Chemical Energy Gravitational Nuclear Energy Thermal Energy
Energy and Work Energy: the capacity to make things happen Work: is the transference of energy F=mg=2000N h Work = Force x Distance W = F d W = Fh = mgh Work is the transfer of energy Where did energy of lifters go? into potential energy of piano! Gravitational potential energy = m g h
Component into ramp Force from ramp Weight = mg Net Force h = mg h W W = F d Work on piano = change in energy of piano = same!
A man loads a refrigerator onto a truck using a ramp. He claims he would be doing less work if the length of the ramp would be longer. Is this true? A.Yes B.No C.Not enough information i-clicker question -4
Work Lifting a Piano Going straight up: work = force · distance Going up ramp: work = force · distance The work is the same, either way!
Black board example 1.3.1: 2000N!RAMP Straight Lift 1.Superman is lifting a piano (mass 100 kg) straight up onto a 1 m high platform. How much work is he doing? 2.You are pushing the same piano along a 10 m long (frictionless) ramp onto the 1m platform. How much work are you doing? 3.How much force do you apply to the piano? 4.How much energy did the piano gain by being lifted 5.Where did that energy come from? i-clicker-4 A.100 J B.200 J C.98 J D.1 J E.980 J
How much work is done when just holding up an object?
What is the work done when lifting? Strongest man lifting up 140 kg boulder by 1 m.
Black board example Angus is pulling a 10,000 kg truck with all his might (2000N) on a frictionless surface for 10.0 m. How much work is the man doing? What kind of energy does the truck gain? What is the speed of the truck if he pulls for ten seconds?