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Monday 9/3 Labor Day No Class Pretest 3 posted Monday 9/10 HW 2 due Tuesday Tuesday Wednesday 9/5 HW 3 posted HE 1 posted Wednesday 9/12 Pretest 3 due Pretest 4 posted Thursday Thursday 9/7 Friday Friday 9/13 HW 3 due HE 1 due HW 4 posted Physics 100 Fall 2007 Ramps
Section 1.3 Net Forces Newton's 3rd law Energy Work Physics 100 Fall 2007 Observations About Ramps It's difficult to lift a heavy cart straight up It's easer to push a heavy cart up a ramp The ease depends on the ramp's steepness Gradual ramps involve gentler pushes Gradual ramps involve longer distances
Physics 100 Fall 2007 Questions about Ramps
1. Why doesn't the cart fall through the ramp? 2. Are both cart and ramp pushing on each other? 3. Why is it easier to push the cart up a ramp? 4. Is there a physical quantity that's the same for any trip up the ramp, regardless of its steepness?
Physics 100 Fall 2007 Question 1 Why doesn't the cart fall through the ramp? Why doesn't a ball fall through a table? Is the table pushing up on the ball? How can an upward push prevent falling? Physics 100 Fall 2007 Support Forces A support force prevents the ball from penetrating the table's surface points directly away from the table's surface Forces along surface are friction (ignore for now) Physics 100 Fall 2007 Net Force
The net force on the ball is the sum of all forces on that ball responsible for the ball's acceleration Physics 100 Fall 2007 Net Force Adding up the Forces As it sits on the table, the ball experiences its weight downward a support from the table upward Since the ball isn't accelerating, the sum of forces (i.e., net force) on the ball is zero the support force must balance ball's weight! Since the cart isn't accelerating into ramp, the ramp's support force must keep cart on surface
Physics 100 Fall 2007 Question 2
Are both cart and ramp pushing on each other?
i. Are both ball and table pushing on each other? ii. Is the table pushing on the ball? iii. Is the ball pushing on the table? iv. Which is pushing harder?
Physics 100 Fall 2007 An Experiment
If you push on a friend, will that friend always push back on you? A. Yes B. No Physics 100 Fall 2007 Answer A 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. Physics 100 Fall 2007 Another Experiment
If you push on a friend who is moving away from you, 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
Physics 100 Fall 2007 Answer C Forces Present (Part 1) For the ball resting on the table, the forces are i. On ball due to gravity (its weight) zero ii. On ball due to support from table acceleration 3rd law iii. On table due to support from ball pair These forces all have the same magnitude Where is the other 3rd law pair? Physics 100 Fall 2007 Forces Present (Part 2)
i. On earth due to gravity from the ball 3rd law pair 3rd law pair ii. On ball due to gravity from the earth iii. On ball due to support from table iv. On table due to support from ball Forces 2 and 3 aren't a Newton's 3rd law pair! when equal in magnitude, ball doesn't accelerate when not equal in magnitude, ball accelerates! Physics 100 Fall 2007 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. Forces ALWAYS exist in pairs Physics 100 Fall 2007 ...
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This note was uploaded on 03/26/2008 for the course PHYS 100 taught by Professor Tsui during the Fall '07 term at UNC.
- Fall '07