Ramps Ramps Yes Yes Y Y A. A. No No B. B. Ramps 3 Ramps - PDF document

Ramps 1 Ramps 2 Introductory Question Introductory Question Can a ball ever push downward on a table with Can a ball ever push downward on a table with � a force greater than the ball’s weight? a force greater than the ball’s weight? Ramps Ramps Yes Yes Y Y A. A. No No B. B. Ramps 3 Ramps 4 Observations About Ramps Observations About Ramps 4 Questions about Ramps 4 Questions about Ramps � It’s difficult to lift a heavy cart straight up It’s difficult to lift a heavy cart straight up � Why doesn’t the cart fall through the ramp? Why doesn’t the cart fall through the ramp? � It’s easer to push a heavy cart up a ramp It’s easer to push a heavy cart up a ramp � Are the cart and ramp pushing on each other? Are the cart and ramp pushing on each other? � The ease depends on the ramp’s steepness The ease depends on the ramp’s steepness � Why is it easier to push the cart up a ramp? Why is it easier to push the cart up a ramp? � Gradual ramps involve gentler pushes Gradual ramps involve gentler pushes � Is there a physical quantity that’s the same for Is there a physical quantity that’s the same for any trip up the ramp, regardless of its steepness? any trip up the ramp, regardless of its steepness? � Gradual ramps involve longer distances Gradual ramps involve longer distances Ramps 5 Ramps 6 Question 1 Question 1 Support Forces Support Forces � Why doesn’t the Why doesn’t the cart cart fall through the fall through the ramp ramp? ? � A support force A support force � � Why doesn’t a Why doesn’t a ball ball fall through a fall through a table table? ? � prevents the ball from penetrating the table’s surface prevents the ball from penetrating the table’s surface � Is the table pushing up on the ball? Is the table pushing up on the ball? � points directly away from the table’s surface points directly away from the table’s surface � How can an upward push prevent falling? � H How can an upward push prevent falling? H n n p n n p rd p rd p h pr h pr nt f llin ? nt f llin ? � Forces F Forces along along surface are friction ( l surface are friction ( ignore for now) f f f i i f i i ( ( ignore for now) 1

Ramps 7 Ramps 8 Adding up the Forces Adding up the Forces Question 2 Question 2 � As it sits on the table, the ball experiences As it sits on the table, the ball experiences � Are the Are the cart cart and and ramp ramp pushing on each other? pushing on each other? � � its weight downward its weight downward � Are the Are the ball ball and and table table pushing on each other? pushing on each other? � a support from the table upward a support from the table upward � Is the Is the table table pushing on the pushing on the ball ball? ? � Since the ball isn’t accelerating, Si Si Since the ball isn’t accelerating, h b ll i h b ll i ’ ’ l l i i � Is the Is the ball I I h b ll h ball pushing on the b ll pushing on the table hi hi h h table? bl ? bl ? ? � the the sum sum of forces (the of forces (the net force) net force) on the ball is zero on the ball is zero � Which is pushing harder? Which is pushing harder? � so the support force must balance ball’s weight! so the support force must balance ball’s weight! � Since cart isn’t accelerating into ramp, Since cart isn’t accelerating into ramp, � the ramp’s support force must keep cart on surface the ramp’s support force must keep cart on surface Ramps 9 Ramps 10 Clicker Question Clicker Question Newton’s Third Law Newton’s Third Law If you push on a friend, will that friend If you push on a friend, will that friend always always � For every force that one object exerts on a For every force that one object exerts on a � push back on you? push back on you? second object, there is an equal but oppositely second object, there is an equal but oppositely directed force that the second object exerts on directed force that the second object exerts on the first object the first object the first object. the first object. Yes Yes Y A. A. No No B. B. Ramps 11 Ramps 12 Clicker Question Clicker Question Forces Present (Part 1) Forces Present (Part 1) If you push on a friend who is moving away If you push on a friend who is moving away For the ball resting on the table, the forces are For the ball resting on the table, the forces are � � from you, how will the force you exert on your from you, how will the force you exert on your friend compare to the force your friend exerts friend compare to the force your friend exerts On On ball ball due to due to gravity gravity (its weight) (its weight) zero 1. 1. on you? on you? on you? on you? acceleration 3 rd law On ball On ball due to support from due to support from table table 2. 2. pair On table On table due to support from due to support from ball ball 3. 3. You push harder You push harder A. A. These forces all have the same magnitude These forces all have the same magnitude � Your friend pushes harder Your friend pushes harder B. B. rd law pair? Where is the other 3 Where is the other 3 rd law pair? � The forces are equal in magnitude The forces are equal in magnitude C. C. 2

Ramps 13 Ramps 14 Forces Present Forces Present (Part 2) Introductory Question (revisited) Introductory Question (Part 2) (revisited) Can a ball ever push downward on a table with Can a ball ever push downward on a table with � 3 rd law a force greater than the ball’s weight? a force greater than the ball’s weight? On earth On earth due to gravity from the due to gravity from the ball ball 1. 1. pair On On ball ball due to gravity from the due to gravity from the earth earth 2. 2. Yes Yes Y Y 3 rd law On On ball ball due to support from due to support from table table A. A. 3. 3. No No On On table table due to support from due to support from ball ball pair B. B. 4. 4. rd law pair! Forces 2 and 3 aren’t a Newton’s 3 rd Forces 2 and 3 aren’t a Newton’s 3 law pair! � when equal in magnitude, ball doesn’t accelerate when equal in magnitude, ball doesn’t accelerate � when not equal in magnitude, ball accelerates! when not equal in magnitude, ball accelerates! � Ramps 15 Ramps 16 Misconception Alert Misconception Alert Question 3 Question 3 � While the forces two objects exert on one While the forces two objects exert on one � Why is it easier to push the cart up a ramp? Why is it easier to push the cart up a ramp? another must be equal and opposite, the net another must be equal and opposite, the net � Why does the ramp’s steepness matter? Why does the ramp’s steepness matter? force on each object can be anything. force on each object can be anything. � Why does the cart tend to roll downhill? Why does the cart tend to roll downhill? � Each force within an equal � Each force within an eq al Each force within an equal-but Each force within an eq al b t but-opposite pair is b t opposite pair is opposite pair is opposite pair is exerted on a different object, so they don’t exerted on a different object, so they don’t cancel directly. cancel directly. Ramps 17 Ramps 18 Forces on a Cart on a Ramp Forces on a Cart on a Ramp Clicker Question Clicker Question � If I pull uphill on the cart just hard enough to If I pull uphill on the cart just hard enough to balance the downhill ramp force, the cart will balance the downhill ramp force, the cart will support force � be motionless. b be motionless. b i i l l ramp force (sum) f ( ) � move at constant velocity. move at constant velocity. weight � Ramp force causes cart to accelerate downhill Ramp force causes cart to accelerate downhill 3