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# Ramps 1 Ramps. Ramps 2 Introductory Question Can a ball ever push downward on a table with a force greater than the ball’s weight? Can a ball ever push.

## Presentation on theme: "Ramps 1 Ramps. Ramps 2 Introductory Question Can a ball ever push downward on a table with a force greater than the ball’s weight? Can a ball ever push."— Presentation transcript:

Ramps 1 Ramps

Ramps 2 Introductory Question Can a ball ever push downward on a table with a force greater than the ball’s weight? Can a ball ever push downward on a table with a force greater than the ball’s weight? A. Yes B. No

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Ramps 5 Observations About Ramps It’s difficult to lift a heavy cart straight up It’s difficult to lift a heavy cart straight up It’s easer to push a heavy cart up a ramp It’s easer to push a heavy cart up a ramp The ease depends on the ramp’s steepness The ease depends on the ramp’s steepness Gradual ramps involve gentler pushes Gradual ramps involve gentler pushes Gradual ramps involve longer distances Gradual ramps involve longer distances

Ramps 6 4 Questions about Ramps Why doesn’t the cart fall through the ramp? Why doesn’t the cart fall through the ramp? Are both cart and ramp pushing on each other? Are both cart and ramp pushing on each other? Why is it easier to push the cart up a ramp? Why is it easier to push the cart up a ramp? Is there a physical quantity that’s the same for any trip up the ramp, regardless of its steepness? Is there a physical quantity that’s the same for any trip up the ramp, regardless of its steepness?

Ramps 7 Question 1 Why doesn’t the cart fall through the ramp? Why doesn’t the cart fall through the ramp? Why doesn’t a ball fall through a table? Why doesn’t a ball fall through a table? Is the table pushing up on the ball? Is the table pushing up on the ball?

Ramps 8 Support Forces A support force A support force prevents the ball from penetrating the table’s surface prevents the ball from penetrating the table’s surface points directly away from the table’s surface points directly away from the table’s surface Forces along surface are friction ( ignore for now) Forces along surface are friction ( ignore for now)

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Ramps 10 Net Force The net force on the ball is The net force on the ball is the sum of all forces on that ball the sum of all forces on that ball responsible for the ball’s acceleration responsible for the ball’s acceleration

Ramps 11 Adding up the Forces As it sits on the table, the ball experiences As it sits on the table, the ball experiences its weight downward its weight downward a support from the table upward a support from the table upward Since the ball isn’t accelerating, Since the ball isn’t accelerating, the sum of forces (i.e., net force) on the ball is zero the sum of forces (i.e., net force) on the ball is zero the support force must balance ball’s weight! 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 12 Question 2 Are both cart and ramp pushing on each other? Are both cart and ramp pushing on each other? Are both ball and table pushing on each other? Are both ball and table pushing on each other? Is the table pushing on the ball? Is the table pushing on the ball? Is the ball pushing on the table? Is the ball pushing on the table? Which is pushing harder? Which is pushing harder?

Ramps 13 An Experiment If you push on a friend, will that friend always push back on you? If you push on a friend, will that friend always push back on you? A. Yes B. No

Ramps 14 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. 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.

Ramps 15 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? 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

Ramps 16 Forces Present (Part 1) For the ball resting on the table, the forces are For the ball resting on the table, the forces are 1. On ball due to gravity (its weight) 2. On ball due to support from table 3. On table due to support from ball These forces all have the same magnitude These forces all have the same magnitude Where is the other 3 rd law pair? Where is the other 3 rd law pair? zero acceleration 3 rd law pair

Ramps 17 Forces Present (Part 2) 1. On earth due to gravity from the ball 2. On ball due to gravity from the earth 3. On ball due to support from table 4. On table due to support from ball Forces 2 and 3 aren’t a Newton’s 3 rd law pair! Forces 2 and 3 aren’t a Newton’s 3 rd 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! 3 rd law pair

Ramps 18 Introductory Question (revisited) Can a ball ever push downward on a table with a force greater than the ball’s weight? Can a ball ever push downward on a table with a force greater than the ball’s weight? A. Yes B. No

Ramps 19 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. 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. Each force within an equal-but-opposite pair is exerted on a different object, so they don’t cancel directly.

Ramps 20 Question 3 Why is it easier to push the cart up a ramp? Why is it easier to push the cart up a ramp? How hard must you push on the cart? How hard must you push on the cart?

Ramps 21 Forces on a Cart on a Ramp If you balance ramp force, cart won’t accelerate If you balance ramp force, cart won’t accelerate weight support force ramp force (sum)

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Ramps 23 Balanced Cart on Ramp If you balance the ramp force, If you balance the ramp force, the cart won’t accelerate the cart won’t accelerate the cart will coast – at rest, uphill, or downhill the cart will coast – at rest, uphill, or downhill The more gradual the ramp, The more gradual the ramp, the more nearly weight and support balance the more nearly weight and support balance the smaller the ramp force on the cart the smaller the ramp force on the cart the easier it is to balance the ramp force! the easier it is to balance the ramp force!

Ramps 24 Question 4 Is there a physical quantity that’s the same for any trip up the ramp, regardless of its steepness? Is there a physical quantity that’s the same for any trip up the ramp, regardless of its steepness? What physical quantity is the same for What physical quantity is the same for a long trip up a gradual ramp a long trip up a gradual ramp a medium-long trip up a steep ramp a medium-long trip up a steep ramp a short trip straight up a vertical ramp a short trip straight up a vertical ramp

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Ramps 26 Energy and Work Energy – a conserved quantity Energy – a conserved quantity can’t be created or destroyed can’t be created or destroyed can be transformed or transferred between objects can be transformed or transferred between objects is the capacity to do work is the capacity to do work Work – mechanical means of transferring energy Work – mechanical means of transferring energy work = force · distance (where force and distance in same direction)

Ramps 27 Work Lifting a Cart (Part 1) Going straight up: Going straight up: Force is large Force is large Distance is small Distance is small work = Force · Distance

Ramps 28 Work Lifting a Cart (Part 2) Going up ramp: Going up ramp: Force is small Force is small Distance is large Distance is large work = Force · Distance

Ramps 29 Work Lifting a Cart (Part 3) Going straight up: Going straight up: work = Force · Distance Going up ramp: Going up ramp: work = Force · Distance The work is the same, either way! The work is the same, either way!

Ramps 30 Mechanical Advantage Mechanical advantage Mechanical advantage Doing the same amount of work Doing the same amount of work Redistributing force and distance Redistributing force and distance A ramp provides mechanical advantage A ramp provides mechanical advantage You can raise a heavy cart with a modest force You can raise a heavy cart with a modest force You must push that cart a long distance You must push that cart a long distance Your work is independent of the ramp’s steepness Your work is independent of the ramp’s steepness

Ramps 31 The Transfer of Energy Energy has two principal forms Energy has two principal forms Kinetic energy – energy of motion Kinetic energy – energy of motion Potential energy – energy stored in forces Potential energy – energy stored in forces Your work transfers energy from you to the cart Your work transfers energy from you to the cart You do work on the cart You do work on the cart Your chemical potential energy decreases Your chemical potential energy decreases The cart’s gravitational potential energy increases The cart’s gravitational potential energy increases

Ramps 32 Summary about Ramps Ramp supports most of the cart’s weight Ramp supports most of the cart’s weight You can easily balance the remaining ramp force You can easily balance the remaining ramp force You do work pushing the cart up the ramp You do work pushing the cart up the ramp Your work is independent of ramp’s steepness Your work is independent of ramp’s steepness The ramp provides mechanical advantage The ramp provides mechanical advantage It allows you to push less hard It allows you to push less hard but you must push for a longer distance but you must push for a longer distance

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