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Newtonian Mechanics – Momentum, Energy, Collisions Gernot Laicher For ACCESS Class 2011

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Quick Review: Vectors and Scalars Vectors: Magnitude and Direction (e.g., Force, Displacement, Velocity, Acceleration) Scalars: Magnitude only (e.g., Temperature, Altitude, Age)

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Adding Vectors Two forces are acting on a mass m: How do we find the total force ?

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Subtracting Vectors An object changes its velocity. What is its change in velocity?

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(Note: F 2y had a negative value)

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Sir Isaac Newton Godfrey Knellers 1689 portrait of Isaac Newton (age 46) (Source:

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Newton's First Law : An object at rest tends to stay at rest and an object in uniform motion tends to stay in uniform motion unless acted upon by a net external force. Newton's Third Law: For every action there is an equal and opposite reaction. Newton's Second Law: An applied force on an object equals the rate of change of its momentum with time.

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Newtons Second Law

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For a non-changing mass:

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Rewriting Newtons Second Law Impulse = Change in Momentum

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Example: A constant force of 5N applied for 0.5 seconds on a mass of 0.2kg, which is initially at rest.

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In general, the force may depend on time Impulse = Area under the F(t) curve (Integrate F(t) over time to get the impulse)

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A Two-body Collision Before the collision During the collision (Newtons third law)

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During the Collision Impulse on m 2 Impulse on m 1 Impulse on m 2 = - Impulse on m 1

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Change of momentum of m 2 = - Change of momentum of m 1 Total momentum after collision = Total momentum before collision The total momentum of the system is conserved during the collision. (This works as long as there are no external forces acting on the system)

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After the collision For a 1-dimensional collision we can replace the vector with + or – signs to indicate the direction. Note: u 1, u 2, v 1, v 2 may be positive or negative, depending on direction and depending on your choice of coordinate system.

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Given: m 1, m 2, v 1, v 2 What can we find out about the final velocities u 1 and u 2 ?

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Given: m 1, m 2, v 1, v 2 What can we find out about the final velocities u 1 and u 2 ? Answer: In general there are an infinite number of possible solutions (combinations of the final velocities u 1 and u 2 that fulfill the conservation of momentum requirement). Which of those solutions really happens depends on the exact nature of the collision.

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Excel Program – Using a Solver 1)Solving a simple math problem (2x + 6y =14) infinitely many solutions 2)Adding a constraint (a second equation) (x+y=3) to get a single solution. 3) Solving a quadratic equation (4x 2 + 2x =20) how many solutions? 4) Pick different initial conditions for x to find all the solutions. 5) Write an Excel program that has the following input fields: m 1, m 2, v 1, v 2, u 1, u 2. 6) Create (and label) fields that calculate p 1initial, p 2initial, p 1final, p 2final. 7) Create (and label) fields that calculate p total initial, p total final. 8) Create (and label) fields that calculate p total final - p total initial. 9) Fill in these values: m 1 =1 (kg), m 2 =1 (kg), v 1 =1 (m/s), v 2 =-1 (m/s), u 1 =some value (m/s), u 2 =some value (m/s). 10) Use a solver that changes u 1 and u 1 and makes p total final - p total initial = 0. How many possible solutions can you find?

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Mechanical Energy of Two Mass System Before the collision After the collision

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Totally Elastic Collision: (no mechanical energy is lost)

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Excel Program – Using a Solver Add the following to your Excel Collision-Solver: Create fields that calculate E total initial, E total final, E total final - E total initial, % Energy change. Now solve your collision problem again. This time, use the constraint E total final - E total initial = 0 Find all possible solutions and interpret their meaning.

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Totally Inelastic Collision: (The two masses stick together after the collision and move with the same velocity)

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Is there Mechanical Energy Conservation in Totally Inelastic Collisions?

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Excel Program – Using a Solver Name the Excel worksheet tab for the previously created elastic collision Elastic Collision. Make a copy of this tab and rename it Inelastic Collision Modify the Inelastic Collision tab so that it calculates u1 from the initial conditions (masses, initial velocities) according to the formula for u on the previous page. Then you can make u2=u1. So now both u1 and u2 will always have the same value.

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Use a lab notebook to record your findings and investigate several scenarios of collisions For both elastic and inelastic collisions you should find solutions for these cases: 1)Equal masses, one mass initially at rest. 2)Equal masses, equal but opposite velocities (head on collision). 3)Very unequal masses, lighter mass at rest. 4)Very unequal masses, heavier mass at rest. 5)Very unequal masses, equal but opposite velocities (head on collision)

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