A Review of Some Newtonian Mechanics

Gernot LaicherFor REFUGES Class 2013

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Sir Isaac NewtonGodfrey Kneller’s 1689 portrait of Isaac Newton (age 46)

(Source: http://en.wikipedia.org/wiki/Isaac_Newton)

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.

Newton’s Second Law

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Rewriting Newton’s Second Law

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Impulse = Change in Momentum

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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A Two-body Collision

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(Newton’s third law)

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Impulse on m2 = - Impulse on m1

Impulse on m2 = - Impulse on m1

Change of momentum of m2 = - Change of momentum of m1

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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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For a 1-dimensional collision we can replace thevector with + or – signs to indicate the direction.

Note: u1, u2, v1, v2 may be positive or negative, depending on direction and depending on your choice of coordinate system.

22112211 vmvmumum

Given: m1 , m2 , v1 , v2

What can we find out about the final velocities u1 and u2 ?

22112211 vmvmumum

Given: m1 , m2 , v1 , v2

What can we find out about the final velocities u1 and u2 ?

Answer: In general there are an infinite number of possible solutions (combinations of the final velocities u1 and u2 thatfulfill the conservation of momentum requirement). Which ofthose solutions really happens depends on the exact natureof the collision.

Excel Program – Practicing the Solver1) 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 (4x2 + 2x =20) how many solutions do you get?

4) Pick different initial conditions for x to find all the solutions.

5) Find a solution to the following set of equations: 3x2 + 4y + 12z = 294x - y2 – z = 12x + 2y - 5z = 5

Excel Program – Using the Solver to determine 1-dimensional collision outcome

1) Write an Excel program that has the following input fields: m1, m2 , v1 , v2 , u1 , u2 . 2) Create (and label) fields that calculate p1initial , p2initial , p1final , p2final .

3) Create (and label) fields that calculate ptotal initial , ptotal final.

4) Create (and label) fields that calculate ptotal final - ptotal initial.

5) Fill in these values: m1=1 (kg), m2=1 (kg), v1=1 (m/s), v2=-1 (m/s), u1=some value (m/s), u2=some value (m/s).

6) Use a solver that changes u1 and u1 and makes ptotal final - ptotal initial = 0. How many possible solutions can you find?

Mechanical Energy of Two Mass System

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Totally Elastic Collision: initialfinal EE

(no mechanical energy is lost)

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Excel Program – Using a SolverAdd the following to your Excel “Collision-Solver”:

Create fields that calculate Etotal initial , Etotal final , Etotal final - Etotal initial ,% Energy change.

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initial

initialfinal

EEE

Now solve your collision problem again. This time, use the constraint Etotal final - Etotal initial = 0Find all possible solutions and interpret their meaning.

Totally Inelastic Collision: finalfinalfinal uuu 21

(The two masses stick together after the collision and move with the same velocity)

Task:

Determine a mathematical formula for the final velocity after a totally inelastic collision as a function of the masses and the initial velocities of two colliding objects.

Totally Inelastic Collision: finalfinalfinal uuu 21

(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 SolverName the Excel worksheet tab for the previously created elasticcollision “Elastic Collision”.

Make a copy of this tab and rename it “Inelastic Collision”

Modify the “Inelastic Collision” tab so that it calculates “u1” from theinitial conditions (masses, initial velocities) according to the formulafor u on the previous page. Then you can make u2=u1. So now both u1 and u2 will always have the same value.

Use a lab notebook to record your findings and investigate several scenarios of collisions

For both elastic and inelastic collisions you should find solutions forthese cases. Imagine each scenario and describe what is happening.

1) m1=5kg, m2=5kg, v1=15m/s, v2=0 (one mass initially at rest)

2) m1=5kg, m2=5kg, v1=15m/s, v2=-15m/s (one mass initially at rest)(head on collision).

3) m1=5000kg, m2=0.01kg, v1=15m/s, v2=0 (very unequal masses, lighter mass at rest).

4) m1=5000kg, m2=0.01kg, v1=0, v2=-15m/s (very unequal masses, heavier mass at rest).

5) m1=5000kg, m2=0.01kg, v1= 15m/s, v2=-15m/s(head on collision)

Quick Review: Vectors and Scalars

• Vectors: Magnitude and Direction(e.g., Force, Displacement, Velocity, Acceleration)

• Scalars: Magnitude only(e.g., Temperature, Altitude, Age)

Adding Vectors

1F

2F

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

1F

2F

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Subtracting Vectors

initialv

An object changes it’s velocity.What is it’s change in velocity?

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(Note: F2y had a negative value)

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