Work%/%Energy%and%their%transformations · 2019. 9. 10. · May 29, 2015...
Transcript of Work%/%Energy%and%their%transformations · 2019. 9. 10. · May 29, 2015...
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Work / Energy and their transformations
in physics work is NOT just any exhausting action, but has a specific definition:
When a force acts on an object to either cause or hinder a displacement, then work is being done upon the object by the force.
practice:
in which of the following situations is work being done?
a) holding up a bucket of water at a 90 degree angle.
b) a book falling from a table to the ground.
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Work equation:
W = F × ∆s × cos Ɵ
need three things in place for work:
1. force
2. displacement
3. force must either cause or hinder the movement
➔ force vector must have non -‐o component along movement axis
➔ force vector cannot be perpendicular to movement axis (Ɵ ≠ 90º)
∆s
Work not a vector but a scalar
unit: 1 J = 1 Joule = 1N × m
F = magnitude of force
∆s = magnitude of displacementƟ = angle between force and displacement
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counter-‐examples:
tiring, but ...
1.)
2.)
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• when force points in the same direction (Ɵ = 0) as displacement -‐-‐> F = Feff , Max work
• Ɵ < 90º -‐-‐> work >0
• 90º force is opposing movement
-‐-‐-‐> work
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4.) An approximately 2-‐kg object is sliding at constant speed across a friction free surface for a displacement of 5 m to the right.
forces doing work:
5.) An approximately 2-‐kg object is pulled upward at constant speed by a 20-‐N force for a vertical displacement of 5 m.
A 10-‐N force is applied to push a block across a frictional surface at constant speed for a displacement of 5.0 m to the right.
3.)
forces doing work:
forces doing work:
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Power
Power is the rate at which work is done:
P = W∆t
A ship that crosses a river in less time compared to another boat is more powerful.
unit: 1 W = 1 J1 s = 1 N × m
1 s
example:
A horse uses a force of 300N to pull a 100 kg sleigh for 5 min over a distance of 500 m on horizontal ground. The rope used to pull the sleigh forms an angle of 30º with the horizontal.
What power is the horse generating?
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Energy
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Kinetic energy
which vehicle has greater kinetic energy?
who has greater kinetic energy?
same velocity, but big mass vs. small mass
same mass but slow vs. fast
Example:
A 60 kg cyclist is cycling at a speed of 20 km/h. What is her kinetic energy?
unit: 1J = N×m
= kg×m2/s2
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Work-‐energy theorem
The total work done upon an object, (between an initial instant and a final instant), is equal to the change in kinetic energy of the object between these two instances.
A 10-‐N force is applied to push a block across a frictional surface at constant speed for a displacement of 5.0 m to the right.
3.)
forces doing work:
total work: = sum of all work done on the object
= work done by resultant force on the objet
Wtot = FR×∆s×cosƟ
example:
Wtot = ∆Ekin = Ekin -‐ Ekinf i
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Example:
A dog uses 50 N to pull a 25 kg sled over a horizontal distance of 4 m. The sled was initially at rest. Assuming that friction is negligible, calculate
a) the total work done
b) the sled's final velocity
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Gravitational potential energyWhat will hurt more:
dropping a stone from a hight of 50 cm on your foot or from a hight of 2 m?
gravitational potential energy : Epg
= energy stored in a body of mass because of its position in relation to another body of mass.
on earth: object has gravitational potential energy if elevated above earth surface.
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Formula:
Epot-‐g = m×g×h
m = mass of the objectg = gravitational acceleration h = height above reference level
hh
unit: 1J = N×m = kg×m2/s2
Example:
a) determine the gravitational potential Energy of a 500g bottle of water on a shelf which is located 1.5 m above the ground.
b) how does the situation change if a 60 cm high chair is placed underneath the shelf?
➔ need to define the reference level for gravitational potential energy
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Example 2:
A 1 kg mass hangs suspended from a 50 cm long cable. you let the mass swing so that the angle to the vertical axis at its highest point is 20º. What is the potential Energy of the mass at this point?
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mechanical Energy= sum of potential and kinetic Energy
Em = Ekin + Epot-‐g
Conservation of mechanical Energy:
If no force is adding or removing (-‐-‐> no friction) energy from an object (or system) its mechanical energy remains constant.
Emi = EmfEkin-‐i + Epot-‐gi = Ekin-‐f + Epot-‐gf
➔ Any loss of gravitational potential energy must result in an increase in kinetic energy
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m = 60 kgv = 8 m/s
max height
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Example (from text book)
An object located at a hight of 50 m has an initial velocity of 5 m/s. What will its velocity be when it hits the ground?
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Hooke's law -‐ helical springs
compressed:
or stretched:
springs are elastic, so they can either be
apply force to spring along its helical axis length increases or decreases.
Force removed spring assumes original length
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∆x
• Fr = -‐ Fapp
Fr
• when a spring is deformed (elongated or compressed) by a force, the spring develops a
force to counteract this deformation= Frestoring
• Frestoring ~ to elongation :
Fr = -‐ k × ∆x
2∆x
Hooke's law
Fr = restoring force (N)
∆x = elongation of spring (m)
k = spring constant of the spring (N/m) -‐ specific for each spring
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