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Transcript of Physics
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Physics
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PHS 5043 Forces & EnergyNewton’s Laws
Force:
Any agent capable of changing the shape of an object or changing its state of rest or motion
Symbol: F
Units: N (newton)
N = kg m/s2
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PHS 5043 Forces & EnergyNewton’s Laws
Force:
Vector quantity
(magnitude and direction)
Forces can be added as any other vector (polygon, parallelogram and component method)
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PHS 5043 Forces & EnergyNewton’s Laws
Force:
At any given moment in time, all objects are subjected to the simultaneous action of many forces
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PHS 5043 Forces & EnergyNewton’s Laws
Equilibrium:
Objects are said to be in equilibrium when they are “at rest” or when they have “uniform rectilinear motion”
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PHS 5043 Forces & EnergyNewton’s Laws
Equilibrium:
When an object is in equilibrium all external forces add up to zero.
If we add all forces vectors (regardless of the method), we should obtain a zero value resultant
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PHS 5043 Forces & EnergyNewton’s Laws
Practice:
Given the following forces values, determine if the object is in equilibrium.
Fa = 30N, 270°
Fb = 10N, 45°
Fc = 20N, 135°
_Coordinates: Fa (0, -30); Fb (7.1,7.1); Fc (-20,20)
_Resultant Force: Fr (-12.9,-2.9)
No, the system is not in equilibrium for the resultant vector (component method) does not have (0.0) coordinates
Try with the polygon method!
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PHS 5043 Forces & EnergyNewton’s Laws
Equilibrant force:
Is the force that balances the resultant force It brings the system (object) to a state of
equilibrium
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PHS 5043 Forces & EnergyNewton’s Laws
Practice:
Given the following forces values, determine the magnitude and direction of the equilibrant force
Fa = 3N, 90°
Fb = 4N, 0°
_Coordinates: Fa (0, 3); Fb (4, 0), therefore Fr (4, 3)
_Magnitude Fr = 5N (Pythagoras), direction = 37°
_Feq = Fr but in opposite direction
Therefore, the equilibrant force would be Feq = 5N, 217°
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PHS 5043 Forces & EnergyNewton’s Laws
Inertia:
Property that causes an object to remain in its state of rest or uniform rectilinear motion, that is, to remain in equilibrium.
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PHS 5043 Forces & EnergyNewton’s Laws
Newton’s First Law:
“If no external or unbalanced force acts on an object, it maintains its state of rest or its constant velocity in a straight line”
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PHS 5043 Forces & EnergyNewton’s Laws
Newton’s Second Law:
“The change of momentum of an object is proportional to the net applied force”
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PHS 5043 Forces & EnergyNewton’s Laws
Newton’s Second Law:
F = m a
F = Δp / Δt F: Net force (N)
F = mΔv / Δt m: Total mass (kg)
F = m a a: acceleration (m/s2)
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PHS 5043 Forces & EnergyNewton’s Laws
Practice:
A 15 kg sled is pulled horizontally with a constant force of 60N. What acceleration would this force impart on the sled if the frictional force (sled-snow) is 10N?
F = m a
Fa – Ff = m a
a = (Fa – Ff ) / m
a = (60N – 10N) / 15kg
a = 3.3 m/s2
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PHS 5043 Forces & EnergyNewton’s Laws
Practice:
A 15 kg sled is pulled horizontally with a constant force of 60N at 30° with respect to ground. What acceleration would this force impart on the sled if the frictional force (sled-snow) is 10N?
F = m a
Fa – Ff = m a
a = (Fax cos 30°– Ff ) / m
a = (60N)(cos 30°) – 10N / 15kg
a = 2.8 m/s2
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PHS 5043 Forces & EnergyNewton’s Laws
Impulse: It can be consider the cause
of the motion of an object Its effect is the change of
Momentum of a moving object It is defined as the product of
Force and time, or more conveniently
as the variation of momentum
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PHS 5043 Forces & EnergyNewton’s Laws
Practice:
A 168 g hockey puck hits the boards of the ice rink perpendicularly at a velocity of 28 m/s and rebounds along same trajectory at 20 m/s. What impulses did the boards impart to the puck?
Δp = m Δv
Δp = 0.168 kg (-20 m/s – 28 m/s)
Δp = 0.168 kg (-48 m/s)
Δp = – 8.1 kg m/s
Δp = – 8.1 N s
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PHS 5043 Forces & EnergyNewton’s Laws
Newton's second law & momentum:
You (50 kg) ride a bike (10 kg) at 10 m/s and speed up to 20 m/s, after 100 m. If frictional force (air & ground) is 30 N, what force did you apply during the acceleration?
F = m a
Fa – Ff = m a
Fa = m a + Ff V22 – V1
2 = 2ad
a = (V22 – V1
2 ) / 2d
a = 1.5 m/s2
Fa = (60 kg) (1.5 m/s2) + 30 N
Fa = 120 N
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PHS 5043 Forces & EnergyNewton’s Laws
Newton’s Third Law:
“If object A exerts a force on object B (action), then object B exerts a force equal in magnitude, but opposite in direction, on object A (reaction)”
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PHS 5043 Forces & EnergyNewton’s Laws
Newton’s Third Law:
Rockets and airplanes are examples of the application of Newton’s third law
Both forces act simultaneously (always)