1
Motion – Newton’s Laws
1. Every body continues in its state of rest or uniform motion unless it is acted upon by a net external force.
2. The acceleration of a body is equal to the net force acting on the body divided by the mass of the body.
a = FNET / m
FNET = ma
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Part 2, B: MOTION 2
Newton's Laws
F = m a
Net force is the vector sum of the individualforces
7 N 2 N 5 N
net
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Part 2, B: MOTION 2
Newton's Laws
F = m a
The direction of the acceleration is the sameas that of the net force.
5 N
net
If the mass is 10 Kg,the acceleration is0.5 m/sec² to the left.
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Part 2, B: MOTION 2
Newton's Laws
Weight: The force of gravity on an object.
How would you measure this?
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Part 2, B: MOTION 2
Newton's Laws
Weight: The force of gravity on an object.
How would you measure this?
Put the object in free fall and measure theacceleration. Then
W = mg.
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Part 2, B: MOTION 2
Observation: Motion in an arc possible onearth.
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Part 2, B: MOTION 2
Newton's Laws
Example: Air resistance
W
Without air resistance,
W mg a = = = g
m m
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Part 2, B: MOTION 2
Newton's Laws
Example: Air resistance
W
With air resistance,
W-R a = < g
m
R
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Part 2, B: MOTION 2
Newton's Laws
If R = W, what will happen?
(a) The object will begin to moveupward.(b) The object will stop immediately.(c) The object will slowly come to a stop.(d) The object will continue to fall, butat a constant velocity.(e) None of the above.W
R
10
Part 2, B: MOTION 2
Newton's Laws
If R = W, what will happen?
(a) The object will begin to moveupward.(b) The object will stop immediately.(c) The object will slowly come to a stop.(d) The object will continue to fall, butat a constant velocity.(e) None of the above.W
R
11
Motion – Newton’s Laws
Special Case: Friction
* Friction resists the motion of an object
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Motion – Newton’s Laws
Special Case: Friction
* Friction resists the motion of an object
* Frictional force is in the opposite direction of the motion of the object
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Motion – Newton’s Laws
Special Case: Friction
* Friction resists the motion of an object
* Frictional force is in the opposite direction of the motion of the object
* Frictional force is proportional to the weight of the object
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Motion – Newton’s Laws
Special Case: Friction
* Friction resists the motion of an object
* Frictional force is in the opposite direction of the motion of the object
* Frictional force is proportional to the weight of the object
FFRICTION α mg
FFRICTION = μmg
The constant of proportionality μ is called the coefficient of friction
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Motion – Newton’s Laws
Special Case: Friction
There are two types of friction:
Static Friction – frictional force that holds an object stationary.
μs → Coefficient of Static Friction
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Motion – Newton’s Laws
Special Case: Friction
There are two types of friction:
Kinetic Friction – frictional force associated with a sliding object.
μk → Coefficient of Kinetic Friction
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Motion – Newton’s Laws
Special Case: Friction
There are two types of friction:
FACT: It requires more force to set an object into motion than it does to keep an object moving.
μs > μk
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Motion – Newton’s Laws
Special Case: Friction
Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2
19
Motion – Newton’s Laws
Special Case: Friction
Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2
F = μk mg → a = μk mg/m = μk g
Δv = a t → t = Δv/a
d = ½ a t2 = ½ a (Δv/a)2 = Δv2 / 2 μk g
d = (2 m/s)2 / (2 * 0.8 * 10 m/s2) = 0.25 m
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Motion – Newton’s Laws
Special Case: Pressure
P = F/A
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Motion – Newton’s Laws
Special Case: Pressure
P = F/A
F
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Motion – Newton’s Laws
Special Case: Pressure
P = F/A
F
Area
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Motion – Newton’s Laws
Special Case: Pressure
P = F/A
F
Area
MKS: N/m2
British: psi