chapter 4. laws of motionkenahn/16fall/phys102/lecture/l8-l9.pdf · chapter 4: the laws of motion....
TRANSCRIPT
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Chapter 4. Laws of Motion
Newton’s 1st Law
Newton’s 2nd, 3rd laws
Force
Net force
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So far, we learned about
Units, 1d motion, 2d motion, vectors, …
How to describe motions
Today,
Chapter 4: The Laws of Motion
Forces, Newton’s 1st, 2nd, and 3rd laws
Motivation
Then, what causes the motion ???
What causes an apple to fall?
What causes a ball to undergo projectile motion?
Answer
Chapter 4: The Laws of Motion
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ForcesThe measure of interaction between two objects (pull or push). Vector – force has magnitude (unit: N, Newton) and direction
Example
Gravitational Force on apple from earth
Force on apple from your hand
apple
Your hand
Earth
Force on one object from the other object
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Net Force netF
If forces from many objects are applied on a single object
Net force : Vector sum of forces on an object
Gravitational Force on apple from earth
Force on apple from your hand
apple
Your hand
Earth
net on apple hand gravityF F F
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iClicker Quiz
Two forces of magnitude 5 N are acting on an object.What is the magnitude of the net force?
A) 10 NB) 0 NC) Not enough information
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iClicker Quiz
Which is closest to the magnitude of the net force?
(a) Zero(b) 1.0 N(c) 4.0 N(d) 5.0 N(e) 7.0 N
3N and 4 N forces are acting on an object as shown.
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Example
2 N
3N
Find the net force and its magnitude.
Three forces are acting on an object.
4 N
90 deg120 deg
+x
+y
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Newton’s First Law of Motion (The Law of Inertia)
the object at rest remains at rest, the object moving continues to move with the same velocity
(same speed and same direction).
An additional force is NOT required to keep a cart moving with a constant velocity on a frictionless track!
If 0, netF
does not change.v
If net force on an object is zero.
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iClicker Quiz v = 2.5 m/s
A block is moving to the right at a constant velocity 2.5 m/son a frictionless horizontal surface.
Two forces F1 and F2 are exerted on the block in the opposite directionsas shown above.
While the block is moving at a constant velocity,Which force has a greater magnitude?
(a) F1
(b) F2(c) They have the same magnitude
1F
2F
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Newton’s Second Law of Motion
• Non-zero net force causes object to accelerate.
• An object of mass m has an acceleration a, equal to the net force F, divided by the mass of the object, m.
netFa
m
If 0, netF
changes, in other words, accelerates.v
m
netF
netFa
m
The acceleration is given by
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In this projectile motion, While the ball is going up, the direction of the net force is
(a)
(b)
(c)
(d)
(e)
iClicker
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Equal magnitude and opposite directionAction-reaction pair:
Action and Reaction forces are acting on different objects, NOT on a single object. Don’t add them to find the net force!!!
A B
A B
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Action-reaction versus net force• The 3rd law pair of forces are always applied to different
objects! Net force in Newton’s 1st and 2nd laws is the sum of forces on the
same object.
Examples of action & reaction
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Find reaction force for the gravity on the cantaloupe(a) Force on the cantaloupe from the table(b) Force on the earth from the cantaloupe(c) Force on the table from the cantaloupe(d) Force on the earth from the table(e) Force on the table from the earth
iClicker Quiz
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Next topic..
Various ForcesGravity forceNormal forceTension
Friction forces (static, kinetic)
Motivation
1 2 ...netF F F ma
To find the acceleration, we need to add all forces applied to an object of mass m.
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Various Forces
Gravitational Force
Normal Force
Tension Force
Friction Force
Spring Force
Other forces…
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Gravitational Force (Weight):
Magnitude:F = mg, where g = 9.8 m/s2
•Weight: The force that the Earth is pulling the object with near the surface of the Earth.
Direction: Pointing downward
Weight: From the Earth on objects near Earth Surface
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In this projectile motion, When the ball is at the point A,the direction of the gravity force is
(a)
(b)
(c)
(d)
(e)
iClicker
A
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m
A block of mass m is on an inclined surface as below. What is the direction of the gravity force on the block?
(a)
(b)
(c)
(d)(e)
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Direction:
Magnitude:Determined by analyzing forces
From surface on objects in contact
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m
A block of mass m is on an inclined surface as below. What is the direction of the normal force on the block?
(a)
(b)
(c)
(d)(e)
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Example: Person in an elevator
A person with a mass 50 Kg in an elevator is moving upward with an upward acceleration 21.1 /a m s
Identify all forces on the person and find magnitude and direction for each.
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A person is in a elevator and the elevator is moving downward but slowing down. The magnitude of the normal force from the floor on the person must be
A) greater than the weight of the personB) less than than the weight of the person.C) equal to the weight of the person.
iClicker Quiz
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A B
Direction: pulls along the ropeMagnitude : Constant along a single rope,
determined by analyzing forces
From the rope on objects at both ends
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M=10 kg Normal force = 50 N
Horizontal surface
Rope
Find the tension force from rope on block.
scale
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A bucket is being lowered by a very light rope with a constant downward velocity. The tension in the rope must beA) equal to the weight of the bucket.B) greater than the weight of the bucket.C) less than the weight of the bucket.
iClicker Quiz