Download Newton`s Second Law

Lec 04: Newton’s Second Law
• Normal Force and Friction
• Gravity and Weight
• Newton’s Second Law
• Nonfree Fall and Air Resistance
Normal Force and Friction
• Normal Force
• Perpendicular to surface
• Away from contact
• Friction
• Parallel to surface
• Opposite to motion
Normal Force and Friction
A person pushes a box across the floor at a constant
speed without changing direction.
In what direction is the friction?
A.
B.
C.
D.
Forward
Backward
Up
Down
Normal Force and Friction
A person pushes a box across the floor at a constant
speed without changing direction.
How much is the friction?
A.
B.
C.
D.
Equal to the push
Equal to the box weight
More than the push
100 N
Normal Force and Friction
A person pushes a box across the floor at a constant
speed without changing direction.
In what direction is the Normal Force?
A.
B.
C.
D.
Forward
Backward
Up
Down
Normal Force and Friction
A person pushes a box across the floor at a constant
speed without changing direction.
How much is the Normal Force?
A.
B.
C.
D.
Equal to the push
Equal to the box weight
More than the push
100 N
Puzzles
P1. If you press a book against the ceiling, in which
direction is the Normal force on the book?
P2. When you drive your car forward, the friction force
applied to the bottom of the tires is in which directions?
P3. When you slide forward across the floor, in which
direction is the friction?
Gravity and Weight
• Gravitational
Force
• Between Earth and object
• F = (mass) x (9.8 m/s2)
• Weight
• Considered as the Gravitational Force
• BUT measured as another force that usually
cancels the gravitational force
Question: In how many different ways have
you seen the weight measured?
What do Scales Read?
Newton’s Second Law
Acceleration =
Net Force
Mass
Question: If you apply 20 N to a 2-kg object,
what is its acceleration?
Ans. 4 m/s2
Question: If you double the force, what will
be the acceleration
Mass and Weight
When you pull down QUICKLY, which string breaks first?
A.
B.
C.
D.
top
bottom
Both simultaneously
I don’t know
Mass and Weight
When you pull down SLOWLY, which string breaks first?
A.
B.
C.
D.
top
bottom
Both simultaneously
I don’t know
Nonfree Fall
• Drag Force depends on
frontal surface
• Drag Force increases with
speed
• Drag Forces increases until it equals
Gravity: terminal velocity
Puzzles
P1. If you apply three times as strong force to an object
what happens to its acceleration?
P2. If the same amount of a force is applied to a twice
as heavy object, what is its acceleration?
P3. You place 2 lb of apples on a scale in an elevator. When
the elevator begins to accelerate upward, the scale reads:
(a) Heavier (b) Lighter (C) The same
P4. A 10 N falling object encounters 5 N resistance.
What is its acceleration?
P5. If a 50-kg person is to fall at terminal speed, what
should be the air resistance force?