1. Lec 04: Newton’s Second Law
Normal Force and Friction
Gravity and Weight
Newton’s Second Law
Nonfree Fall and Air Resistance

2. Normal Force and Friction
Perpendicular to surface
Away from contact
Friction
Parallel to surface
Opposite to motion

3. 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?
Forward
Backward Up
Down

4. Normal Force and Friction
A person pushes a box across the floor at a constant speed without changing direction.
How much is the friction?
Equal to the push
Equal to the box weight
More than the push
100 N

5. 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?
Forward
Backward Up
Down

6. 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?
Equal to the push
Equal to the box weight
More than the push
100 N

7. 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?

8. Gravity and Weight Gravitational Force Weight
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?

9. What do Scales Read?

10. 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

11. Mass and Weight When you pull down QUICKLY, which string breaks first?
top
bottom
Both simultaneously
I don’t know

12. Mass and Weight When you pull down SLOWLY, which string breaks first?
top
bottom
Both simultaneously
I don’t know

13. Nonfree Fall Drag Force depends on frontal surface
Drag Force increases with speed
Drag Forces increases until it equals Gravity: terminal velocity

14. 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?