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Lecture 1: Forces
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3. How many kids does Curtis Have?
None
One
Two
Three
Who’s Curtis

4. Newton’s Laws of Motion
If the sum of all external forces on an object is zero, then its speed and direction will not change. Inertia
If a nonzero net force is applied to an object its motion will change F= ma
In an interaction between two objects, the forces that each exerts on the other are equal in magnitude and opposite in direction.
Lets do all demos here. Table cloth, Fred and cart, Equal and Opposite forces 30

5. Forces Non-Contact ---- Gravity (F = G m M /r2 ) Example: (two people)
G = 6.7x10-11 m3 / (kg s2)
Example: (two people)
F = (6.7x10-11 ) (60) (60)/ (.5)2 =
Weight of 100g = 31

6. Example Weight of Person
Calculate the gravitational force on a 60 kg person near the surface of the earth.
Earth: Mass = 6x1024 kg, radius = 6.4 x106 m.
F = G M m / r2
= (6.7x10-11 m3 / (kg s2)) (6x1024 kg) (60 kg)/ (6.4x106 )2 m2
= kg m/s2 (Newtons)
Most common gravitational calculation involves object close to surface of the earth
F = (G M / r2) * m
F = * m = 33

7. Newton’s 2nd Law
Fnet = m a
What if Gravity is the only force?

8. Normal Force
Perpendicular to Surface
Repulsive
Reactive
Contact

9. Example: (person in elevator) What is the magnitude of the Normal force?
Constant velocity
M = 60kg

10. Example: (person in elevator) What is the magnitude of the Normal force?
Accelerating up at 2 m/s2
M = 60kg

11. Example: (person in elevator) What is the magnitude of the Normal force?
Accelerating down at 3m/s2
M = 60kg

12. Tension Similar to normal force (Reactive)
Can only pull (can’t push on rope)
Constant throughout rope

13. Example: Box in Elevator What is the magnitude of the tension?
a= 2 m/s2
10kg

14. Springs 10kg Similar to tension, normal Reactive
Changes length according to tension
F = -k Δx
k=50 N/m
10kg

15. Free Body Diagrams Choose Object (book) Label coordinate axis
Identify All Forces
Hand (to right)
Gravity (down)
Normal (table, up)
Friction (table, left)
Normal
friction
hand
Gravity y x
Physics 40