1. Unbalanced Force (Net Force) Model Aka: Newton’s Second Law (N2L)
Unit 5
Unbalanced Force (Net Force) Model
Aka: Newton’s Second Law (N2L)

2. First Law – No Un-balanced Force…
No change in motion
No change in speed
No change in direction
MASS is a measure of the object’s tendency to retain its current state of motion.

3. Newton’s First Law
If there is no “left over” force in a particular direction, there will be no change in velocity (speed or direction).
But what if there IS a left over “net” force? What will be its effect?

5. Combining these ideas…
F = m a
But, since more than one force may be acting on an object…

6. The second law Fnet = m a A “force” is an interaction between two objects.

7. Units Force Mass Acceleration Metric Unit Newton (N) kilogram (kg)
m/s/s

8. F = m a N = (kg) (m/s/s) Does this look right?
Does this mean…
F = m a
N = (kg) (m/s/s)
Does this look right?

9. Practice Problems #1 Given: F=140N a=20 m/s/s Find: mass
Solution : F=ma
140=m(20)
m=7 kg
Where did kg come from?

10. Algebra Triangle

11. Newtons 2nd Law Triangle

12. Net Force

13. Fnet Causes Acceleration

14. Weight – the Force of Gravity
It ALWAYS acts downward.

15. But gravity is different
Gravity applies a force proportional to the mass
More mass means more force
No other force seems to behave in this way

16. The acceleration produced by gravity does not depend on mass
The ratio F/m remains constant
Its value is 10 m/s/s at the surface of the earth (9.8 m/s/s)
It is true, but still very amazing!

17. Fg = m g
This formula is still Newton’s Second Law, just rewritten to be specific for gravity.

18. How much does 1.00 kg weigh?
Look familiar?

19. What is the mass of an object that weighs 575 N?

20. How much would the same 58. 7 kg object weigh on the moon (g=1
How much would the same 58.7 kg object weigh on the moon (g=1.6 m/s/s)?

21. Adding Easy Vectors

22. Important Lesson!!!

23. One Force Acting on One Object
Newton’s Second Law
One Force Acting on One Object

29. Assignment
Worksheet 5a due tomorrow (only some questions will be graded, not all of them)