1. Free body diagram (FBD)
Zero net Force implies constant velocity

2. Identifying Forces acting on an object

3. Free Body Diagram (FBD)

4. What is the “natural state” of matter?
Aristotle (384 – 322 BCE)
Natural state of matter is to be “at rest”
What force objects to move?
Why they prefer (move) to stay away from their “natural state”
Galileo ( ) B’day: 15th January
“at rest” is a limiting case of the “natural state” of matter which is in motion with uniform velocity
Presence of external forces such as air resistance, friction, and gravity keep the objects in motion
Objects need an external force for motion
When no force is acting (incl. air resistance, friction etc.) objects move with constant (uniform) velocity
His experiments are limited to horizontal surfaces ONLY

5. Newton’s Era (1643-1727) B’day 4th January
Generalized both Galileo’s and Aristotle’s theories/observations and signed “in to the law” of constitution of science. It states
“Objects at rest continues to be at rest, and objects in motion continues to move with uniform velocity along a straight line if and only if the net force on the object is zero”
This is popularly known as Newton’s First Law

6. Motion when net Force is zero
Objects can have two states when net force is zero
State of rest (static equilibrium)
State of motion with zero acceleration (dynamic equilibrium)

7. Examples

8. Trick Question
We know from our daily life experience that an external force is needed to move the object
Newton’s law suggests that objects in motion continue to move with constant velocity although net external force is zero!
What is wrong?
The law of inertia
If an accelerating car suddenly stops…why would the driver run into windshield?

9. Chapter 5: Motion Along a Line
Dynamic equilibrium
Static equilibrium
T1=100 N, T2=200 N, Find direction and magnitude of T3
Find T if angle of inclination is 20o

10. Speed of a car when exerted by a Force

11. Difference between weight and apparent weight
The weight of an object is the force of gravity on that object
If you are hanging from a rope, your sensation of weight is due to the tension force pulling up on you
If you measure your weight in an elevator will it be the same as your normal weight?

12. Measuring instruments for weight

13. Apparent weight
Weight of a person in a moving elevator

14. Friction