1. Newton’s First Law
Level 1 Physics

2. CHAPTER 4 ESSENTIAL QUESTIONS
What are Newton’s Laws of Motion?
How do forces cause motion?
How can the Law of Universal Gravitation be applied?
How can one weigh the earth?
What effect does friction have on the motion of an object?

3. OBJECTIVES FOR NEWTON’S LAWS

4. Force Facts Basic definition: a push or a pull
Can we do a better job? Probably.
An interaction between bodies.
Forces can be classified by two types;
1. Contact Force
i.e. Normal, Tension, Friction
2. Field Force (at-a-distance)
i.e. Gravitational, Magnetic, electric

5. Newton’s First Law The Law of Inertia
The natural tendency for all objects is motion
Newton’s First Law (N.F.L.)
An object remains at rest or in a state of motion at constant speed
along a straight line unless compelled to change that state by a net
force (SF).
Forces are balanced
Object at rest
vo = 0 m/s
Object in motion
v m/s/s
a = 0 m/s/s
a = 0 m/s/s
Object in motion
Same speed/direction
Stays at rest

6. Inertia Inertia – Tendency to resist motion
The more massive an object is, the more inertia it will have
It is harder to get boulder in moving (also harder to stop
the boulder from moving) than it is to bet a pebble moving
Inertia is a quantity that is dependent upon the mass of the object

7. Free Body Diagrams (FBD)
A pictorial representation of all of the forces acting on an object N
Normal force (N): Surface
force that is always drawn
perpendicular to the
surface T f
Weight (mg): Always drawn
Straight down from center mg T
Tension (T): Force in
ropes/chains drawn
away from object
Friction (f): Drawn
opposing motion mg

8. FBD for slopes N f mg

9. Example
A 10-kg box is being pulled across the table to the right at a constant speed with a force of 50N.
Calculate the Force of Friction
Calculate the Force Normal N F f mg

10. Example
Suppose the same box is now pulled at an angle of 30 degrees above the horizontal.
Calculate the Force of Friction
Calculate the Force Normal F N Fy f 30 Fx mg

11. A Final Note