The stretch of the spring is a measure of the applied
force.
Every object continues in its state of
rest, or of uniform motion in a straight
line, unless it is compelled to change the
state by forces impressed upon it.
Net Force
The combination of forces that act
on the object is the net force
Units of Force
1 newton (N) = 1 Kg *m/s2
acceleration ~ net force
a~F
1. You push on a crate that sits on a smooth
floor and it accelerates. If you apply four
times the pushing force, how much greater
will be the acceleration?
2. Same for a rough surface.
Friction
If you apply a force to an object, a force of friction
usually reduces the net force and resulting acceleration.
The direction of friction force is always in the direction
opposing motion.
The force of dry friction between solid surfaces doesn’t
depend on speed or area of contact.
Fluid friction is called drag
Drag does depend on the speed and
area of contact. For slow motion
through water or air drag is
approximately proportional to the
speed of the object.
Question
A jumbo jet cruises at constant velocity of 1000 km/h
when the thrusting force of its engines is constant
100000 N.
1. What is the acceleration of the jet?
2. What is the force resistance on the jet?
Mass and Force of Gravity
Mass: The quantity of matter in an object. It is also the
measure of the inertia or sluggishness, that an object
exhibits in response to any effort made to start it, stop
it, or change its state of motion in any way.
Force of Gravity= mg
Newton’s Second Law
The net force on an object is equal to its mass times its
acceleration and points in the direction of acceleration
Fnet = ma
Important: acceleration of a body is
always in the direction of net force!!!!
Free-body diagrams
Free fall revisited
When Acceleration is Less Than
g – Nonfree Fall
The force of air drag experienced by the
falling object depends on two things:
1. Frontal area of the falling object
2. The speed of the object
When the force of air resistance
completely balances the force
of gravity we say, that an object has
reached its terminal speed.
Examples
1. Forces of 4 N and 6 N act on the object. What is the
minimum value for the sum of these two forces?
2. Two ropes are being used to pull a car out of a ditch. Each
rope exerts a force of 700 N on the car. Is it possible for
the sum of these two forces to have a magnitude of
1000N? Explain your reasoning.
3. If the net force on a boat is directed due east, what is the
direction of the acceleration of the boat? Would your
answer change if the boat had a velocity due north but the
net force still acted to the east?
Examples
1. Describe the force(s) that allow you to walk across the
room.
2. You are analyzing a problem in which two forces act on an
object. A 200 N force pulls to the right, and a 40 N force
pulls to the left. Your classmate asserts that the net force is
200 N because that is the dominant force that is acting.
What is wrong with that assertion?
Summary of Terms
Mass
Newton (units of force)
Force
Friction
Free fall
Terminal speed