Transcript Forces

Unit 9 Lecture
Newton’s
st
1
Law of Motion
 An object at rest will stay at rest, and
an object in motion will stay in motion
UNLESS acted on by an unbalanced force
 In other words: An
object will keep doing what
it’s doing until a force changes its motion
Forces
 A force is a push or a pull on an object
 All changing motion is caused by ONE or
MORE forces.
 The SI unit for force is Newton (N).
Kinds of forces we will talk about
 Frictional Force
 Electrical Force
 Air Resistance
 Normal Force
 Gravitational Force
 Spring Force
 Tensional Force
 Magnetic Force
 Applied Force
Newton’s Second Law of Motion
 An object will accelerate in the same direction as
the net force on an object!
Force = mass x acceleration
F=mxa
Net Force
 The net force on an object is the sum of all
the forces on the object in each direction.
Net force = F1 + F2 + F3 + …
Practice Problem
Bob the squirrel is mowing his lawn. A lawn mower is
pushed across the ground with a force of 25 N. The ground
provides 14 N of resistance. What is the net force on the
lawn mower in the x-direction (along the ground)?
Net Force = F1 + F2 + F3……
F1 = + 25 N
F2 = - 14 N (negative because it is resisting)
Net Force = (+25N) + (-14N)
= 25N – 14N
= 11N
TRY IT!
If the mass of a helicopter is 4,500 kg. and the net force on it is
18,000 N, what is the helicopter’s acceleration?
Given:
m = 4,500 kg
F = 18,000 N
Finding?
a = ? (acceleration)
F
m
a
F=mxa
a = F_
m
= 18,000 N
4,500 kg
a = 4 m/s2
Gravity
 Gravity is the attractive
force between two objects
 Gravity’s strength depends
on the masses of the objects
and the distance between
them
Force of Earth’s Gravity
F = mg
 m is the mass of the object gravity is
acting on
 g is the gravitational acceleration
constant: g
= 9.8m/s2
 The “F” on Earth is weight.
Let’s try one!
 On earth, what is the weight of a television having a
mass of 75 kg?
F = mg
where ‘F’ is weight
m= 75 kg g = 9.8 m/s2 weight =?
weight = (75 kg) (9.8 m/s2)= 735 kg x m/s2
weight = 735 N
Try one by yourself…..
Find the acceleration due to gravity on Mars if a
person with a mass of 60.0 kg weighs 22.2 N on
Mars?
Normal Force
 When gravity acts on an object, but the object
does not fall, there is a normal force pushing up
on the object
Fnormal + Fgravity = 0 N
Balanced Forces
 If an object is not accelerating,
either:
1. no forces are acting on it,
or
2. all the forces balance out
 Two forces are balanced if they
have BOTH:
 equal sizes
 opposite directions
Unbalanced Forces
 Every force has a magnitude and a direction
 If the net force (sum of the magnitude in each
direction) is not zero, the object accelerates
 Objects can have the same direction, but
different magnitudes.
Force Diagrams
 Force diagrams use different sized arrows to represent
the Forces acting on an object
 Including: sizes, directions, and type of force
 The object is usually represented by a box
EXAMPLE 1
Draw the forces affecting a textbook on the table
BOOK
BOOK
EXAMPLE 2
Draw the forces affecting a textbook being
pushed across the table at a constant velocity
BOOK
BOOK
Example 3
Draw the force affecting a magnet being held to
close to another magnet.
N
S S
N
N
S
S
S
N
Newton’s Third Law
 For every action force, there is an equal
and opposite reaction force
 Action and Reaction forces do not cancel
-they can act on different objects or spread
in different directions
Air Resistance
 Air resistance is a force that
opposes the motion of objects that
move through air
 Air resistance depends on an
object’s size, shape, and speed
 Ignore air resistance in this class
unless you are told to include it
Friction

Friction is the force that opposes motion.
1.
Sliding friction – opposes the motion of 2 surfaces
as they slide against each other
Static Friction
 Static friction – prevents 2 surfaces from
sliding against each other at all
 Rolling friction – slows down the motion of an
object rolling on a surface
Static friction keeps a wheel from spinning in
place and makes it turn instead
The “nitty-gritty” of projectiles
 The only forces that acts on a projectile is gravity.
 So, the only acceleration is in the downward direction.
Types of Projectiles
 Drops- no initial velocity in the horizontal or vertical
direction
 Throw vertically- initial velocity in the vertical, but
not in the horizontal
 Launch- initial horizontal and vertical velocity