Forces
Change in motion?
 What causes an object to start moving, stop moving, or
change direction?
UNBANLANCED FORCE
What is a force?
 A push or pull
 A force is described by strength and direction
 The strength of a force is measured in the Newton
(N)
 Arrows are used to represent force:
Combining forces
 Most of the time, there are more than 1 force acting on an
object at a time
 Net force- the combination of all forces acting on an
15 N
15 N
15 N
15 N
Unbalanced vs. Balanced Forces
Unbalanced Forces
 Result in a net force
 Can cause a change in the
object’s motion
Balanced Forces
 Equal forces acting in
opposite direction
 Do not cause a change in
the object’s motion
 They cancel each other out
Draw Force Diagrams in Notebook
A book is at rest on a tabletop. Diagram the forces acting
on the book
2. A gymnast is holding on the rings suspended from the
ceiling waiting to being.
3. A flying squirrel is gliding (no wing flaps) from a tree to the
ground at constant velocity. Consider air resistance.
4. A rightward force is applied to a book in order to move it
across a desk at constant velocity. Consider frictional
forces. Neglect air resistance
1.
Examples Continued
5. A student rests a backpack upon his shoulder. The pack is
suspended motionless by one strap from one shoulder.
6. A skydiver is descending with a constant velocity. Consider
air resistance
7. A force is applied to the right to drag a sled across loosely
packed snow with a rightward acceleration
8. A car is coasting to the right and slowing down
Formula
F=m•a
Force = mass x acceleration
Units: Newtons (N)
N=(kg)•(m/s2)
Friction
The force that one surface exerts on
another when the two surfaces rub
against each other
Force that oppose motion- always in opposite
direction of motion
Friction…
 Friction produces heat! – how have you seen this?
 The amount of friction depends on:
 How hard the surfaces push together (force)
 Types of surfaces involved
SURFACE AREA DOES NOT AFFECT THE AMOUNT OF
FRICTION
Static Friction
 Acts on an object that are
not moving
 Strongest- most difficult to
overcome
Examples:
 Desk on the floor
 Parked car on the
pavement
Sliding Friction
 Occurs when one solid
surface slides over another
solid surface
 Not as strong as static
friction
Examples:
 Bicycle brakes
 Pushing a box along the
floor
Rolling Friction
 Occurs when an object
rolls over a surface
 Not as strong as sliding
friction
Examples:
 Shopping cart
 Car traveling on the road
Fluid Friction
 Occurs as an object moves
through a fluid
 Examples:
 Surf boarding in the ocean
 Car as it moves through the
air
Gravity
 Is the force that pulls objects toward each other
 The law of Universal Gravitation- the force of gravity acts
between all objects in the universe!
 That means: any two masses exert an attractive force
on each other
Factors affecting gravity
 Mass- a measure of the amount of matter in an object
 The more mass an object has, the greater its gravitational force
on other objects is
 Distance
 The farther apart objects are, the lesser the gravitational force
between them
 The sun has a gravitational force on you..why don’t you feel it?
 The pencil in front of you has a gravitational force on you…why
don’t you feel it?
Gravitational Acceleration
 The gravitational attraction of Earth causes all falling objects
to have an acceleration of 9.8 m/s2
 Which direction is Earth’s gravitational force?
Weight
 What is it?
 The gravitational force exerted on an object
 W=m x g  g=a
 Calculate the weight of a truck with a mass of 600 kg.
G: m = 600 kg
g = 9.8 m/s/s
Find: w = ?
F: W=m x g
W.S.:
w = 600kg x 9.8 m/s/s
Answer: 5,880 N
Mass vs. Weight
 Mass is _________
 Weight is a FORCE
 the more mass an object has, the stronger the
gravitational force between the object and Earth.
So, the more mass an object has, the more it weighs
 Weight can be different if the gravitational force is different
Weightlessness
 When an object is influences only by the force of gravity, it is
said to be in free fall.
Air resistance
 Air resistance acts in the opposite direct of an object’s
motion
 The force it opposes is GRAVITY
 The amount of air resistance depends on:
 Speed
 Size
 Shape
 Terminal velocity- when the force of air resistance
= weight of object no more acceleration
What happens if you drop 2 objects?
 What would happen if there was no air resistance?
REMEMBER:
 ALL OBJECTS IN FREE FALL ACCELERATE AT THE
SAME RATE, REGARDLESS OF MASS!!
Projectile Motion
 A projectile is an object that has been projected through the
air
 A projectile has horizontal and vertical motion
 Neglecting air resistance, all falling objects accelerate at the
same rate
 No matter how fast and object is moving horizontally, the
force of gravity acts the same as if it was falling straight down