# Forces

Document Sample

```					     Forces
•   Objectives
•   What is a force?
•   What is gravity?
•   What is friction?
•   What is pressure?
Forces
• Force is a push or a pull, that causes a change in
the motion or shape of an object.

• Words that indicate
the use of a force:
• Pushing , Pulling,
Stretching, Squeezing,
Bending, and Falling.
• Force is measured in
units of: Newtons (N)
Forces
• Force is a push or a pull.
• Example:

Electric
Forces
• Force is a push or a pull.
• Example:

Gravitational
Forces
• Force is a push or a pull.
• Example:

Magnetic
Forces
• Forces can be equal. (Balanced Forces):
• If the two girls push with the same amount of force
on the stack of books,
• the books do not move
due to balanced forces.
• Other examples:
• Book laying on a
table, person sitting in
a chair, or a light
signal hanging from a
power line.
Forces
• Forces can also be unequal. Unbalanced forces.
• When an unbalanced force is applied to an object
motion is produced.
• Any object in motion is
due to an unbalanced
force.
• Motion requires a force.
• The overall amount of
force used to move an
object is called net
force.
Forces
• Finding the net force applied to an object
requires the sum of all applied forces.

• Forces that are
applied in the same
• Forces that are
applied in opposite
directions are
subtracted.
Forces
• Remember No motion = NO net force!
net Force examples
• If Mary pushes with 260 newtons of force
and Janet pushes 170 newtons of force, what
is the net force on the books?
260 n
-170 n
90 n
To the right
Mary             Janet
net Force examples
• If Mary pushes with 210 newtons of force
and Janet pushes 235 newtons of force, what
is the net force on the books?
235 n
-210 n
25 n
To the left
Mary             Janet
net Force examples
• If a plane travels north with an engine thrust of 25N
of force and hits a cross wind going east with 5N of
force. What is the net force and overall direction of
the plane?
5N
East               C=     a2 + b2

C=     252 + 52
25 N
North           25.5 N
Northeast   C=     625 + 25

C = 25.5 N
Gravity
• Gravity is a Universal Force.
• Gravitational Force causes every object to
attract every other object;
• a ball falling to the ground
• the moon orbiting the Earth

Greater Mass = greater force
Friction
• Friction is a force that opposes motion.
• Friction acts in a direction opposite to the
object's direction in motion.
• Without friction, the object would continue
to move at a constant speed forever.
• There are 3 types of friction.
Friction
• Static Friction
• In static friction the two surfaces are not
moving past each other.

• Here the friction
between the 2
surfaces is so great
that there is no
motion.
Friction
• Sliding Friction
• The opposing force of the two surfaces
slides past each other.
• Friction is overcome so that there is motion.
Friction
• Rolling Friction
• The opposing force of
the two surfaces roll
past each other.
• Friction is overcome
so that there is motion.
Force: F=ma
• Force is connected with Newton’s 2nd law of
motion.

F = ma
Units: N = kg m
s 2
Force: F=ma
• What is the force of impact of a 800kg car that
is traveling at 10 m/s2 when it hits another car?
• Force = ?
• Mass (m) = 800kg
• Acceleration (a) =
10 m/s2
Formula:       F = ma
Substitution: F = (800kg) (10m/s2)
Results:       F = 8,000 N
Force: F=ma
• How much force is needed to accelerate a
7000kg car at a rate of 1.5 m/s2 ?
• F= ?
• m = 7000kg
• a = 1.5 m/s2

Formula:      F = ma
Substitution: F = (7000kg) (1.5m/s2)
Results:      F = 10,500 N
Force: F=ma
• A weightlifter raises a 150kg barbell with an
acceleration of 2.0 m/s2. How much force does
he use to lift the weights?
• F= ?
• m = 150kg
• a = 2.0 m/s2

Formula:      F = ma
Substitution: F = (150kg) (2.0m/s2)
Results:      F = 300 N
Force: F=ma
• A bowler takes a 32kg ball and rolls it with an
acceleration of 2.8 m/s2. How much force does
the bowler use?
• F= ?
• m = 32kg
• a = 2.8 m/s2
Formula:       F = ma
Substitution: F = (32kg) (2.8m/s2)
Results:       F = 89.6 N
Force: F=ma
• A dancer lifts his partner above his head with a
force of 100N and an acceleration of 4.0 m/s2.
What is the mass of the dancer?
• m= ?
• F = 100 N
• a = 4.0 m/s2
Formula:     m = F/a
Substitution: m = 100N /4.0 m/s2
Results:     m = 25.0 kg
Force: F=ma
• If a car with a mass of 2,500 kg hits a tree with a
force of 500,000 N. What is the acceleration of
the car as it hits the tree?
• a= ?
• m = 2,500 kg
• F = 500,000 N
Formula:       a = F/m
Substitution: a = 500,000 N / 2,500 kg
Results:       a = 200 m/s2
Force: F=ma
• A rocket, with a mass of 20 kg, is shot into the
air with 2,000 N of force. What is the
acceleration of the rocket?
• a= ?
• m = 20 kg
• F = 2,000 N
Formula:       a = F/m
Substitution: a = 2,000 N / 20 kg
Results:       a = 100 m/s2
Force: F=ma
• Another rocket is shot into the air with 2,000 N
of force, but it’s acceleration was only 10 m/s2.
What is the 2nd rocket’s mass?
• m= ?
• F = 2,000 N
• a = 10 m/s2
Formula:       m = F/a
Substitution: m = 2,000 N / 10 m/s2
Results:       a = 200 kg
Pressure
• Pressure is the force per unit area.
• Pressure is the amount of force acting on an
area.

• Each book exerts the
same force on the table.
• However, which book
exerts the greater
pressure? Why?
Pressure
• Is it possible for a 1 pound force to exert the
same amount of pressure as a 50 pound force?
• Yes, because P = F/A
• Notice that when a
force acts on a
smaller area the
pressure is larger.
• And when a force is
acting on a larger
area, the pressure is
smaller.
Pressure
• Pressure exerted by liquids and gases is known as
Fluid pressure.
• 2 common forms of fluid pressure: Air & Water
pressure

• Air pressure is
measured with a
barometer.
• Water pressure is
measured with a
manometer.
Pressure
• Air pressure changes with altitude.
• As the elevation increases the air pressure will decrease.
• As the elevation decreases the air pressure will increase.
Pressure
• Water pressure changes with depth.
• As the depth increases the water pressure will increase.
• As the depth decreases the water pressure will decrease.
Bernoulli’s Principle
• This principle that states that as the speed of
a fluid increases, the pressure decreases.

• This principle is what allows the wing of an airplane
to produce lift and makes flight possible.
Bernoulli’s Principle
• There are 4 forces on a plane.
• Gravity pulls the airplane down while lift works
against gravity to push the plane up.

• The thrust of the
plane’s engine
pushes the plane
forward, while drag
tends to slow the
plane down due to
air resistance.

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 33 posted: 8/13/2011 language: English pages: 33