Unit II Forces
Force = a push or pull on an object
measured in Newtons (N)
4 Ways Force Effects an Object
1. Increase v
2. Decrease v
3. Change Direction
4. Change Shape
What are the forces action on box
being pushed?
Which box is moving?
A
B
Net Force = sum of forces acting
on an object



Forces are vectors
Same direction add
Opposite direction
subtract
Balanced vs. Unbalanced Forces
Balanced Forces: the
forces on an object are
equal and opposite.
Unbalanced Forces: the
forces are not equal and
opposite
-no change to the
motion
-the object will change
-object does not
accelerate
-the object will
accelerate
Friction: force that opposes motion
of an object
1. Static
Friction
3. Sliding
Friction
2.
Rolling
Friction
4. Fluid
Friction
Laws of Motion
12.2-12.3
1st Law of Motion

An Object in
motion will remain
in motion, or an
object at rest will
remain at rest
unless acted upon
by an unbalanced
force
teachertech.rice.edu/.../Newton
/law1.html
Inertia

The tendency of an
object to keep
doing what it is
already doing
2nd Law of Motion

Force, mass and
acceleration are all
related
2nd Law
Force = mass x acceleration
F = ma
3rd Law of Motion


Action and
Reaction
For every
action there
is an equal
and opposite
reaction
http://upload.wikimedia.org/wikipedia/commons/f/f6/Skaters_showing_newtons_third_law.png
How is weight different than mass?

Weight  pull of gravity acting on an
object
Weight = mass x acceleration due to gravity

Mass  amount of matter in an
object
Sample Problems
1. A boy pushes a cart of groceries
forward with a total mass of 40 kg.
What is the acceleration of the car if
the net force on the cart is 60 N?
1.5 m/s²
2. What is the upward acceleration of
a helicopter with a mass of 5000 kg
if a force of 10000 N act on it in an
upward direction?
2 m/s²
3. An automobile with a mass of 1200
kg accelerates at a rate of 3 m/s/s
in the forward direction. What is
the net force acting on the
automobile?
3600 N
4. A 25 N force accelerates a boy in a
wheelchair at 0.5 m/s/s. What is
the mass of the boy and the
wheelchair?
50 kg
Sample Problems
5. A girl pushes a box of books
forward with a total mass of 80 kg.
What is the acceleration of the box
if the net force on the box is 40 N?
0.5 m/s²
6. An truck with a mass of 3600 kg
accelerates at a rate of 2 m/s/s in
the forward direction. What is the
net force acting on the automobile?
7200 N
7. A 30 N force accelerates a boy on a
tricycle at 0.5 m/s/s. What is the
mass of the boy and the tricycle?
60 kg
14.1 Work




Definition: the product of distance and
the force in the direction an object
moves
W=Fxd
SI unit for work: newton-meter (N·m)
or joule (J)
If there is no movement, no work is
done
Power

What is power?
• The rate of doing work
• Doing work faster requires more power

Power = Work
Time

SI Unit – Watt (W)
Momentum



Momentum = mass x velocity of an
object
Momentum is large if product of
mass and velocity is large
Objects with large momentum’s are
hard to stop
Momentum


Momentum of an object at rest = 0
SI Units = kg*m
s
Momentum
Law of Conservation of Momentum
If no net force acts on the system, the
total momentum of the system does
not change.


In a closed system, momentum is
conserved.
Momentum

Problem
Which has more momentum, a
0.046 kg golf ball with a velocity of
60 m/s or a 7.0 bowling ball with a
velocity of 6 m/s?
Remember: Momentum = mass x velocity
12.4 Universal Forces

What are the 4 universal forces?
• Electromagnetic
• Nuclear (Strong and Weak)
• Gravity

How are these different from forces
talked about in the 12.1-12.3?
• All act over a distance between particles
Describe each universal force
and what makes them unique.
1.
Electromagnetic Force
• only force that can attract and
repel
2.
Gravitational force
• An attractive force between 2
masses
Describe each universal force
and what makes them unique.

Nuclear force: act within
nucleus to hold it together
3. Strong – acts only on neutrons
and protons in the nucleus (100x
stronger than charges)
4. Weak – attractive force that acts
over a short range
Force
Can be
represented by
Can be
Measured in
units of
unbalanced
Causes no change to
the object
Causes the
object to
Force
Can be
represented by
arrows
Can be
Measured in
units of
unbalanced
Newtons
balanced
Causes no change to
the object
State of
Motion
Causes the
object to
Accelerate