1. CHAPTER 12
FORCES AND
MOTION
12.1 FORCES

2. 12.1 FORCES
There are 4 distinct forces in our universe:
Gravitational, electromagnetic, strong nuclear
and weak nuclear forces.
Ex: everyday force – wind
Force – is a push or pull that acts on an object.
A force can cause a resting object to move, or it
can accelerate a moving object by changing
the object’s speed or direction.

3. *
Gusts of wind push you either
way.
Weight is a type of force (spring
scale) measure force

4. *
Units of Force
Force is measured in Newtons (N)
One Newton is the force that causes a l-
kilogram mass to accelerate at a rate of
1 meter per each second (1 m/s2)
1N = 1kg x m/s2
Named after Sir Isaac Newton, English
scientist who explained how force, mass
and acceleration are related.

5. *
Representing Force
Use arrows to represent the direction and
strength, or magnitude, of a force.
Figure 2 page 357
Force arrows represent the weight of items on
the scale
The longer the arrow the more weight acting on
the scale.
Combining Forces
Net force is the overall force acting on an
object after all the forces are combined.

6. *
Balanced Forces (ex: tug of war)
Sometimes the net force acting on an object is
zero.
These are called balanced forces.
Equal and Opposite Forces
=0
When the forces on an object are balanced the
net force is zero and there is no change in the
object’s motion.
EX: tug of war ~ Where no one wins

7. *
Unbalanced forces
Is a force that results when the net force acting
on an object is not zero.
When an unbalanced force acts on an
object, the object accelerates.
Forces acting in opposite directions can also
combine to produce an unbalanced force.
The net force equals the size of the larger force
minus the size of the smaller force.
Accelerate in the direction of the unbalanced
force ( larger force)

8. *
Ex: Adding forces
=
Subtracting forces
=
Equal and Opposite Forces
=0

9. FRICTION
Friction (4 types)- it is a force that
opposes the motion of objects.
It acts on any surface where 2 objects or
more are in contact.
Without friction all surfaces would be
more slippery than ice.
All objects experience friction.
We need friction to walk. Finger prints
helps up pick things up.

10. *
4 main types
1. Static Friction : Is the friction force that
acts on the objects that are not moving. It
always acts in the opposite direction of the
force being applied. Ex: foot and ground
allows you to walk. Ground pushing up.
2. Sliding Friction: It is the force that opposes
the direction of motion of an object as it slides
over a surface. This force is less than static
friction. Less force is needed to keep
something sliding. Ex: Pushing something
along the floor.

11. *
3. Rolling Friction: The friction force that acts
on a rolling object
100 to 1000 times less than the force of static
friction. Why we use dollies to move heavy
objects. Ball bearings are used in machines to
reduce friction. Ex: Bikes, cars, inline
skates, etc.
4. Fluid Friction: Opposes the motion of an
object through a fluid. Can be air or water.
Air = airplane Water = submarine
Fluid Friction increases as the speed of the
object moving through the fluid increases.

12. *
Gravity – a force that acts between any two
masses
- Attractive force that pulls objects together
- Can act over long distances
- Acts downward toward the center of the Earth

13. *
Falling Objects
- 2 forces act upon this object
- Gravity and Air resistance
- Reach terminal velocity when gravity and air
resistance equal each other (forces balance
each other out)
Projectile Motion – a curved path of an object in
free fall.
Force – initial force forward, force of gravity
pulling it down.

14. 12.2 NEWTON’S FIRST AND SECOND LAWS
OF MOTION
Aristotle thought force was needed to keep
an object moving at constant speed. Not
correct and held up the progress in the
study of motion for about 2 thousand years.
Galileo found out how gravity produced
constant acceleration. Concluded that
moving objects that are not subjected to
friction or any other force would move
indefinitely.
Newton defined mass and force. Wrote his
laws of motion

15. *
Newton’s first law of motion- (the state of
motion)
• The state of motion of an object does not
change as long as the net force acting on the
object is zero.
• Basically an object in motion stays in motion
with the same speed and direction.
• An object at rest stays at rest unless an
unbalanced force acts on it.
• Sometimes called the law of inertia.
• Inertia is the tendency of an object to resist
change in its motion.

16. 12.2
Newton’s Second Law of Motion
The acceleration of an object is equal to the net
force acting on it divided by the objects mass.
Acceleration of an object depends upon its
mass.
Mass is a measure of the inertia of an object
and depends on the amount of matter the object
contains.
Acceleration is directly proportional to the net
force acting on it.

17. 12.2
Formula:
Acceleration = Net Force a=F
Mass m
Units- N/kg and m/s2 are equivalent because
N= kg·m/s
Acceleration of an object is always in the same
direction as the net force
The 2nd law also applies when a net force acts
in the direction opposite to the objects motion.
Force produces a deceleration.

18. WEIGHT AND MASS
An object’s weight is the product of the
object’s mass and acceleration due to
gravity.
We have a weight formula which also
considered Newton’s second law.
Weight = Mass × Acceleration due to gravity
W = mg (just a different form of a=F/m)
g= 9.8m/s2

19. SECTION 12.3 NEWTON’S 3RD LAW OF MOTION AND
MOMENTUM
Forces never exist alone. Forces all
exist in pairs.
Newton’s 3rd Law:
Whenever one object exerts a force on a
second object, the second object exerts
an equal and opposite force on the first
object.

20. *
Action and reaction forces
Not all action-reaction force pairs result in
motion.
Ex: Swimmer in water- yes
Push on wall-no
Why don’t action-reaction forces cancel out
and produce a net force of zero?
It is because reaction and action forces do
not act on the same object.

21. *
Ex: Figure 16 P. 373
The action force of the swimmers arms
act upon the water.
The reaction force acts upon the
swimmer ( the water pushing on the
swimmer)
Only equal and opposite forces act on
the same object and give a net force of
zero.

22. *
Momentum- is the product of an object’s mass
and its velocity.
An object with a large momentum is hard to
stop.
An object has a large momentum if the product
of its mass and velocity is large.
Formula:
Momentum = Mass × velocity

23. *
Law of conservation of momentum
If no net force acts on a system, then the
total momentum of the system does not
change.
See diagrams page 376

24. SECTION 12.4 UNIVERSAL FORCES
Electromagnetic forces
• Associated with charged particles
• Electric force and magnetic force are the only
forces that can attract and repel.
Electric forces- between charged objects or
particles such as electrons and protons.
• Same charge – repel
• Opposite charge - attract

25. *
Magnetic forces – act on certain metals, on
the poles of magnets, and on moving charges.
• 2 poles- N and S, attract each other
• 2 like poles (N,N or S,S) repel each other
Nuclear forces
2 forces, the strong and weak nuclear forces
act within the nucleus to hold it together.

26. *
Strong nuclear force- overcomes the electric
force that acts among the protons.
• goes the diameter of a proton (distance it
acts on)
• Powerful force of attraction
• Acts only on the neutrons and protons in the
nucleus, holding them together.

27. *
Weak nuclear force- involved in
certain types of radioactive
processes
• Weaker than strong nuclear force
• Also located in the nucleus of an
atom
• Acts over a shorter range than the
strong nuclear force.

28. *
Gravitational force
• Is the weakest universal force
• Is an attractive force that acts between any
two masses
• Keeps the moon in its orbit
• Pulls the moon towards the Earth.
• Newton’s law of universal gravitation states
that every object in the universe attracts
every other object.

29. *
• Gravitational force between 2
objects is proportional to their
masses.
• It decreases as the distance
between the masses increase.
• Greater the mass of the objects, the
greater the gravitational force.
• Most effective universal force over
long distances.

30. *
Why do we 2 high tides and 2 low tides.
Gravitational pull from the moon produces 2
bulges in the Earth’s oceans.
One on the side of the Earth closest to the
moon.
One on the side of the Earth farthest from
the moon.
This is also why satellites stay in the sky.