Forces

1
Forces

• Types of Forces
• Newtons Laws of Motion
• Weight
• Friction
• Misconceptions

2
Forces

• A force is a push or a pull.
• weight
• pulling a wagon
• friction
• Objects do not always have to be in contact with
  each other.
• The sun exerts a force on the Earth.

3
Four Fundamental Forces

• Gravity
• Anything that has mass has a gravitational
  attraction. The larger the mass, the more
  gravity.
• It is the weakest of the four.
• Electromagnetic
• Force caused by charged particles.
• Attractive or Repulsive

4
Four Fundamental Forces cont

• Strong
• Nuclear (inside the nucleus) holds the nucleus
  together.
• Strongest of the four.
• Only acts over a small distance.
• Weak
• Nuclear
• Form of EM force.
• Responsible for radioactive decay.

5
Newtons Laws of Motion
6
Newtons First Law

• Three parts
• An object in motion stays in motion.
• An object at rest stays at rest.
• Unless acted on by an outside force.
• Inertia- property of matter based on mass says
  the same thing as Newtons 1st Law

7
Newtons 1st Law Example

• Reason for Seatbelts.
• In a front-end-collision, a unrestrained person
  will continue to go forward until something stops
  him/her. Maybe the windshield.
• You can pull a table cloth out from underneath
  dishes an they stay.

8
A Word about Inertia

• Inertia is a property of matter that is
  independent of gravity.
• i.e If someone throws a can of soup at you in
  space, it would still hurt if it hit you in the
  head.
• Likewise, after you get hit you kick the wall of
  the shuttle and it still hurts.

9
Newtons Third Law

• For every action there is an equal and opposite
  reaction.
• example A book resting on a desk exerts a force
  down equal to the force exerted up on the book.

10
Newtons 3rd Law Example

• A bullet applies the same force on a gun as the
  gun applies to the bullet.
• The Earth applies the same force on a person as
  the person applies to the Earth.

11
Newtons 3rd Law Example

• A baseball bat applies the same force to a
  baseball as the baseball applies to the bat.
• So why do they respond differently?
• Mass.
• We will use Newtons 2nd law to explain why.

12
Newtons Second Law

• The acceleration of an object is directly
  proportional to the net Force and inversely
  proportional to the mass of the object.
• a F / m
• F ma
• Net force sum of all the forces acting on the
  object

13
Units

• Look at the units for force.
• F ma kg m/s2
• This is way too much to write so lets give Newton
  some credit.
• Force is measured in Newtons (N).

14
Using Newtons 2nd Law

• Back to the baseball and bat question. If the
  forces acting on each other are the same, why
  does each respond differently.
• A smaller mass means a higher acceleration. A
  larger mass means a lower acceleration.

15
Using Newtons 2nd Law

• Fbat Fbaseball
• Ma mA

16
Weight is a Special Force.

• Weight and mass are different!!!
• Start with F ma
• Gravity accelerates objects at a rate of 9.8
  m/s2.
• Call the force weight.
• Wmg

17
Friction

• Friction is a force that oppose motion.
• Friction is due to the electromagnetic attraction
  between the two surfaces.
• Is friction a good thing?
• Could be. You need it to walk and not fall on the
  ice.
• Much of the time its turned into heat that is not
  needed. (waste)

18
Two types of Friction

• Static friction the frictional force that
  prevents an object from starting to move
• Kinetic friction the frictional force of a
  moving object
• Kinetic friction is less than static friction.
• This is the idea behind anti-lock breaks.

19
Frictional Strength

• What determines the strength of the friction?
• how hard the two surfaces are pushing on each
  other (N, normal force)
• the type of materials (m coefficient of
  friction)
• Items that have no effect
• speed of the object
• contact area

20
Coefficient of Friction

• Determined by the type of material
• Steel on ice has a low m
• Rubber on asphalt has a high m
• Ff m FN

21
Some Misconceptions

• An object that is moving must have a net force
  exerted on it.
• Nope. An object could be moving and not
  accelerating.
• If the velocity is zero then the acceleration and
  force must be zero.
• Nope. Consider a ball at the top of its path.
• An object must move in the direction of the net
  force.
• Nope. An object must accelerate in the direction
  of the net force.

22
Some More Misconceptions

• Heavier objects fall faster than light objects.
• All objects accelerate at 9.8 m/s2 (neglecting
  air resistance)
• When two objects collide, the bigger object
  always applies a larger force.
• Nope. Think Newtons Third Law
• If an object accelerates it must be changing its
  speed.
• Nope. It could be changing directions.