Warm Up What actually causes motion? In other words, how do objects start, stop, or change direction?

Dynamics - the study of what causes motion

Newton’s First Law of Motion – “Inertia”

Galileo For 2000 years, people believed objects were naturally at rest. For 2000 years, people believed objects were naturally at rest. Galileo used experimentation to disprove 2000 years of accepted belief in motion Galileo used experimentation to disprove 2000 years of accepted belief in motion

Galileo ( ) “An object, once placed in motion, will move further until another force brings it to a stop”. “An object, once placed in motion, will move further until another force brings it to a stop”. Implied that the natural state of an object is constant motion. Implied that the natural state of an object is constant motion.

What does this actually mean? “Implied that the natural state of objects is continuous motion.” “Implied that the natural state of objects is continuous motion.”

What does this actually mean? “Implied that the natural state of objects is continuous motion.” “Implied that the natural state of objects is continuous motion.” Things stay in motion unless something stops it. Things stay in motion unless something stops it. Galileo figured out the identity of that something and called it “Resistance” Galileo figured out the identity of that something and called it “Resistance”

Galileo’s Breakthrough #1 He realized that things slowed down due to friction. He realized that things slowed down due to friction. Friction opposes all motion. Friction opposes all motion.

Galileo’s Breakthrough #2 In the real world, we have friction. What if there was no friction?  What would happen to motion? Galileo hypothesized and experimented discovered something entirely new.

In a frictionless world … Objects will never stop, will go forever Objects will never stop, will go forever Objects needs no force to keep it moving Objects needs no force to keep it moving

Why don’t things move on their own on a frictionless surface?

Something keeps them from moving That “something” must be universal Photograph by Beverly Joubert

In a frictionless world … All objects have “it” even on a frictionless surface. All objects have “it” even on a frictionless surface.

“INERTIA” Newton incorporated this into his work and called the property “inertia”.

Newton’s First Law of Motion The Law of Inertia When no external, unbalanced force acts on an object its velocity remains constant. or When no external, unbalanced force acts on an object its velocity remains constant. or If no net force acts on an object, it maintains its state of rest or its constant speed in a straight line or If no net force acts on an object, it maintains its state of rest or its constant speed in a straight line or An object at rest will remain at rest, an object in motion will remain in motion in a straight line unless they are acted upon by an external force. An object at rest will remain at rest, an object in motion will remain in motion in a straight line unless they are acted upon by an external force.

What is Inertia? Inertia is a property of matter like color and mass. Inertia is directly related to mass  More mass, more inertia Mass is a measurement of the amount of inertia and matter.  mass = matter = inertia

But, what is inertia? Inertia is the “laziness” of matter Matter does not like to change its motion The more matter, the lazier it gets  More matter = harder to change

Example An elephant at rest has a lot of “laziness” so it has a greater tendency to remain at rest. An elephant at rest has a lot of “laziness” so it has a greater tendency to remain at rest. (hard to move) (hard to move)

Example A runaway train at constant velocity has a lot of “laziness” so it has a greater tendency to stay in motion. A runaway train at constant velocity has a lot of “laziness” so it has a greater tendency to stay in motion. (hard to stop) (hard to stop)

Implications External force is required to accelerate an object. External force is required to accelerate an object.  Remember what accelerates mean? External force must be unbalanced. External force must be unbalanced.

NET Force – resultant force. Which object will remain at rest, according to Newton’s First Law? Which object will remain at rest, according to Newton’s First Law? Which object will continue moving with constant velocity, according to Newton’s First Law? Which object will continue moving with constant velocity, according to Newton’s First Law? 5 N 8 N This car will accelerate. The cat will move

Look

True or False?

6 Notions of Force and Motion 1. If there is motion, there is a force acting to keep an object in motion. 2. There cannot be a force without motion- if there is no motion then there is no force acting. 3. When an object moving there is a force in the direction of motion.

6 Notions of Force and Motion 4. A moving object has a force within it which keeps it moving. 5. A moving object stops when its force is used up. 6. Motion is proportional to force acting. Therefore a constant speed result from a constant force.

Answers? NOT one of these notions are correct. These are 6 commonly held misconceptions of force and motion, and they all originate from Aristotle’s views.

Difference between Mass and Weight

Differentiating Mass and Weight Mass measures the amount of matter  Mass is a scalar Does the amount of matter change when an object is moved from the earth into space?

Differentiating Mass and Weight Weight depends on the mass of an object TIMES the acceleration due to gravity.  Weight is a FORCE and a vector.

Compare your mass and your weight Divide your weight in pounds by 2.2.  This is your mass in kg What is your weight on the moon where gravity is 1/6 th the earth’s? What is your mass on the moon?

Forces

Force SI unit – Newton; N A force needed to accelerate a 1 kg mass 1 m/s every second. 1kg*m/s 2 = 1N A vector quantity

A force is a push or a pull A force can cause  1. a stationary object to move  2. a moving object to stop  3. an object to accelerate (change speed or direction)

Net Force - Resultant Force Net force – the combination of all the forces acting on an object. The net force changes an object’s state of motion.

Net Force Balanced force – Net force is equal to zero  Object remains at rest  Object maintains its motion 5 N

Net Force Unbalanced force – Net force is NOT equal to zero  The cat will move  The car will accelerate 5 N 8 N

Net Force Objects move in the same direction as the Net Force  What is the Net Force on the cat?  Which way does it move?  And the car? 5 N 8 N 3 N

What are the forces acting on this book? 1. Gravitational Force (Weight) 2. Normal Force (Support Force) The net force on the book is 0N. The forces acting on the book are balanced and the book is in a state of equilibrium.

Types of Forces Normal ForceNormal Force Magnetic ForceMagnetic Force Electrical ForceElectrical Force Gravitational Force Gravitational Force Frictional Force - F fFrictional Force - F f Tension Force - F tTension Force - F t Normal Force - F nNormal Force - F n Air Resistance Force - F airAir Resistance Force - F air Applied Force - F appApplied Force - F app Spring Force - F springSpring Force - F spring Gravitational Force (Weight) – F gGravitational Force (Weight) – F g Resultant or Net Force - F netResultant or Net Force - F net Contact Force Field Force

Free body Diagrams Determine the resultant vector (net force) for these objects. 400 N, Up200 N, Down20 N, Left

Determine the net force for each situation. 0 N – balanced force 5 N, Left 0 N – object in equilibrium 15 N, Up

Exit Ticket Determine the magnitude of the unknown forces.

Light Bulb Moment No matter how many applied forces are acting on an object AND the net force is equal to zero, the object will maintain its inertia. No matter how many applied forces are acting on an object AND the net force is equal to zero, the object will maintain its inertia. Balanced forces on an object, maintains the state of inertia of an object. Balanced forces on an object, maintains the state of inertia of an object.

If the forces acting on an object are cancelled, the object is said to be in a state of equilibrium. Meaning the net force is equal to zero and the object maintains its inertia. Weight Air Friction The object is at rest Normal Force by the ground Falling objects will experience terminal velocity

Example If you are standing, at rest, the balanced forces acting on you are: If you are standing, at rest, the balanced forces acting on you are: a.) your weight and b.) the upward support force of the ground called the normal force. normal force.

Questions 1. Why do professional photographers use heavy cameras for their shots? 1. Why do professional photographers use heavy cameras for their shots? 2. While making a turn to the right, why is your body “moving towards the left side of the car”? 2. While making a turn to the right, why is your body “moving towards the left side of the car”? 3. Why do we use safety seat belts? 3. Why do we use safety seat belts? 4. In which location will it be difficult to shake a 50 kg rock, on earth or in space? 4. In which location will it be difficult to shake a 50 kg rock, on earth or in space?

Questions 5. Which object has more inertia, a 15 kilogram bowling ball or a 15 kilogram bag of pure cotton? 5. Which object has more inertia, a 15 kilogram bowling ball or a 15 kilogram bag of pure cotton? 6. While standing still what is your inertia with 6. While standing still what is your inertia with respect to the sun? with respect to the ground? respect to the sun? with respect to the ground? 7. What is the value of the normal force if you are standing on the table? 7. What is the value of the normal force if you are standing on the table? 8. If you throw a ball horizontally and we neglect air friction and the force of gravity, how will you describe the motion of the ball? 8. If you throw a ball horizontally and we neglect air friction and the force of gravity, how will you describe the motion of the ball?

Helps explain the behavior of objects in motion- they resist change in motion due to their inertia. Helps explain the behavior of objects in motion- they resist change in motion due to their inertia.