 A scientific law is something that will happen in the physical world each and every time that conditions allow for it. ◦ Gravity ◦ Conservation of momentum.

Presentation on theme: " A scientific law is something that will happen in the physical world each and every time that conditions allow for it. ◦ Gravity ◦ Conservation of momentum."— Presentation transcript:

 A scientific law is something that will happen in the physical world each and every time that conditions allow for it. ◦ Gravity ◦ Conservation of momentum ◦ Newton’s laws

An object in motion or at rest will stay in constant motion or at rest forever and ever unless an outside force acts on it.

 The moon will keep revolving around the earth forever, unless acted on by an unbalanced force.

Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force?

Moon in orbit around earth

 Once airborne, unless acted on by an unbalanced force, it would never stop!

 Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

 There are four main types of friction: ◦ Sliding friction: friction between a sliding object and the surface it is sliding on. ◦ Rolling friction: friction between a rolling object and the surface it is rolling on. ◦ Static friction: friction that keeps a stationary object in place ◦ Fluid friction (air or liquid): air or water resistance What is this unbalanced force that acts on an object in motion?

 Sliding ◦ + ◦ -  Rolling ◦ + ◦ -  Static ◦ + ◦ -  Fluid ◦ Air  +  - ◦ Water  +  -

 Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force.  What force?  Sliding Friction  In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.)

Don’t let this be you. Wear seat belts. The Law of Inertia states that an object in motion stays in motion until acted on by an outside force. Let’s make that outside force the seatbelt and not the windshield or pavement!

 Newton’s 1 st Law in Action. Newton’s 1 st Law in Action.  An person will remain in motion until acted upon by an outside force!  Can you identify the outside forces?

 An object will accelerate in the direction of the greatest force acting on it.  The total force of an object is equal to its mass times its acceleration.

 F = m x a ◦ Mass in kilograms ◦ Acceleration in m/s 2 ◦ Force is kg x m/s 2 or Newtons  One Newton is equal to the force required to accelerate one kilogram of mass at one m/s 2.

 How much force is needed to accelerate a 1400 Kilogram car 2 m/s 2 ?  Write the formula  F = m x a  Fill in given numbers and units  F = 1400 kg x 2 m/s 2  Solve for the unknown  2800 kg X m/s 2 or 2800 N

 1. How much force is needed to accelerate a 66-kg skier 1 m/s 2 ?  2. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/s 2 ?  3. What acceleration will result when a 12 N net force applied to a 3-kg object?  4. A net force of 16 N causes a poodle to accelerate at a rate of 5 m/s 2. Determine the mass of the poodle.

 1. How much force is needed to accelerate a 66 kg skier 1 m/s 2 ? 66 N = 66 kg x 1 m/s 2  2. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s 2 ?  9800 N = 1000 kg x 9.8 m/s 2  3. What acceleration will result when a 12 N net force applied to a 3 kg object? 12 N = 3 kg X 4 m/s 2  4. A net force of 16 N causes a poodle to accelerate at a rate of 5 m/s 2. Determine the poodle’s mass. 16 N = 3.2 kg x 5 m/s 2

 The fastest speed/velocity a falling object can go.  This occurs when downward gravity is balanced by upward air resistance.  If enough time exists, all falling objects eventually reach a constant speed.  This is the only time on earth that a moving object has balanced forces acting on it.

Gravity = 9.8m/s 2 Terminal Velocity of Feather Air Resistance Terminal Velocity of Ball Questions: What is the only difference between the two objects? Which object will accelerate for a longer period of time? Which object will have the greatest terminal velocity? Why?

 Occurs when objects move in a circular/orbital path.  The force that keeps these objects moving in that path.  These objects always have an inertia that is perpendicular to the orbit.  Example: The Moon and Earth. ◦ What would happen if we had a switch to flip and turn off Earth’s gravity?

 How much force does a 2000 kg meteor hit the earth with?  What is the acceleration of a 55 kg swimmer jumping off a cliff at Raystown?  On Earth, when dropped, why does a bowling ball hit the ground before a feather?  On the moon, when dropped, which one will hit the lunar surface first and why?  http://www.youtube.com/watch?v=5C5_dOEyAfk http://www.youtube.com/watch?v=5C5_dOEyAfk

 For every action, there is an equal force and opposite direction reaction.  Draw a diagram for each of the following slides.

 The action force is the arrow pushing backward on the string.  The reaction force the string pushing forward on the arrow.  The mass of the bow + Indian and the acceleration of the string is transferred to the mass of the arrow giving it acceleration.

 Projectile Motion  The action force is the mass of the bullet accelerating forward.  The reaction force is the mass of the gun accelerating backward.  Therefore, guns kick!  The bigger and faster the bullet, the more kick.  The lighter the gun, the more it kicks.

 Pitching a baseball/softball.  The action force is the pitcher pushing backward on the mound.  The reaction is the mound pushing the pitcher toward home plate!

 The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

 Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.  The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

 Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. 

The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

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