The Law of Inertia
Galileo said Marbles rolled down a ramp will reach the same height as that from which they are released no matter how long the ramp. So, what if the ramp is flat?
Newton said Objects at rest remain at rest unless acted upon by an outside force. Objects in motion will remain in motion unless acted upon by an outside force. What causes the tire to stop rolling?
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Inertia is dependent on mass. The larger the mass, the more inertia it has.
Forces can change the motion of an object. A force is a push or pull. What kinds of changes occur?
Contact Forces Any force applied when things are in contact. Ex. Push, pull, air resistance, friction Field Forces Any force applied, but there is no contact. Ex. Gravity, Magnetism, electric field
The unit of force is a newton. (Appropriately named) One Newton = the amount of force needed to accelerate 1 kilogram by 1 meter per second squared. 1 kg a = 1 m/s/s
Force is a vector quantity. 100 Newtons The length of the arrow shows the strength of the force. The direction of the arrow shows the direction that the force is applied. Force vectors are added and subtracted just as velocity vectors are The sum of the force vectors is called the net force.
Finding Net Force: If the net force is zero. There is no change in motion. The system is at equilibrium. The forces are balanced. An unbalanced force results in a change in motion in the direction of the larger force.
Finding Net Force: Dumb & Dumber moving a piano examples: http://www.youtube.com/watch?v=-s4nvg_w
Finding Net Force: Drawing free-body diagrams floor pushing up Gravity pulling down
A box being pushed across a table. Draw the free diagram.
Weight is a force. It is the force of gravity acting on an object s mass. Weight (in newtons) = mass x acceleration due to gravity. 1 kg x 9.8 m/s/s = 9.8 Newtons Can you calculate your weight in Newtons?
Friction is a force which is caused by objects touching and that opposes motion, but is necessary for basic movement. Friction helps us stand Friction helps go forward and stop Friction helps us walk and run
Friction can also hinder motion. Can you think of situations where friction is a disadvantage?
Friction helpful or harmful?
Newton s second law explains the relationship between the net force on an object, it s mass and the acceleration that results from the force:
Let s look at the relationships: If mass is constant, the larger the force on an object, the faster the objects accelerates. Large force = mass x large acceleration Small force = mass x small acceleration This is a direct proportion.
Let s look at the relationships: If the force is constant, the larger the mass of an object, the slower the object accelerates. force = small mass x large acceleration force = large mass x small acceleration This is an indirect or inverse proportion.
Let s look at the relationships: If the acceleration is constant, the larger the mass of an object, the stronger the force must be to change the motion. small force = small mass x acceleration large force = large mass x acceleration This is a direct proportion.
Solving problems related to Newton s second law: The basic equation is: Force = mass x acceleration or F = ma Try this: Rearrange the variables so that you can solve for acceleration or mass. a = f / m m = f / a
For Practice: Example 1 How much force must a 30,000 kg jet plane develop in order to achieve an acceleration of 1.5 m/s/s?
Example 2 What acceleration is produced by a force of 500 N applied to a 1000 kg car?
Example 3 If a grocery cart is pushed with a force of 75 N, and the cart accelerates from rest at a rate of 1.5 m/s/s, what is the mass of the cart? What is the weight of the cart?
What about friction? Friction is a force that opposes motion. The force applied must be greater than the force of friction in order to accelerate an object.
Net force Fnet = Fapplied - Ffriction Ffriction Fapplied Consider this box. In order to make it accelerate, an applied force larger than the force of friction is needed to make it accelerate.
Example problem: A 500 kg cart is pushed across the floor with a force of 40 N. The floor and wheels oppose the motion with a force of 10 N. What is the acceleration of the cart?
What about upward motion? Gravity is a force pulling down on an object. In order to accelerate an object upward, the applied force must be larger than the weight of the object.
Net force Fnet = Fapplied - Fweight Fapplied Fweight Consider this rock. In order to make it accelerate, an applied force larger than the weight of the rock is needed to make it accelerate.
Example problem: A 1000 kg elevator is accelerated upward. The weight of the elevator opposes the motion. What force must be applied in order to cause an acceleration of 1m/s/s?
Newton s Third Law For every Action there is an equal and opposite Reaction Forces ALWAYS come in pairs.
Examples: Hammer and nail Your chair and your seat. You and the ground Pushing against the wall Tires and the tar Rocket and propelling Gasses Why does the nail go into the board?
A batter exerts some 6000-8000 pounds of force on the ball. This force is required to change a 5 1/8th-ounce ball from a speed of 90 mph to a speed of 110 mph, this distorts the baseball to half its original diameter and the bat is compressed one fiftieth of it's size. How many Newtons of force are applied to the ball? How much force does the ball apply to the bat?
The horse pulls on a wagon with a force of 10,000 Newtons, with what force does the wagon pull on the horse? How is it possible that the horse and wagon move?