martes, 1 de noviembre de 2011

Newton's Laws of Motion


Objetivo de la secuencia:
1. Interpreta y aplica las Leyes de Newton como un conjunto de
reglas para describir y predecir los efectos de las fuerzas en
experimentos y/o situaciones cotidianas.
2. Valora la importancia de las Leyes de Newton en la explicación de
las causas del movimiento de los objetos.


Isaac Newton (a 17th century scientist) put forth a variety of laws that explain why objects move (or don't move) as they do. These three laws have become known as Newton's three laws of motion. The focus of Lesson 1 is Newton's first law of motion - sometimes referred to as the law of inertia.


Newton's first law of motion is often stated as

* An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
* Every object persists in its state of rest or uniform motion­ in a straight line unless it is compelled to change that state by forces impressed on it.
* ­An object at rest will stay at rest, forever, as long as nothing pushes or pulls on it. An object in motion will stay in motion, traveling in a straight line, forever, until something pushes or pulls on it.

*Objects keep on donig what they are doing.

1. A magician takes the table clothe without breaking the dishes.
2. When your automobile starts its motion, you move backward and if you brakes ypu go forward.
3. At "Recórcholis" there is a game wich uses discs that float over a table and continue in motion becasuse of the air that goes upward.
4. Blood rushes from your head to your feet while quickly stopping when riding on a descending elevator.
5. The head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface.
5. A brick is painlessly broken over the hand of a physics teacher by slamming it with a hammer. (CAUTION: do not attempt this at home!)
6. To dislodge ketchup from the bottom of a ketchup bottle, it is often turned upside down and thrusted downward at high speeds and then abruptly halted.
7. Headrests are placed in cars to prevent whiplash injuries during rear-end collisions.
•While riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb or rock or other object that abruptly halts the motion of the skateboard.

Mass as a Measure of the Amount of Inertia

All objects resist changes in their state of motion. All objects have this tendency - they have inertia. But do some objects have more of a tendency to resist changes than others? Absolutely yes! The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is solely dependent upon the inertia of an object. The more inertia that an object has, the more mass that it has. A more massive object has a greater tendency to resist changes in its state of motion

Physicists use the term inertia to describe this tendency of an object to resist a change in its motion. The Latin root for inertia is the same root for "inert," which means lacking the ability to move. So you can see how scientists came up with the word. What's more amazing is that they came up with the concept. Inertia isn't an immediately apparent physical property, such as length or volume. It is, however, related to an object's mass. To understand how, consider the sumo wrestler and the boy example in "How stuff works"

Which person in this ring will be harder to move? The sumo wrestler or the little boy?

Inertia: the resistance an object has to a change in its state of motion (constant velocity or repose).
Inertia: tendency of an object to resist changes in its velocity
Inertia: tendency of an object to resist accelerations


Newton's second law of motion can be formally stated as follows:

*The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
*­When a force acts on an object, the object accelerates in the direction of the force. If the mass of an object is held constant, increasing force will increase acceleration. If the force on an object remains constant, increasing mass will decrease acceleration. In other words, force and acceleration are directly proportional, while mass and acceleration are inversely proportional.
*For a constant mass, force equals mass times acceleration

So what can you do with Newton's second law? As it turns out, F = ma lets you quantify motion of every variety. Let's say, for example, you want to calculate the acceleration of the dog sled shown below.

If you want to calculate the acceleration, first you need to modify the force equation to get a = F/m. When you plug in the numbers for force (100 N) and mass (50 kg), you find that the acceleration is 2 m/s2.

Now let's say that the mass of the sled stays at 50 kg and that another dog is added to the team. If we assume the second dog pulls with the same force as the first (100 N), the total force would be 200 N and the acceleration would be 4 m/s2
Notice that doubling the force by adding another dog doubles the acceleration. Oppositely, doubling the mass to 100 kg would halve the acceleration to 2 m/s2.

Finally, let's imagine that a second dog team is attached to the sled so that it can pull in the opposite direction
If two dogs are on each side, then the total force pulling to the left (200 N) balances the total force pulling to the right (200 N). That means the net force on the sled is zero, so the sled doesn’t move

How do we measure forces?

1 Newton = 1 Kg * m/s^2

The definition of the standard metric unit of force is stated by the above equation. One Newton is defined as the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s.

*Even today people still believe that a force is required to keep an object moving.

Newton's Third Law of Motion - LAW OF FORCE PAIRS

Newton's third law is probably the most familiar. Everyone knows that every action has an equal and opposite reaction, right? Unfortunately, this statement lacks some necessary detail. This is a better way to say it:

*A force is exerted by one object on another object. In other words, every force involves the interaction of two objects. When one object exerts a force on a second object, the second object also exerts a force on the first object. The two forces are equal in strength and oriented in opposite directions.

*For every action, there is an equal and opposite reaction.

*The statement means that in every interaction, there is a pair of forces acting on the two interacting objects.
*The size of the forces on the first object equals the size of the force on the second object.
*The direction of the force on the first object is opposite to the direction of the force on the second object.
*Forces always come in pairs - equal and opposite action-reaction force pairs.


1. Propulsion of a fish through the water.
2. Flying motion of birds
3. You move your feet backward. Your body moves forward
4. Motion of a car on the way to school
5. Interaction between a baseball bat and a baseball
6. Baseball pushes glove leftwards. The glove pushes the baseball rightward.
7. Bowling ball pushes pin leftwards. Pin pushes bowling ball rightward
8. Enclosed air particles push balloon wall outwards. Balloon wall pushes enclosed air particles inwards


1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will
a. gradually stop.
b. continue in motion in the same direction at constant speed.

2. A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

3. Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain why.

4. Supposing you were in space in a weightless environment, would it require a force to set an object in motion?

5. Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and consuming large quantities of food. What effect (if any) does this practice have upon his inertia? Explain.

6. If the forces acting upon an object are balanced, then the object

a. must not be moving.
b. must be moving with a constant velocity.
c. must not be accelerating.
d. none of these

7. Determine the accelerations that result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.

8. A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.

9. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is doubled, then what is the new acceleration of the sled?

10. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is halved, then what is the new acceleration of the sled?

11. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?

12. For years, space travel was believed to be impossible because there was nothing that rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ...

a. ... space is void of air so the rockets have nothing to push off of.
b. ... gravity is absent in space.
c. ... space is void of air and so there is no air resistance in space.
d. ... nonsense! Rockets do accelerate in space and have been able to do so for a long time.

13. Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...

a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.

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