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NEWTON’S LAWS I inertia |
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Classical physics is sometimes called Newtonian physics in honor of Sir Isaac Newton who lived from 1642 to 1727 and formulated three laws of motion that summarize much of the behavior of moving bodies. Forces may cause motion. Inertia tends to resist the influence of an applied force. Forces, inertia, friction, and how they relate to motion are considered now. |
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FORCES AND THE LAW OF INERTIA (first Newton’s Law)
A force is a push or a pull that tends to cause motion or tends to prevent motion.
Force is a vector quantity and thus has both magnitude and direction. The unit is the newton. For example: The force due to gravity, acting on 1kg mass, is 9.8N down 9.8N is the magnitude and down is the direction.
The force tends to produce an acceleration in the direction of its application. For example: A free-fall body accelerated downward because of the force of gravity.
Some force do not produce motion because they are balanced by other forces. In that case, the net force is zero
Draw the forces acting on:
· A book on a table · A car moving at a constant speed · A car accelerating · A resting load on a string · A rock falling. Neglect air resistance. · A feather falling with a constant speed · A sled pulled by you. Your sister is sitting on it. The speed is constant. · A wagon pulled by your friend. The wagon is accelerating.
Only 4 fundamental forces can explain any interaction between 2 objects:
· the force of gravity Newton found that the same force (gravitational force) explain the motion of planets or other objects in space and the motion of object on earth (like a projectile). He showed that the moon falls toward the Earth the same way an apple falls toward it. It was the first unification of 2 forces. · The electromagnetic force. Maxwell unified the electric force and the magnetic force. He showed that electricity and magnetism were 2 aspects of the same phenomenon. He wrote 4 elegant mathematical equations to show this unification. More extraordinaire: his equations lead to the value of the speed of light. This time, mathematics showed the way to physicists. · The strong nuclear force. This force was discovered in the 20th discovery what physicists started to understand atoms and their structure. This force hold the protons together despite their strong repulsion. That's why this force is very strong but only over a very short distance. · The weak nuclear force. This force acts inside the particles of the nucleus. Physicist were able to unify this force with the electromagnetic force leading to the electroweak force. The weak force is involved in breaking apart some nuclei and changing neutrons into protons.
One of the challenge of Physics is to unify all the 4 fundamental forces of nature. That was the big problem that puzzled Einstein during years. It is what string theory is about. Unification of the 4 forces. So far, string theory has not been proven right. Just ideas. |
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The units for measuring force are the newton (N) in the metric system and the pound (lb) in the English system. The conversion factor is 4.45N = 1 lb 1 kg mass is acted upon by a force of gravity of 9.8N on Earth or 2.2 lbs
If a large object is at rest (car), it resists being moved. That is, you have to produce a tremendous push to get it moving. Similarly, if a car is moving, it takes a large force to stop it (the brakes). This property of resisting a change in motion is called inertia.
Inertia is the property of a body hat causes it to remain at rest if it is at rest or to continue moving with a constant velocity unless an unbalanced acts upon it.
Every object continues in its state of rest or motion, in a straight line, at a constant speed, unless compelled to change that state by forces exerted upon it.
Try to do some of the demonstrations shown in the movie: - Use a cart. Push a cart with 500g inside. Then with 1000g inside - Hang 1 string at each end of a 1,0000 mass. Try to break the top one and then the bottom one. - Hang 1 string to a large mass and lift it without breaking the string. And breaking the string. -Put your hand under a physics book and hit the physics book - Put your hand on a stick and hit the table with the stick. You hand keeps moving it hurts! Try to do some of the demonstrations shown in the movie: - A penny on an index card. The index card is on a glass. Give a kick to the index card. - A newspaper flat on a paper. Insert a flat ruler between the table and the newspaper. Push down quickly on the ruler to lift the newspaper. - Fill a glass with water. Insert a paper between the glass and the paper. Try to pull the paper without spilling the water. -Put a block in a cart. Push the cart then stop it abruptly. What happens to the block/ - attach a soft ball to a string. Turn the ball in a vertical plan. Let the ball goes when it is at the top. watch this one extra credits if you can do the same ! Try to answer the questions without the hints. Unless you are confused.
1) If so why when you stop pushing the accelerator of a car, the car will slow down and stop instead of keeping going with a constant speed ? Hint: think friction
2) demonstration: Have you tried to drop a quarter on a card in a glass? Using the law of inertia?
3) demonstration: How could you hit your hand with a hammer without breaking any bone ? Using a big Physics book ?
Inertia will resist : (think of a car)
· Changing speed (going faster or slower) · Changing direction (like stirring the wheels of a car), even if the speed stay constant · Stopping (final speed =0) or getting started (initial speed =0)
Galileo was the one who explained the concept of inertia. He used incline planes. Let’s say you have 3 incline planes and you let go a ball at one edge. There is no friction between the ball and the plane. Can you predict the position of the ball next ? In the 3 following cases:
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Questions:
4) An elephant has a large inertia and can run very fast. What could you do to escape a charging elephant ? Hint: a large inertia (mass ) resists change in direction.
5) The principle of Inertia is not intuitive. Aristotle got it wrong and his ideas were dominant until Galileo came along. How long is that ? WIKIPEDIA.ORG
6) Year Galileo died ? Year Newton was born ? WIKIPEDIA. ORG
7) If suddenly the force of gravity of the Sun stopped acting on planets, in what king of path would the planets go ? Make a sketch
8) The principle of Inertia explains why we need to use the safety belts in a car. Can you ? What about the air bag ?
9) Can the principle explain head injury when struck by a car from behind?
10) Did you try the magic trick with a table cloth and dishes ? Can you explain it now ?
Underline the correct answer or fill the gaps:
11) An astronaut in outer space away from gravitational or frictional forces throws a rock. The rock will : gradually slow to a stop continue moving in a straight line at constant speed
12. The rock’s tendency to do so is called: __________________
13. Suppose you are standing in the aisle of a bus that travel along a straight line road at 100 km/h, and you hold a pencil still above your head. Then relative to the bus, the velocity of the pencil is 0km/h and relative to the road, the pencil has a horizontal velocity of: Less than 100km/h 100km/h more than 100km/h
14. You drop the pencil. Relative to the road, its horizontal velocity is : ____________
15 Relative to you, does the way the pencil fall depends on the velocity of the bus ? _______ The laws of Physics are he same is a referential at rest or moving at a constant speed.
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