Determine whether each example below is a case of Newton s first law or Newton s second law. EXPLAIN

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Adela Fitzgerald
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1 QOD: 3/6/12 Determine whether each example below is a case of Newton s first law or Newton s second law. EXPLAIN a. a skydiver accelerating toward the ground b. a skydiver falling with constant velocity c. a skydiver on the ground at rest Mar 6 7:06 AM 1

2 page 401: Newton's Second Law Review F = m x a Mar 6 7:13 AM 2

3 Textbooks page 403: weight: a measure of gravitational force exerted on an object formula: w=mg weight=mass x free fall acceleration g represents gravity: 9.8m/s2 on earth SI unit: N Newtons mass vs. weight: mass is the amount of matter in an object. The number of atoms in an object this does not change. weight varies depending on the amount of gravity. Mar 6 7:09 AM 3

4 triple beam balance vs. spring scale measures mass measures weight measures force What is the difference between mass and weight? Mar 6 7:12 AM 4

5 Blood Pressure in Space Blood Pressure Gravity plays a role in your body. Blood pressure, for example, is affected by gravity. Therefore, when you are standing, your blood pressure will be greater in the lower part of your body than in the upper part. Doctors and nurses take your blood pressure on your arm at the level of your heart to see what your blood pressure is likely to be at your heart. &safe=active Mar 6 7:12 AM 5

6 QOD 3/7/12 Calculate the mass in kg of an object that weighs 1,225 N on Earth. Show your work. Explain why free fall acceleration can be regarded as a constant for objects falling within a few hundred miles of Earth s surface. Mar 7 6:48 AM 6

7 What Really Happened with the Apple? Probably the more correct version of the story is that Newton, upon observing an apple fall from a tree, began to think along the following lines: The apple is accelerated, since its velocity changes from zero as it is hanging on the tree and moves toward the ground. Thus, by Newton's 2nd Law there must be a force that acts on the apple to cause this acceleration. Let's call this force "gravity", and the associated acceleration the "accleration due to gravity". Then imagine the apple tree is twice as high. Again, we expect the apple to be accelerated toward the ground, so this suggests that this force that we call gravity reaches to the top of the tallest apple tree. Mar 7 7:06 AM 7

Sir Isaac's Most Excellent Idea Now came Newton's truly brilliant insight: if the force of gravity reaches to the top of the highest tree, might it not reach even further; in particular, might it not

8 Sir Isaac's Most Excellent Idea Now came Newton's truly brilliant insight: if the force of gravity reaches to the top of the highest tree, might it not reach even further; in particular, might it not reach all the way to the orbit of the Moon! Then, the orbit of the Moon about the Earth could be a consequence of the gravitational force, because the acceleration due to gravity could change the velocity of the Moon in just such a way that it followed an orbit around the earth. This can be illustrated with the thought experiment shown in the following figure. Suppose we fire a cannon horizontally from a high mountain; the projectile will eventually fall to earth, as indicated by the shortest trajectory in the figure, because of the gravitational force directed toward the center of the Earth and the associated acceleration. (Remember that an acceleration is a change in velocity and that velocity is a vector, so it has both a magnitude and a direction. Thus, an acceleration occurs if either or both the magnitude and the direction of the velocity change.) Mar 7 7:07 AM 8

9 But as we increase the muzzle velocity for our imaginary cannon, the projectile will travel further and further before returning to earth. Finally, Newton reasoned that if the cannon projected the cannon ball with exactly the right velocity, the projectile would travel completely around the Earth, always falling in the gravitational field but never reaching the Earth, which is curving away at the same rate that the projectile falls. That is, the cannon ball would have been put into orbit around the Earth. Newton concluded that the orbit of the Moon was of exactly the same nature: the Moon continuously "fell" in its path around the Earth because of the acceleration due to gravity, thus producing its orbit. Mar 7 7:08 AM 9

10 The Law of Universal Gravitation On the Apollo 15 mission, astronaut David Scott released a hammer and a feather on the moon at precisely the same moment. Hypothesize about what happened in the experiment based on your knowledge of free fall acceleration. Because there is no air resistance on the moon, the hammer and feather fell with the same acceleration and hit the moon at the same instant. &safe=active Mar 7 6:54 AM 10

11 Mar 7 6:58 AM 11

12 Why do objects fall toward Earth? What keeps the planets in motion in the sky? All objects in the universe attract each other through the force of gravity. The same force that causes objects to fall to Earth controls the motion of planets in the sky. Newton stated his observations on gravity in a law known as the law of universal gravitation. Mar 6 7:59 AM 12

13 This equation says that gravitational force increases as one or both masses increase. It also says that gravitational force decreases as the distance between two masses increases. The symbol G in the equation is a constant. Mar 6 8:00 AM 13

14 What is the direction of the force of gravity??? Mar 7 6:57 AM 14

15 Gravity Launch Game!!! Read all instructions! Mar 7 7:04 AM 15

More examples: Summary of previous lecture

More examples: Summary of previous lecture More examples: 3 N Individual Forces Net Force 5 N 37 o 4 N Summary of previous lecture 1 st Law A net non zero force is required to change the velocity of an object. nd Law What happens when there is