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Chapter 12: Forces

Chapter 12: Forces. Section 2. Another acceleration. What happens to the speed of an object as it falls to the ground? It speeds up. What happens to the speed of an object when it is thrown straight up into the air? It slows down Until it stops

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Chapter 12: Forces

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  1. Chapter 12: Forces Section 2

  2. Another acceleration • What happens to the speed of an object as it falls to the ground? • It speeds up. • What happens to the speed of an object when it is thrown straight up into the air? • It slows down • Until it stops • And then it speeds up again as it falls back towards Earth’s surface

  3. Gravity • Gravity is an acceleration. • It is given a special name, because it is the acceleration of an object as it falls. • With no air, objects fall to Earth’s surface in the same way… because of Earth’s pull, which we call “gravity.”

  4. Gravity Every planet and moon has gravity, but since we live on Earth, we are going to use Earth’s gravitation pull. Acceleration due to the gravity of Earth is 9.81 m/s2. This number is close to 10m/s2. We only use this number when we are talking about falling objects, because gravity only works in the vertical direction.

  5. Weight Fw = m x g • Weight is a special case of Newton’s 2nd Law… • Our weight is due to Earth’s gravitational pull. • Earth’s gravitational pull causes objects to accelerate towards the surface at 10m/s2 • g = 10m/s2 • Even when object’s are not falling, a force is keeping the object in place on Earth’s surface • weight force • To find the weight of an object, mass is multiplied by the acceleration due to gravity.

  6. Example 3: Weight force Given: Fw = 670N g = m/s2 Fw = m X g 670 = m x (10) m = 670/10 m = 67kg • The weight of an average person is 670N (or 150 pounds). What is this person’s mass?

  7. Weight is different from mass. • Mass and weight are easy to confuse • Mass is the measure of how much matter makes up an object • Weight is the force from the earth that the object experiences • The 2 are proportional to each other, because we are measuring mass on Earth’s surface. • The gravitational acceleration on Mars is 3.7m/s2, so an object on Mars weighs less than an object on Earth • The mass of the object stays the same, whether it is on Mars or on Earth

  8. Weight influences shape. Gravity pulls the surface of the earth toward the center, making the earth spherical. Elephants and flamingos have different leg structures. Why?

  9. Law of Universal Gravitation All objects in the universe attract all other objects in the universe because of the force of gravity In other words, all matter is affected by gravity

  10. This is an inverse square law • What happens to the force if • One of the masses increases by 2? • Both of the masses increase by 2? • The distance between the masses increases by 2?

  11. Free Fall • When only Earth’s gravitational pull is acting on an object, pulling it to the ground, that object is said to be in free fall. • All objects, in the absence of air, accelerate towards earth at the same rate, or about 10m/s2 • This number is constant because of the Universal Law of Gravitation, for distances away from Earth’s surface that are less than the radius of the Earth. • Why “in the absence of air?” Example of this…

  12. Gravity We use the same equation for gravity as we do for acceleration, but instead of “a,” we put in “g.” g = 10m/s2 every time something is going vertically. g = Δv / t

  13. Example Given: g = 10m/s2 t = 3s Vi = 0 g = Δv / t g = (vf – vi) / t 10 = (vf – 0) / 3 10 x 3 = vf – 0 Vf = 30m/s If you drop a pebble into a well, and it takes 3 seconds to hear the pebble hit the water, how fast was the pebble moving when it hit the water?

  14. Other equation • Replacing “a” for “g” works in the other equation also… • Instead of • d = vit + ½ at2 • We have • d = vit + ½ gt2

  15. Example Given: t = 3s g = 10m/s2 Vi = 0 d = vit + ½ gt2 d = 0 + ½ (10)(3)2 d = 45m If you drop a pebble into a well, and it takes 3 seconds to hear the pebble hit the water, how far did the pebble drop?

  16. For you to do: Determine your reaction time using data.

  17. Terminal Velocity • Because we have air on Earth’s surface, we have air resistance. • When air resistance equals the weight of the falling object, • The object STOPS ACCELERATING • The object is moving at its maximum velocity, or TERMINAL VELOCITY

  18. v = 0 For symmetrical situations… • Look at each side separately… • The shape of the position vs. time graph is a parabola… • Up Side • Initial velocity is upward. • a = g = -10 m/s2(slowing down) • The velocity at the maximum height is zero • Final velocity is zero. • Down Side • Initial velocity is zero. • a = g = 10 m/s2(speeding up) • The velocity at the maximum height is zero • Final velocity is downward. Time up =Time down

  19. Projectile Motion • Projectile motion is • The curved path followed by an object that is thrown or launched near Earth’s surface • Examples: • Baseball pitch • Vomit • Driving a golf ball

  20. Projectile Motion, continued • Projectile Motion has 2 components: Horizontal and Vertical • When these motions are combined, they form a parabolic shape • Either half of a parabola • Or a whole parabola

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