# A what is the cars acceleration b if the car started

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a. What is the car's acceleration? b. If the car started at 29 m/s and ended at 5 m/s after 4 seconds, what would its acceleration be? How is this different from the answer above? 10. What is the acceleration of a truck with a mass of 2,000 kg when its brakes apply a force of 10,000 N? 11. A 20 N force accelerates a baseball at 140 m/s 2 (briefly!). What is the mass of the baseball? 12. Gina is pushing a 10-kg box with 50 N of force toward the east. Dani is pushing the same box at the same time with 100 N of force toward the west. Assuming there is no friction, what is the acceleration and direction the box moves? 13. A cheetah can accelerate at 7 m/s 2 , and the average cheetah has a mass of 40 kg. With what average force does the cheetah push against the ground? 14. If you have a mass of 75 kg, what is your weight in newtons on Earth? 15. When a ball is first dropped off a cliff in free fall, it has an acceleration of 9.8 m/s 2 . What is its acceleration as it gets closer to the ground? Assume no air friction.

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THIS AT HOME Astronomy Chapter 15 The Solar System Chapter 16 The Sun and Stars Chapter 17 Galaxies and the Universe How do satellites orbit Earth? Place a world globe on the floor, or use a large ball placed on top of an empty flowerpot. Choose a small ball, such as a tennis or racquetball. Carefully make a small slit in the ball, and shove the knotted end of a piece of string though the slit. Imagine the large ball is Earth and the small ball is a satellite. Hold the satellite’s string over the large ball, and figure out how to make the satellite move in a direction and at a speed that will allow it to orbit Earth. What did you have to do to make this work?
Chapter 15 The Solar System Earth is a planet that is just right for living things — and among them are people who have long wondered if other planets have life. Mars and Europa (a moon of Jupiter) are good candidates for having extraterrestrial life, but are only just candidates. Space probes have explored only a tiny fraction of the surfaces of Mars and Venus looking for signs of life, and the small amount of evidence collected gives no definite answers. If you were asked to describe a creature that could live on each of the planets (or moons) in the solar system, what characteristics would it have? What would it eat? How would it move? A creature on Venus might have to live at a surface temperature of 500 °C . Pluto’s environment is rock and ice; what type of creature could live there? In this chapter, you will learn about the vast, unexplored territories that are the planets and moons of the solar system. 1. What is the solar system and how does it stay together? 2. How do the other planets in the solar system compare with Earth? Could they support life? 2. What else is there in the solar system besides the sun and planets?

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312 U NIT 6 A STRONOMY Figure 15.1: Two of Galileo’s discoveries that helped prove that Earth and the other planets orbit the sun. The top diagram shows how the phases of Venus are due to its orbit around the sun. The bottom diagram depicts moons orbiting Jupiter. This observation
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