Why: according to Newton’s first law of motion, an object that is free from all outside forces travels at a constant velocity, covering equal distances in equal times along a straight-line path, C is true.

2. D

Why: not matter an object is still or is in motion, the object has inertia all the time. Inertia is a nature/property of any objects.

3. D

Why: the ship is moving at a constant speed, which is in a balanced status. For the athlete, it would be likely to do a long jump on ground. Since he uses the same strength every time he tries, the distance should be equal.

4. A

Why: before he jumps, he moves as the car does (with a same velocity); after he jumps off and lands, he still has the same velocity with the car, therefore, he could easily fall toward the

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Let the block’s original location be the original point, then present position will be 0 + 0*4 + ½ * 35 * 42 = 280. So the block moves 280 cm from the original point, which should – 65 to get the final answer 215 cm. It is easy to solve the question by graph a picture.

14. C

Why: according to Newton’s third law of motion, F1 and F2 are a pair of equal-but-opposite forces.

15. B

Why: F1 = (mb + mc) * a = 2ma, F3 = mc * a = ma. Therefore, F1 = 2F3

16. C

Why: when the weight lifer is going exert a force on his body, his body also exerts a equal but opposite force (according to Newton’s third law of motion).

17. C

Why: using the formula of work: work ＝ force * distance, we have worki = workii = workiii = F *s.

18. B

Why: to determine negative or positive work sliding friction does, we first need to figure out the angle between direction of sliding friction and direction of its displacement. If the angle is greater than 90o, sliding friction does negative work. If the angle is equal to 90o, sliding friction does not do work. If the angle is less than 90o, sliding friction does positive work.

19.