物理问题。。最好能用英文回答。中文也没关系。

1.If an object has no acceleration, can you conclude that no forces are exerted on it? Explain

Ans: According to the second Newton's law, if an object has no acceleration, then we can conclude that the sum of external forces exerted on it is 0.

2.What is the acceleration of a rock at the top of its trajectory when thrown straight upward? Explain whether or not the answer is zero by using the equation a=f/m as a guide to your thinking.

Ans: the acceleration is equal to -g

3. A rocket fired from its launching pad not only picks up speed, but its acceleration also increases significantly as firing continues. Why is this so?( hint: about 90% of the mass of a newly launched rocket is fuel)

Ans: the velocity of the rocket increases is because of the acceleration. Assume that the driving force remains unchanged, then because the mass of rocket decreases, the acceleration is also increasing.

4. When blocking in football