physics problems(3)

5. a ball is thrown at an angle of 38 degree to the horizontal. what happens to the magnitude of the ball vertical acceleration during the total time interval that the ball is in the air
(1) it decreases, then increases
(2) it decreases, then remains the same
(3) it increases, then decreases
(4) it remains the same
不變因為物體在空中時，它仍受重力(地球對物的體吸引力)作用。w = mg，所以它在垂直方向的加速度為
a = mg / m = g
(等於重力加速度，所以保持不變。)

6. a 5N force could have perpendicular components of
(1) 1N and 4 N
(2) 2N and 3N
(3) 3N and 4N
(4) 5N and 5N
若兩分量互相垂直，則可以用畢氏定理求它的合向量，所以
答案(1)
√[1^2+4^2] = 4.12N ≠ 5N
所以不正確
答案(2)
√[2^2+3^2] = 3.61N ≠ 5N
所以不正確
答案(3)
√[3^2+4^2] = 5N
所以正確
答案(4)
√[5^2+5^2] = 7.07N ≠ 5N
所以不正確

The answer for number 5 is one, because when the ball is thrown upward, the air resistance and the gravity is slowing the ball down, and once the ball reaches its peak, it stops accelerate. And when it comes back down, it is accelerating because the gravity is also pulling the ball downward. But the truth is, the time that takes the ball to go up and come down is the same.

As for number 6, you have to give me the direction of where the force is exerting to.

5：

Case (1):
If you mean throwing upwards at an angle of elevation of 38 degrees, it will first have a downward acceleration greater than 10ms-2 and gradually decreases to 0.

Case (2):
If you mean throwing downwards at an angle of depression of 38 degrees, it will first have a downward acceleration smaller than 10ms-2 and gradually decreases to 0.

So the answers in both cases are (5) It decreases.

6：

By trial-and-error, we found that when 3N and 4N are acted perpendicularly, we will have a resultant force of magnitude of 5N.