X 8 joe and bill throw identical balls vertically upward joe...

X 8.) Joe and Bill throw identical balls vertically upward. Joe throws his ball with twice the initial speed as Bill's ball. The maximum height of Joe's ball will be: A) two times that of Bill's ball. B) four times that of Bill's ball. C) equal to that of Bill's ball. D) eight times that of Bill's ball. E) roughly √2 times that of Bill's ball. X 9.) A child pulls on a wagon with a horizontal force of 75 N. If the wagon moves horizontally a total of 42-m in 3.0 min, what is the average power supplied by the child? A) 18 W B)22 W MS C) 24 W D) 27 W E) 30 W

Transcript

00:01 Okay, first we know the impulse can be equal to change the momentum, which is equal to m -datav, and when the delta -v is a change of speed, which is equal to final speed, minus initial speed.

00:10 So therefore, we have impulse is equal to mv -final minus v -i.

00:14 So this is the motion diagram before the collision.

00:18 So before the collision, as you can tell, the motion is pointing downward, okay, since the ball was dropped from a point that is 1 .8 meter away from the ground.

00:28 And as you can tell, the final side, speed of this motion is the initial speed in the momentum, okay? and the direction of the acceleration of gravity is always pointing downward.

00:43 So therefore, we can determine the initial speed during the change of momentum, which is vi squared minus zero square over 2g equal to h .i.

00:52 Therefore, we have v .i is equal to square over 2g hi.

00:55 And this is the motion diagram after collision.

00:58 As you can tell, the final speed for the motion diagram after collision is zero.

01:04 Because eventually the ball will reach a maximum altitude above the ground.

01:11 So therefore, the initial speed for this motion should be the final speed in the change of momentum.

01:17 And we know the final high here is two -third of the initial high, which is 1 .2 meter.

01:22 Okay? and the direction of the acceleration of gravity is always pointing downward.

01:27 That means the direction of acceleration is having an opposite direction when compared with the direction of the motion.

01:35 So if we consider the upward direction is positive direction, that means the direction of the acceleration of gravity should be negative.

01:43 So therefore, we have the final speed of motion with zero square minus the initial speed of the motion, which is the final speed in a change of momentum, we final square, over negative 2g is equal to the final height.

01:54 So therefore, we have we final square is equal 2g hf.

01:57 So we final is equal to square root 2ghf.

01:59 And as you can tell, initial speed of the momentum and the final speed of the change of momentum are having opposite direction.

02:07 So therefore, i pick the final speed direction is positive, which is upward direction.

02:13 So therefore, the direction of the initial speed should be negative, which is pointing downward.

02:20 Okay? so therefore, if we rearrange the equation, we have jx is equal to mvf minus v .i, which is equal to m times square root 2ghf minus negative, square root 2ghi, okay? and this will give us, eventually give us m10 square root 2g, 10 square root hf plus square root hi.

02:43 So we know the mass of the ball is 40 grams.

02:45 If we convert the kilograms, it's about 0 .04 kilograms.

02:49 And the acceleration of gravity is 9 .8 meter per second square.

02:53 And we know the, oh, oops...

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