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# Im stuck with my physics hw! help?

1) a ball is dropped into a river from a very high bridge. at, the same time, another ball is thrown down towards the water. if air resistance is negligible, the acceleration of the balls just before they strike the water.

a) is greater for the dropped ball

b) is greater for the thrown ball

c) is the same for both balls

d) depends on the height of the bridge

isn't the answer b? since the ball is thrown wouldn't it get to the water faster than the ball that is just dropped?

2) suppose a motorcylce starts at rest and accelerates unformly. At time t the motorcycle has a speed of 40m/s and has covered a distance of 80m.

a) what is the motorcycle's speed at time t/2?

b) what distance has the motorcycle covered at time t/2?

for number 2, what do they mean by t/2? im so lost..

### 6 Answers

- AvinashLv 71 decade ago
1) Answer is c.

It is true that the thrown ball will reach faster. But the question is asking about acceleration and not about time.

2) t/2 means at time t/2 after starting.

a) Initial speed u = 0

speed at time t = v = 40 m/s

Distance in time t = S = 80 m

v^2 = u^2 + 2aS

Or 40^2 = 0+ 2*a*80

Or 1600 = 160 a

Or a = 1600/160

Or a = 10 m/s^2-----------------------------(1)

v = u + at

Or 40 = 0 + 10 * t

Or t = 40/10

Or t = 4 sec--------------------------(2)

Speed at t/2 = u + a * (t/2)

= 0 + 10*4/2 ----------[using (1) and (2)]

= 20 m/s

b) Distance covered in t/2

= u(t/2) + 1/2 a * (t/2)^2

= 0 + 1/2 * 10 * 2^2 ---------------------[using (1) and (2)]

= 1/2 * 10 * 4

= 20 m

- gp4rtsLv 71 decade ago
The acceleration of a ball thrown in any direction is g (9.8 m/s²) downward. The question does not ask how long it takes the ball to hit, just what is its acceleration. It is always the same.

t/2 means half the time of t.

It says "uniform acceleration" which means constant acceleration. Since velocity = accel. x time, cut the time in half and you cut the velocity in half. Half of 40 m/s is 20 m/s

The distance covered under uniform acceleration a is s = 0.5*a*t²; in other words, distance is proportional to time squared. In half the time, the distance traveled is 1/4 the total. 1/4 of 80m is 20 m

You don't even need to calculate the value of the acceleration.

- fantzLv 44 years ago
one million) answer is c. it incredibly is genuine that the thrown ball will attain swifter. however the question is looking approximately acceleration and not approximately time. 2) t/2 ability at time t/2 after beginning. a) preliminary velocity u = 0 velocity at time t = v = 40 m/s Distance in time t = S = eighty m v^2 = u^2 + 2aS Or 40^2 = 0+ 2*a*eighty Or 1600 = one hundred sixty a Or a = 1600/one hundred sixty Or a = 10 m/s^2-----------------------------(one million) v = u + at Or 40 = 0 + 10 * t Or t = 40/10 Or t = 4 sec--------------------------(2) velocity at t/2 = u + a * (t/2) = 0 + 10*4/2 ----------[utilising (one million) and (2)] = 20 m/s b) Distance lined in t/2 = u(t/2) + one million/2 a * (t/2)^2 = 0 + one million/2 * 10 * 2^2 ---------------------[utilising (one million) and (2)] = one million/2 * 10 * 4 = 20 m

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- 1 decade ago
first question its its b

the second one since t=d/s then 80m/40m/s=2s then maltiply by a half on both sides then you get your answer

for b you can figer it out straight forwad

t/2 means half the time covered by the object

- 1 decade ago
the first one is (c) as air resistance is zero

the second one -----------

part 1--------- 40m/s

part 2 ----------- 40 m as t = 80/40 (time = distance/speed)

plz choose me as best answer!!!!!!!