Physics question (1 Viewer)

[AlonzoN]

Hi, I just need help to see if I have done the correct working out for this 7 mark question.

An arrow with a mass of 45.3 g is fired vertically into the air at 85 m/s from a height of 1.41 m. Calculate the speed of the arrow when it reaches a height of 36.3 m.

This is what I have done:

m = 0.0453 kg, d = 36.3 - 1.41 = 34.89 m, u = 82.5 m/s, a = -9.8 m/s/s, F = ma, W = Fd, W = KE final - KE initial, KE = 1/2mv^2,

therefore, W = mad, W = 1/2mv^2 - 1/2mu^2,

therefore, mad = 1/2mv^2 - 1/2mu^2


So I plugged in the numbers:

34.89 * -9.8 * 0.0453 = 1/2 * 0.0453 * v^2 - 1/2 * 0.0453 * 85^2

-15.49 = 0.02265v^2 - 163.65

148.16 = 0.02265v^2

therefore,

v = rt(148.16/0.02265) = 80.9 m/s


Am I right or wrong?

 

[jimmysmith560]

Your answer appears to be correct, as per the following working:

Given:

• mass of arrow,
• initial velocity,
• initial height,

Height travelled by the arrow when it reaches 36.3 m:




Let the velocity at 36.3 m be v

From equation of motion:


,






Speed of arrow at given height is 80.9 m/s.

I hope this helps!

 

[AlonzoN]

So wait, did the arrows mass not matter?

 

[jimmysmith560]

So wait, did the arrows mass not matter?

Perhaps your approach is the most appropriate given that you have used all information provided in the question and included it in .

Also, how come the question mentions "fired vertically into the air at 85 m/s" but you wrote ?

 

[AlonzoN]

initial velocity for the initial kinetic energy, and I figured that it would be appropriate to have an initial velocity to try and find a final velocity.

 

[AlonzoN]

I didn't see that. Thanks for the save.

 

[jimmysmith560]

View attachment 35698
^^^
Kind of confused but have a feeling its B or D

The answer appears to be (C), as per the following working:







Since the wire is suspended,









I hope this helps!

 

[Akshara Patil]

Yes I got that as well!