Arrow being fired to centre of target

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Homework Help Overview

The discussion revolves around the motion of an arrow being fired towards a target, specifically focusing on the acceleration due to gravity and its implications on the arrow's trajectory. The subject area includes kinematics and the application of SUVAT equations.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the use of the SUVAT equations to analyze the motion of the arrow, questioning the sign of the acceleration due to gravity and its consistency throughout the arrow's flight. There is a focus on understanding why acceleration is considered negative when the arrow moves upward initially.

Discussion Status

The discussion is active, with participants seeking clarification on the assumptions regarding the direction of acceleration and the treatment of gravity in the problem. Some guidance has been provided regarding the conventional treatment of upward motion as positive and gravity as negative, though explicit references in the original question are noted as missing.

Contextual Notes

Participants mention that the problem does not explicitly state the direction conventions for positive and negative values, leading to confusion about the treatment of acceleration. The original poster also refers to previous parts of the question for context.

hello478
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Homework Statement
part b and part c of the question
Relevant Equations
suvat equations
i solved it like this...
s = ut + 1/2 at^2
t= 1.08 (from part a)
u= 65 sin4.30
a= 9.81? or -9.81
the answer said -9.81
why? wouldn't acceleration change from -9.81 to +9.81 because it moves up then down???
its soo confusing...

1711481019245.png

1711481043968.png
 
Last edited:
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hello478 said:
Homework Statement: part b and part c of the question
Relevant Equations: suvat equations

i solved it like this...
s = ut + 1/2 at^2
t= 1.08 (from part a)
u= 65 sin4.30
a= 9.81? or -9.81
the answer said -9.81
why? wouldn't acceleration change from -9.81 to +9.81 because it moves up then down???
No, the acceleration wouldn't change unless gravity suddenly reversed.
hello478 said:
its soo confusing...

View attachment 342395
 
Mark44 said:
No, the acceleration wouldn't change unless gravity suddenly reversed.
ok... so why is acceleration -9.81?

i fixed the picture now... 🙃
 
hello478 said:
ok... so why is acceleration -9.81?
Because they're assuming that the "up" direction is positive, and gravity is acting downward.
 
Mark44 said:
Because they're assuming that the "up" direction is positive, and gravity is acting downward.
is it in the question? i cant find it...
 
hello478 said:
is it in the question? i cant find it...
It's not explicitly given. What they tell you is that the speed (i.e., ##|\vec v|##) of the arrow is 65 m/sec, at an angle of 4.3° above horizontal. The arrow's velocity vector can be decomposed into a vertical component and a horizontal component.

The usual approach for problems of this sort is to treat upward velocities as positive, with gravity acting downward (so g = -9.81 m/sec^2). During the flight, the arrow's vertical component of velocity will start off positive, slowly decrease to zero at its high point, and then become negative as it continues on to the target.
 
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ok thank you, i got it now
 

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