projectile motion - answer wrong? (1 Viewer)

[shkspeare]

A particle is projected to just clear two walls of height 7 m and distance 7m and 15m from the point of projection

Prove that if @ is the angle of projection, then tan@ = 3/2

ok i think the question is wrong.. or worded really badly

i get tan@ = 1/2 o.o

...
and i need help with this part..


Prove that if the walls are h metres high and distant b and c metres from the point of projection... then : tan@ = h(b+c)/bc

 

[CM_Tutor]

The question should say that the walls are 7 m and 14 m from the point of projection. Furthermore, this question is lifted straight from Fitzpatrick (Ex 25(d), q 12), and in my copy of Fitzpatrick, it does say 14 m, not 15 m. So, if you got it from somewhere other than Fitzpatrick, then your source has got the question wrong. If you got it directly from Fitzpatrick, then check if you've made a typo. (Note for 7 m and 15 m, as you have, you should get tan@ = 22 / 15)

As for the second part, put y = h into the equation for the y co-ordinate. You get two times, t₁ and t₂, with t₁ < t₂. At t = t₁, x = b, from which you can get V, then use x = c at t = t₂ to get tan@.

Alternately, form the path equation, and require that it pass through (0, 0), (b, h), (c, h) and (if needed) (b + c, 0).

 

[kpq_sniper017]

The question should say that the walls are 7 m and 14 m from the point of projection. Furthermore, this question is lifted straight from Fitzpatrick (Ex 25(d), q 12), and in my copy of Fitzpatrick, it does say 14 m, not 15 m. So, if you got it from somewhere other than Fitzpatrick, then your source has got the question wrong. If you got it directly from Fitzpatrick, then check if you've made a typo. (Note for 7 m and 15 m, as you have, you should get tan@ = 22 / 15)

As for the second part, put y = h into the equation for the y co-ordinate. You get two times, t₁ and t₂, with t₁ < t₂. At t = t₁, x = b, from which you can get V, then use x = c at t = t₂ to get tan@.

Alternately, form the path equation, and require that it pass through (0, 0), (b, h), (c, h) and (if needed) (b + c, 0).

just on "clearing wall" questions....
i did a couple of those from fitz. and got them right, but how do ensure that the particle clears the wall and doesn't just touch it?
e.g. if a wall were 5m from the origin and 5m high, u can't just sub in (5,5) into the path equation to find the appropriate velocity or angle of projection....what would u do in this situation?