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# Quick C4 Question Watch

1. OABC is a triangular pyramid. The position vectors of A, B, C with respect to O are given by OA=3i, OB=4j, and OC=5k. The points M and N are the feet of the perpendiculars from O to AC and BC respectively (see diagram).

Find AC and hence verify that the vector OM is parallel to 5i + 3k.

- I have found AC to be -3i + 5k
So I can show that (5i + 3k) and AC are perpendicular by using the dot product. I thought of using that and since both (5i + 3k) and OM are perpendicular to AC they must be parallel but I don't think this is sufficent as we are working in three dimesions and I'm not sure if I can state that OM is in the x-z plane

Vector questions like this are my weak point >.<
2. Anybody?
3. hmmmm I seem to be making some stupid arithmetic mistake I will post back when I can spot it
4. (Original post by ElMoro)
OABC is a triangular pyramid. The position vectors of A, B, C with respect to O are given by OA=3i, OB=4j, and OC=5k. The points M and N are the feet of the perpendiculars from O to AC and BC respectively (see diagram).

Find AC and hence verify that the vector OM is parallel to 5i + 3k.

- I have found AC to be -3i + 5k
So I can show that (5i + 3k) and AC are perpendicular by using the dot product. I thought of using that and since both (5i + 3k) and OM are perpendicular to AC they must be parallel but I don't think this is sufficent as we are working in three dimesions and I'm not sure if I can state that OM is in the x-z plane

Vector questions like this are my weak point >.<
But you can state that OM in the x-z plane.
OM is in the OAC plane and OAC is part of the x-z plane,
because C is on the z axis (5k) and A is on the x axis (3i) and O is in all
the three base plane, so AC is in the x-z plane and OM is in it, too.
SImilarly N is in the y-z plane and MN is in the ABC plane which is a
triangular and MN is parallel with AB, and ...... and so on
5. C4 is often used in demolition charges such as the US M183 and M112. The demolition charge M183 is used primarily in breaching obstacles or demolition of large structures where large charges are required (satchel charge). The M183 charge assembly consists of 16 M112 block demolition charges, four priming assemblies, and M85 carrying case. Each priming assembly consists of a five-foot length of detonating cord assembled with two detonating cord clips and capped at each end with a booster. The components of the assembly are issued in the carrying case. The demolition charge M112 is a rectangular block of Composition C4 approximately 2 inches (51 mm) by 1.5 inches (38 mm) and 11 inches (280 mm) long, weighing 1.25 lb (0.57 kg). When the charge is detonated, the explosive is converted into gas. The gas exerts pressure in the form of a shock wave, which demolishes the target by cutting, breaching or cratering.
C4 is often used in demolition charges such as the US M183 and M112. The demolition charge M183 is used primarily in breaching obstacles or demolition of large structures where large charges are required (satchel charge). The M183 charge assembly consists of 16 M112 block demolition charges, four priming assemblies, and M85 carrying case. Each priming assembly consists of a five-foot length of detonating cord assembled with two detonating cord clips and capped at each end with a booster. The components of the assembly are issued in the carrying case. The demolition charge M112 is a rectangular block of Composition C4 approximately 2 inches (51 mm) by 1.5 inches (38 mm) and 11 inches (280 mm) long, weighing 1.25 lb (0.57 kg). When the charge is detonated, the explosive is converted into gas. The gas exerts pressure in the form of a shock wave, which demolishes the target by cutting, breaching or cratering.
what in the world are you talking about
7. (Original post by ztibor)
But you can state that OM in the x-z plane.
OM is in the OAC plane and OAC is part of the x-z plane,
because C is on the z axis (5k) and A is on the x axis (3i) and O is in all
the three base plane, so AC is in the x-z plane and OM is in it, too.
SImilarly N is in the y-z plane and MN is in the ABC plane which is a
triangular and MN is parallel with AB, and ...... and so on
I did understand why OM was in the x-z plane but I wasn't sure how much justification I would need, thanks.

(I still can't rep you but I swear I will when I can! )
8. Yes sorry on the late reply started having dinner and got distracted...yes they are parallel

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Updated: November 29, 2010
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