# Kinematics

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• Nov 30th 2009, 11:23 AM
BabyMilo
Kinematics
Can some one help me with 5 ii) a)?

Here is what i've done.

Trailer:
fx = P-250gsin3-150=250a
fy = R-250gcos3=0

Car:
fx = 980-900gsin3-600-P=900a
fy = R-900gsin3=0

so therefore

980-900gsin3-600=P+900a
-250gsin3-150=250a-P

right? simultaneous equation then solve for P and a.
which comes out as:

a=-0.313 ms^-2 (3s.f.)
P=200N

but the answer from the mark scheme says P= 200N but a = 0.713ms^-2 which suggests the equation should be

980+900gsin3-600=P+900a
+250gsin3-150=250a-P

can someone explain what i've done wrong and which one is right.

Sorry for being quite long. THanks for your help.
• Nov 30th 2009, 01:09 PM
Paul46
For the acceleration i got 0.713 m/s^2 also.

((980-750) + (mgsin(3)) = 819.826 then divide by 1150
• Nov 30th 2009, 01:19 PM
BabyMilo
Quote:

Originally Posted by Paul46
For the acceleration i got 0.713 m/s^2 also.

((980-750) + (mgsin(3)) = 819.826 then divide by 1150

why is it plus? mgsin(3)

since the weight is going down?
• Nov 30th 2009, 01:20 PM
Paul46
potential energy.
• Nov 30th 2009, 01:24 PM
BabyMilo
Quote:

Originally Posted by Paul46
potential energy.

can you explain that?
• Nov 30th 2009, 01:29 PM
Paul46
You have the force plus the PE of which will be converted into KE, so the resultant acceleration will be 0.713m/s^2
• Nov 30th 2009, 01:32 PM
BabyMilo
Quote:

Originally Posted by Paul46
You have the force plus the PE of which will be converted into KE, so the resultant acceleration will be 0.713m/s^2

how do i know when to use +/- ?
how come this never came up from previous examples?
• Nov 30th 2009, 01:37 PM
Paul46
Sorry? i'm a bit confused to what you are asking?
• Nov 30th 2009, 02:02 PM
BabyMilo
Here is a diagram
• Nov 30th 2009, 02:03 PM
BabyMilo
Quote:

Originally Posted by Paul46
You have the force plus the PE of which will be converted into KE, so the resultant acceleration will be 0.713m/s^2

how can you turn Gravitational PE into KE in the opposite direction?
• Nov 30th 2009, 02:07 PM
Paul46
question you gave on 5 ii) a) said its going downhill.
• Nov 30th 2009, 02:08 PM
BabyMilo
Way to go.! lol
thanks!