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Aqa as physics mechanics - urgent help required!

A car bonnet, represented by QP, of mass 12 kg is pivoted at P. Its weight acts at G where QG = GP = 1.0 m.

What force, F, acting perpendicular to QP as shown, is required to hold the bonnet at 30° to the horizontal?

Picture1.png
(edited 7 years ago)
Original post by d010534
A car bonnet, represented by QP, of mass 12 kg is pivoted at P. Its weight acts at G where QG = GP = 1.0 m.

What force, F, acting perpendicular to QP as shown, is required to hold the bonnet at 30° to the horizontal?

Picture1.png


Have you tried resolving vertically? seems pretty straightforward to find out what the vertical component of F is.
For those struggling with this question: Note that the system is in equilibrium, everything is balanced.

Make a triangle using G, the point below G (on the dotted line) and P. Now, the force that pulls the bonnet down, is the weight of the bonnet. This acts at the centre of mass of the bonnet, at G. W=Mg , we know the mass of the bonnet is 12kg and gravity is 9.8, multiply the two and you get 117.6N. Thats the force pulling the bonnet down remember, the weight. Ok, so remember that Moment = Force x distance from pivot perpendicular to the line of action of the force. The distance perpendicular to the pivot IS NOT 1m. The distance perpendicular from the pivot to the line of action of the force is the bottom of the triangle, the distance from P and to the point on the dotted line below G. We find the distance using cos, cos is adjacent over hypotenuse, we have the hypotenuse, so 1 x cos(30 = 0.866m. Now we find out the moment pulling the bonnet down. That is the force x the perpendicular distance from the pivot to the line of action of the force: 0.866 x 117.6 = 101.8445875Nm. Now the force applied on the bonnet to keep it up is perpendicular to the pivot already seen on the diagram, note that that is 2m from the pivot. All you do now is divide that moment you calculated by 2 to get the force. We are able to this as (remember what was said at the beginning) the system is in equilibrium, it is balanced, so in the end you get a rounded value of 51N.
(edited 3 years ago)
Reply 3
you're getting an a*

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