Torque produced by friction when braking in a car

[jimkers]

Hi

So I am busy studying rotational motion and there's an example in the book where they explain that when a car brakes the force of friction acts as a torque force which explains why a car lifts up at the back when braking.

Because the car does no rotational motion they say the net torque is zero and that force N₁ counteracts the counterclockwise rotation and the rest count towards the counterclockwise rotation.

Here's my problem, when the calculations are being made, they put in the values for all the forces, multiplied by the distance shown on the figure: 1,5 m and 0,75 m. Shouldn't they account for the angle the force makes with the line between the axis of rotation and the tires (point where the force acts)? I thought only the force acting perpendicular to this line serves in this net force equation.

Thanks in advance.

 

[Doc Al]

Shouldn't they account for the angle the force makes with the line between the axis of rotation and the tires (point where the force acts)?

That's true, but there are several equivalent ways of calculating the torque.

I thought only the force acting perpendicular to this line serves in this net force equation.

You can either multiply the distance by the force component perpendicular to that line (L*Fsinθ) or multiply the force by the perpendicular distance to the line of action of the force, called the 'moment arm' (Lsinθ*F). Same result either way.

 

[jimkers]

Hmm, I think I understand now, thank you very much