Question

When tightening cylinder-head bolts in a car , the crucial quantity is the torque applied to the bolts.Why is this important than the actual force applied to the wrench handle?

Answer 1

In short, torque is the rotational force applied to the bolt and is basically the ultimate result of the force applied to the handle and the moment arm.

Answer 2

Imagine lifting a plate with your fork and a toothpick. You are using the strength of your hand, so you're using the same force, but a different distance produces a different torque. So as you can see, you need to apply a higher force on the toothpick to get the same torque as you do with the fork. Or conversely, to get the same amount of torque out of the fork as the toothpick you have to apply less force on the fork to get a torque of equal amount.

I suggest you play with different length things and see how they change. Another good experiment with torque is to push a door from the handle side and then push the same door from the hinge side. You'll find there's actually a physics reason for putting the handle on the far side of the door other than just being convenient for being near the open side of the door lol.

Answer 3

Torque is independent of the lever arm -- the length of the wrench you use on the bolts. The force required to snap the bolts will be constant IF it is applied to the circumference of the bolt.

But since the diamater of the bolt is constant, that means the torque (force x lever arm) required to snap the bolts is constant. The force you must apply to the end of a wrench will not, however -- because the lever arm might be longer or shorter.

Or, to put it another way, consider an extreme case: a 4-year-old child can snap any bolt ever made, even the strongest bolts used to bolt aircraft carriers together, if he has a wrench handle long enough.