[Physics] Can a crane lift part of itself off the ground

forcesnewtonian-mechanics

I was looking at some construction work involving a crane and I thought of something…

Two construction workers are driving a crane on the road when suddenly one of the front tire gets a flat. While they have a spare tire, they don't have a farm jack to raise the front end of the vehicle. One the workers says I have an idea, I'll raise the boom enough so that it becomes a hypotenuse of a right triangle and I lower the hook on the cable so we can attach it to the bumper. So the cable and the bumper make a 90 degrees angle with the boom as he hypotenuse. I will then pull the cable, and, with the pivot at the end of the boom, we should be able to lift the frontend and change the tire. The second construction workers says, I have a better idea, let's make the cable the hypotenuse with the boom and the pivot at 90 degrees and then lift the front bumper.

Never mind the details such as whether the bumper is strong enough, etc. Would any of the above plan work to raise the frontend of the crane? If not, would any other triangle shape work? I just can't get my head wrapped around this puzzle that I created for myself. Thanks!

Best Answer

If I understand the question correctly, the crane is firmly attached to the chassis of the truck. The bumper is assumed to be firmly attached to the chassis as well. Attaching the hook anywhere on the bumper will not lift the wheels off the ground.

The hand waving argument: This is like trying to lift yourself off the floor by using your hands to pull up on your own knees.

The formal argument would be this: The cable provides an upward force on the bumper that is exactly cancelled by the downward force at the tip of the crane. Because of vector addition, no change in angle of the crane can alter this exact cancellation. In practice you could deform the bumper, but never lift the whole truck.

If I am misunderstanding the question and the crane is attached to the ground and not the chassis, then yes any upward force would lift the front of the vehicle.

Many cranes have retractable support legs that stabilize the crane laterally. Even if the legs are engaged the crane cannot provide a torque about the rear wheels. However, the support legs often have enough travel and strength to lift the vehicle on their own. In that case, front mounted legs could lift the front of the vehicle. I've seen this done. It may even be standard procedure.

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