# Robotic arm 'Canadarm' in space that doesn't work on earth

The mechanical arm on the space shuttle, or what's called the Canadarm, can handle a 2200-kg satellite but, on the ground, it cannot support its own weight (Resnick, Halliday). Why is this?

What I am able to arrive at is that since it is moving a mass around in space, it is exerting a huge force. Second, on Earth, it collapses because the gravity overwhelms the normal force of the particles in the Canadarm.

Could anyone please explain it from a newtonian mechanics perspective (because the textbook chapter I got this problem was just about newton's laws)?

• In orbit, a human can handle 2200kg satellite just as well, provided some favorable handles, training and a lot of common sense. On Earth, not every human can support its own weight with their arms. Jul 18 at 19:15
• On Earth, you can push (and pull) a (small) rowboat that's floating in the water with your hands, while you yourself are standing on the pier. But you would not be able to lift the same rowboat in the air with just your hands. Jul 19 at 4:47
• Do you mean "Halliday" rather than "Hillary"? Jul 19 at 19:54
• @Michael Seifert whoops Jul 20 at 0:05

In orbit both the arm and the 2200 kg satellite are weightless, so to hold the satellite stationary the arm does not need to apply any force.

The only time the arm needs to apply a force is if it is accelerating the satellite, and provided it uses only low accelerations it needs to apply only low forces i.e. a small fraction of what the satellite's weight would be on Earth.

That's why the arm does not have to be very strong. Unlike the cranes used to load the satellite onto the space shuttle on Earth, the Canadarm only has to apply small forces.

• Comments are not for extended discussion; this conversation has been moved to chat. Jul 19 at 12:53

Imagine a car on a flat surface, without brakes and with the transmission in the neutral position. An able-bodied human can push that car and get it moving -- although the maximum speed will be limited by the rolling resistance. There is no way, however, for a normal person, to lift a normal car off the ground. The Canadarm isn't very strong, but you would expect it to be able to push a car on a flat surface. In orbit, there is no significant gravity, so moving a 2200 kg object (about the weight of an average car) is like pushing a car on a flat surface -- but in space, the object will not slow down on its own since there is no rolling resistance. It means that the maximum speed that the object will reach is limited by the force of the arm and its reach. If the object accelerates too much, the arm will not be able to slow down the object before it gets out of reach. I haven't read much on the Canadarm, but I would not be surprised if there were a lot of safeguards to prevent this from happening.

"it cannot support his own weight" it means it will break down if you apply a compressive force strong enough on some of its components (hinges, arms themsleves, and so on).

The weight of the arm itself is the compressive force I mentioned.

Since the mass of the robotic arm is the same, on the Earth or in the space ... I think you should try harder to get someone solve the problems for you :D