Visual Arts/Reviewed: Flowers & Questions, Fischli & Weiss, Tate Modern, London, until Jan 14 (0044-207-8878888); David Smith: Sculptures, Tate Modern, London, until Jan 21

Some years ago, Limerick's EV+A featured a work by Swiss artistic duo Fischli & Weiss. It was a metaphorically rich film called The Way Things Go, made during 1986 and 1987, and it was extraordinarily good. The premise was laughably simple. In a huge warehouse-like space, the artists set up an apparently non-stop chain reaction involving a succession of workaday objects and materials: car tyres, tables, chairs, planks and boards, kettles, bottles, tin cans, stepladders, newspaper, string, expanding foam filler, water and other, rather more combustible liquids.

At every stage it seemed as if the next stage in the chain of cause and effect would not quite happen: then it did, often explosively. The whole thing went on for 30 minutes and combined the appeal of slapstick comedy with that of a nail-biting thriller.

The Way Things Go features in Flowers and Questions, Fischli and Weiss's retrospective at Tate Modern. As you might expect, the exhibition as a whole is a consistently amusing, playful experience. At the same time it is a bit disappointing, because nothing else they've made comes close to the quality of the 30-minute film, which surely stands as a contemporary art masterpiece.

It is, incidentally, a masterpiece you can get hold of for a very reasonable £25, as a DVD, a far cry from forking out millions on a Pollock. The closest thing to The Way Things Go in mood is its immediate predecessor, and to a large extent its inspiration, Quiet Afternoon. This takes the form of a series of photographs of the kind of things that feature in the video, balanced in witty and impossibly precarious combinations.

Perhaps the conceit is that to pass the time one afternoon Fischli & Weiss started playing around with objects and photographing them. It could be true. They come across as a pair of perpetual adolescents with a low boredom threshold and a facetious sense of humour. They are, individually, Peter Fischli and David Weiss, both born in Zurich, both in their 50s. They have worked collaboratively since the late 1970s, when they made The Sausage Photographs, a set of pictorial tableaux featuring sausages and cold meats. The strategy of focusing on everyday objects, often giving them an anthropo-morphic spin, has remained central to their projects.

Very ambitiously, Suddenly this Overview, a sprawling collection of unfired clay sculptures mounted on plinths, offers an unofficial history of humankind, a historical work comprising out-takes, so to speak. It's funny, but there are dozens and dozens of the things, the jokes are mostly weak, and eventually the law of diminishing returns kicks in. Recurrently, Fischli & Weiss seem in need of an editor. Often they embark on exhaustive series that ramble aimlessly on: as in their photographs of flowers and fungi, which, as with their collection of sunsets, flirt with kitsch. That is partly the point, but not much else is going on.

Better is their hypnotic video Kanalvideo, which documents the viewpoint of an electronic probe as it negotiates the Zurich sewer system. It reflects their interest in the unseen underside of things, but surely it also parodies pretentious video art.

Also outstanding is their roomful of sculptural pieces that collectively resemble a sloppy studio working space. Except that everything we see is painstakingly sculpted from polyurethane and painted to resemble the messy, chaotic reality, rubbish and sweet wrappers and all. Perversely, The Way Things Go is screened in tandem with Making Things Go, which documents work on the film. It's a bit like displaying a Rembrandt self-portrait next to a video of Rolf Harris doing his version of a Rembrandt.

The David Smith exhibition that shares the fourth floor of Tate Modern with Fischli & Weiss is also a pleasure. Smith (1906-1965), a Hemingwayesque, larger-than-life figure, created a sensation with his innovative work, which culminated in his stainless steel Cubi series, made in the years immediately preceding his untimely death. Like Jackson Pollock, he died in a car crash.

The son of an engineer, he challenged sculptural convention in several ways, introducing industrial materials and methods, for example, into the fine art vernacular. Early on he worked as a welder on cars and again later, during the war years, building locomotives and tanks. He co-opted fragments of agricultural implements and other tools, and standard steel beams in boldly stated constructed sculptures. And he painted his sculptures with commercial gloss finishes.

As with many of those who transformed the scope of American art, Smith drew on European influences. Julio Gonzalez and Picasso were particularly important exemplars when he eschewed the idea of a central sculptural mass in favour of a kind of soaring sculptural drawing in space. Surrealism informed his free-flowing narrative pieces. Face to face with his work, one can feel his incredible attack and energy.

The lines can be amazingly flowing and calligraphic, but equally he relishes and engages with the intractable qualities of his material. Rather than starting from scratch, he liked dealing with given forms, most famously when he was invited to Spoleto in 1962 and turned out 27 sculptures in a month, using the off-cuts and discarded tools he found in an abandoned welding shop on site.

Many of his works are, effectively, landscapes or abstracted figures, and he sited his sculptures in the fields around his home in the Adirondacks, so it is hardly surprising that they look best out in nature, subject to atmospheric conditions and the play of changing light, rather than in the formalised gallery environment.

Still, it must be said, they look terrific in the Tate. His influence was enormous. On this side of the Atlantic, he set Anthony Caro off on a fruitful path of exploration. Closer to home, Cork's John Burke emerged as a formidable exponent of constructed steel and, more recently, John Gibbons has reinvigorated the idiom with great flair and inventiveness.