In the reference frame of the interface between two surfaces

In the reference frame of the interface between two surfaces, static friction does no work, because there is never displacement between the surfaces. In the same reference frame, kinetic friction is always in the direction opposite the motion, and does negative work.[24] However, friction can do positive work in certain frames of reference. One can see this by placing a heavy box on a rug, then pulling on the rug quickly. In this case, the box slides backwards relative to the rug, but moves forward relative to the frame of reference in which the floor is stationary. Thus, the kinetic friction between the box and rug accelerates the box in the same direction that the box moves, doing positive work.[25]

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r Rutavi Mehta told the BBC he was shopping nearby and heard the explosion. He grabbed his camera and ran to the scene.

"I took a couple of photographs. I think they might be too graphic for broadcast," he said.

"Bodies and limbs were strewn everywhere. People were crying and screaming. The area was packed with shoppers at the time of the blast. A few offered assistance to the blood-soaked victims, while others looked on in a state of shock," he said.

"It was totally chaos. There were pools of bloo