A:It was pretty good.
B:I visited my grandparents.
C:I played soccer.
一张质量为20kg的单人床垫平放在床上,和床的接触面积为2m2,求:①床垫的重力;②床垫对床的压强(g取10N/kg).
如图所示,已知AB为⊙O的直径,AC为弦,OD∥BC,交AC于D,BC=4 cm.