Which TWO statements about the substances in the experiment are true? A. Quartz is harder than chalk.
B. Water has a higher surface tension than salty water.
C. Soapy water has a higher surface tension than plain water.
D. Graphite is harder than quartz.

Aplayground slide is 8.80 ft long and makes an angle of 25.0° with the horizontal. a 63.0-kg child, initially at the top, slides all the way down to the bottom of the slide. (a) choosing the bottom of the slide as the reference configuration, what is the system's potential energy when the child is at the top and at the bottom of the slide? what is the change in potential energy as the child slides from the top to the bottom of the slide? (include the sign of the value in your answer.)