Why is it more difficult to do a sit up with your hands behind your head than when your arms are stretched out in front of you?

When many atoms are split in a chain reaction, a large explosion occurs. this is an example of what type of energy conservation

What happens to the light rays when they hit the specimen?

What would be the result of an alpha particle coming into a magnetic field? a) the alpha particle will stop moving. b) the alpha particle will reverse its direction. c) the alpha particle will be deflected in a curve path. d) the alpha particle will continue to travel in a straight line.