A320.9 ng sample of an unknown radioactive substance was placed in storage and its mass measured periodically. after 47 days the amount of radioactive substance had decreased to 40.11 ng. how many half‑lives of the unknown radioactive substance have occurred?

The place you get your hair cut has two nearly parallel mirrors 6.00 m apart. as you sit in the chair, your head is 2.00 m from the nearer mirror. looking toward this mirror, you first see your face and then, farther away, the back of your head. (the mirrors need to be slightly nonparallel for you to be able to see the back of your head, but you can treat them as parallel in this problem.) how far away does the back of your head appear to be?

Mechanical energy is when the amount of kinetic and potential energies added together remains the same. conserved created lost

If there's a small amount of friction between two surfaces, the result could be select all that applya. no movement b. heatc. a little bit of movementd. sliding around