Why don't birds fall off of trees when the sleep?

When you push downward on a book at rest on a table, you feel an upward force. does this force depend on friction? defend your answer.

You have a resistor and a capacitor of unknown values. first, you charge the capacitor and discharge it through the resistor. by monitoring the capacitor voltage on an oscilloscope, you see that the voltage decays to half its initial value in 2.70 miss . you then use the resistor and capacitor to make a low-pass filter. what is the crossover frequency fc?

4. if you were an astronaut on the moon, what would you experience? what would you see from your perspective?