21. True or False: Static friction prevents you from slipping when you walk on a sidewalk.

Yes it is true because static friction help us to make grip between the foot and the ground surface

Explanation:

Yes, as we know that the friction between two surface is of two types

1) Static friction

2) Kinetic friction

When two surface moves relative to each other then the friction force between two surface is kinetic friction and it always opposes the relative motion of two rough surface

While when one surface has tendency to move with respect to its contact surface but if it can not move on it then in this case it is static friction

So relative motion is not possible in this type of friction

So static friction helps to prevent slipping

freezing

deposition

condensation

explanation;

melting also called fusion is the process where solid becomes liquid, sublimation is the process where solid becomes gaseous without first changing to liquid, deposition involves change from gaseous to solid state, freezing involves change from liquid to solid, boiling or evaporation involves change of liquid to gaseous state, while condensation is the change from gaseous state to liquid state.

endothermic reactions involves absorption of heat from the surroundings or the environment while exothermic reactions involves loss of heat energy to the surroundings.

in this case, sublimation, evaporation and melting are endothermic reactions (gain energy from the environment) while, freezing, deposition and condensation are exothermic (loss of energy to the surroundings) reactions.

massxgravity x heght=weightxheight= pe

4900=490xh

h=10