Why is water cooler then sand at the beach

This is because large specific heat of water as compared to the specific heat of sand. due to large specific heat , it take water longer to heat Up  and cool down. on the other hand, the sand has smaller specific heat , and hence heats up or cools down faster. So on a hot day , the sand on the beach , heats up quickly while the water heats up slowly. hence the water is cooler as compared to sand at the beach.

they start with the same

the box does not move because you need to use a force greater than 300 n. you are only using 50 n. it needs to at least be 301 n for the box to move because then it is an unbalanced force and it will move the way that the greater force is pushing it. if you use only 300 n it will be a balanced force and will not move because the force you are exerting on the   box is the same as its force.

hope this !

im sorry if i went too in depth, i am going to include a link to a video on khan academy that will explain this.   hope that you like the video and that it explains more in detail to you.

that is the link.