With a pulley of 5 strings, why is each string experiencing 10 N of force and not 2 N apiece (when you pull on the string with 10 N of force)?

When you pull on the string with a 10 N force, you create 10 N of tension in that string. If there is less tension anywhere in the string, then that portion of the string will accelerate toward the side with more tension. That’s why the tension in each string of a multiple pulley is 10 N when you pull on its loose end with a force of 10 N. The 5 strings are really just parts of the same string and that string has to have 10 N of tension in it.

Why do many buses use air brakes instead of hydraulic brakes?

As you have noticed, buses, trucks, and trains often use air as the hydraulic fluid in their braking systems. That’s because air is cheap and non-toxic, so that spilling it isn’t a problem. While air’s compressibility makes it a bit more complicated to work with than a liquid hydraulic fluid, it still works well in power braking systems.

What is the difference between a multiple pulley system in which the string you pull on comes down from the top pulley and the one in which the string you pull on comes up from the bottom pulley?

When the string you pull on comes down from the top pulley, it doesn’t exert its tension on the thing being lifted so it doesn’t count when add up the strings. But when the string you pull on comes up from the bottom pulley, that string is also helping to lift the object. That string does count. Thus if the multiple pulley has 5 segments going up and down between the two pulleys and one more segment going up to your hand, the total number of segments lifting the object is 6 and that object experiences an upward force equal to 6 times the tension in the string.

If you pulled on the bottom of a multiple pulley while an object was hanging from the end of the multiple pulley’s rope, would that object feel heavier than it really is?

Yes. If there were 5 segments in the multiple pulley, then you would have to pull down on the bottom of the multiple pulley with a force that was 5 times the magnitude of the object’s weight in order to lift the object at constant velocity. But the object would also rise 5 times as fast as the end of the multiple pulley would descend.

How does a jackhammer work?

A jackhammer (or pneumatic hammer) uses compressed air to drive a metal piston up and down inside a cylinder. Each time the piston nears the top of the cylinder, it opens a valve that allows compressed air to flow above it and push it downward. Each time the piston reaches the bottom of the cylinder, it opens a valve that allows compressed air to flow below it and push it upward. Thus the compressed air makes the piston shuttle up and down very rapidly.

But while the piston rebounds gently from a cushion of air at the top of the cylinder, it collides suddenly with a metal bar at the bottom of the cylinder. That metal bar is the top end of the drill bit that the jackhammer uses to cut into pavement. Each time the piston moves downward, it pounds the drill bit a little farther into the pavement. The enormous force that pushes the bit into cement comes from the enormous force needed to stop the descending piston and to accelerate it upward. The drill bit pushes up on the piston very hard and the piston pushes down on the drill bit very hard. These two forces are equal and opposite, as they must be (Newton’s third law of motion.) The piston ends up moving upward and the drill bit ends up moving downward.