0.17 Phy1150: force and motion -- units of force  (Page 6/9)

The measurement that we normally think of as the weight of an object with a given mass isthe force exerted on that object at the surface of the earth by the gravitational attraction between that object and the earth.

The weight of that same object on the surface of the moon would be the force exerted on that object by the gravitational attraction between that object andthe moon.

Weighing a package of hamburger

Assume that you have a package that contains a one-pound mass of hamburger. You put it on a scale on the surface of the earth and the display reads 1 pound.

Now assume that you take the same scale and the same package of hamburger to the surface of the moon and place thepackage on the scale. The display would no longer read 1 pound.

The weight indicated by the scale would be different because the gravitational attraction between the mass of the hamburger and the mass of the moon would be less thanthe gravitational attraction between the mass of the hamburger and the mass of the earth.

According to the calculator at (External Link) , the scale would read 0.2 pounds on the moon.

What does the output of the scale really mean?

When you see a scale with a display that reads pound, it should read pound-force instead. If it reads kilogram, it should read kilogram-forceinstead. Pound-force, kilogram-force, and gram-force are units that are tied directly to the gravitational attraction between the earth and other objects.

Weightlessness

When astronauts go into space and speak of being weightless, they probably aren't completely weightless. However, their weight is probably so low that it seemsto them to be zero.

The reduction in an astronaut's weight occurs because the distance between the astronaut and any large massive object (such as theearth or the moon) is so great that the gravitational attraction between them is very small.

Sample problems

Let's work through several sample problems involving forces. The first will be a statics problem and the last three will be dynamics problems.

A static scenario

It will probably help you to keep track of everything if you draw the scenario on your graph board.

Draw a side view of two cubes with different masses on the top of a flat level horizontalfriction-free table. Label them Mass C and Mass B. Mass C is on the left and Mass B is on the right.

Mass B is close to the rightmost edge of the table and Mass C is to the left of Mass B.

Label Mass C as 3 kg and label Mass B as 2 kg.

A mass hanging on a cord

Draw a strong but lightweight cord, connected to the right side of Mass B and thread the cord over a verylight frictionless pulley that changes the orientation of the cord from horizontal to vertical. The pulley is attached to the right edge of the table.

Draw a triangle-shaped mass connected to the cord that hangs down from the pulley. Label this Mass A and label it as 5 kg.

Two additional cords

Draw a strong but lightweight cord connecting Mass B to Mass C.

Draw another strong but lightweight cord connecting Mass C to a vertical wall on the left side of Mass C. That cord prevents any of the masses from moving and keeps the entiresystem in equilibrium with the 5-kg mass suspended from the cord that is threaded over the pulley.