# Block on a Horizontal Table

• thedark_master
In summary, a small block of mass 0.91 kg moves in a circle of radius 0.63 m with a speed of 1.5 m/s on a horizontal table. It is held in place by a string passing through a hole at the center of the circle. The string is then pulled down, decreasing the radius to 0.51 m. The question asks for the amount of work done by the force in changing the radius. The concept of conservation of energy and the work energy theorem are discussed in the conversation.

#### thedark_master

https://chip.physics.purdue.edu/protected/IEalgebraMimg/uiuc101-09-block.ie/pic.gif

A small block of mass 0.91 kg slides without friction on a horizontal table. Initially it moves in a circle of radius r0 = 0.63 m with a speed 1.5 m/s. It is held in its path by a string that passes through a small hole at the center of the circle. The string is then pulled down a distance of r0 - r1 = 0.12 m, leaving it at a radius of r1 = 0.51 m. It is pulled so slowly that the object continues to move in a circle of continually decreasing radius.

How much work was done by the force to change the radius from 0.63 m to 0.51 m?

basically i am completely lost, any help would be appreciated

Well, you know that two things are always conserved, energy and momentum. A conservation theorem may be a good place to start.

ya i am still stuck

First get a sense of what is happening. The ball is going around in a circle with a constant velocity. Some kind of force is keeping the ball from simply shooting outward. The whole time, energy will be conserved. Now, as the ball moves towards the center by whatever force, is energy still conserved?

no its not

Id start by looking at the work energy theorem.

You don't believe that energy is still conserved? What would it lose energy to?

Well, one can't make a statement about energy conservation until the system is defined.

robb_ said:
Well, one can't make a statement about energy conservation until the system is defined.

Not really sure what you mean by this, but the system is defined enough to tell whether or not non-conservative forces are at play, and whether or not the work energy theorem can be used.

You and I may have a system in mind where energy is conserved, but the OP may not, i.e. just the rotating block?

## 1. What is a block on a horizontal table?

A block on a horizontal table refers to a physical scenario in which a rectangular or square-shaped object, commonly known as a block, is placed on a flat surface such as a table.

## 2. What is the purpose of studying a block on a horizontal table?

The study of a block on a horizontal table is important in understanding the fundamental concepts of physics, such as forces, motion, and equilibrium. It also helps in practical applications such as designing structures and machines.

## 3. What are the main forces acting on a block on a horizontal table?

The main forces acting on a block on a horizontal table are gravity, normal force, and friction. Gravity pulls the block towards the center of the Earth, while the normal force from the table pushes the block upwards. Friction acts in the opposite direction to the motion of the block.

## 4. How does the weight of the block affect its motion on a horizontal table?

The weight of the block affects its motion on a horizontal table by determining the magnitude of the force of gravity acting on the block. A heavier block will experience a greater force of gravity and therefore require more force to move it.

## 5. Can a block on a horizontal table be in equilibrium?

Yes, a block on a horizontal table can be in equilibrium if the net force acting on it is zero. This means that all the forces acting on the block, including gravity, normal force, and friction, are balanced and the block will remain at rest or move with constant velocity.