The Effect of Inertia

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Concepts to investigate:

Inertia,weight and mass.


Block of wood (apprx 1kg),string or thread,screw eyes.

Principles and procedures:

Attach two screw eyes to two ends of a block of wood. Use one piece of thread to suspend the object from a suitable support such as a hook. The thread should be barely strong enough to support the object when suspended. Attach another thread to the bottom and pull steadily untill one of the threads breaks. Again,support the object,grasp the bottom thread and give it a quick jerk rather than a steady pull in in the first step. Repeat the procedure 5 times. Which thread breaks when you pull slowly?

Which thread breaks when you pull rapidly? Why?


(1) Explain why the top thread breaks when the bottom string is pulled slowly but the bottom string breaks when you jerk the bottom string.

(2) If a massive object such as a black smith's anvil is placed on the stomach of a person lying on the floor and a second person strikes the anvil hard, with a sledge hammer,the reclining person is unhurt. However,if the anvil were not on the stomach,the person would certainly have been injured by the blow. Explain.(NB: Don't attempt this activity!!)

(3) Suppose you have a hammer with a loose head. Although you could strike the hammerhead with a solid object to secure it on the handle,an easier method is to grasp the handle of the hammer and repeatedly strike the butt(end opposite the handle) on a solid surface. Explain.

(4) Imagine you are in deep space holding a large boulder in your hands. You would find it quite difficult to move this boulder back and forth even though it has no weight. Explain.