When all the air inside the tin are taken out, the air outside the tin becomes greater than the air inside causing inward pushing of the tin which results in collapse of the tin.
What is unbalanced force?
This is a situation when the forces acting on an object are unequal.
A tin will remain firm when air inside it is prevented from escaping. During this time, the forces of air outside the tin balances the forces of air inside the tin, this process creates equilibrium condition that cause the tin to be firm.
When all the air inside the tin are taken out, the air outside the tin becomes greater than the air inside causing inward pushing of the tin which results in collapse of the tin.
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