Test: Turning effect of a force - Class 8 MCQ

Metals are good conductors of heat and electricity because their free electrons allow efficient transfer of thermal and electrical energy.

In a displacement reaction, a more reactive metal can replace a less reactive metal from its compound. For example, if you place a piece of zinc in a copper sulfate solution, zinc will displace copper from the solution, forming zinc sulfate and copper metal. This is because zinc is more reactive than copper.

The moment of a force, or torque, causes an object to rotate about a specific point (pivot) because it applies a turning effect proportional to the force and the distance from the pivot.

In a displacement reaction, zinc, being more reactive, displaces copper from copper sulfate, forming zinc sulfate and copper metal. The correct word equation is: Zinc + copper sulfate → zinc sulfate + copper.

When metal carbonates react with acids, carbon dioxide gas is produced. This gas can be tested for using limewater (calcium hydroxide solution). Carbon dioxide gas turns limewater milky/cloudy due to the formation of calcium carbonate, which is insoluble in water. This reaction is a classic test for the presence of carbon dioxide gas.

Torque (or moment of force) is the term used to describe the turning effect of a force around a pivot point. It depends on both the magnitude of the force and the distance from the pivot point where the force is applied.

Torque depends on the force’s magnitude, the perpendicular distance from the pivot (lever arm), and the direction of the force (specifically, the perpendicular component, determined by the sine of the angle between the force and the lever arm).

The reactivity series ranks metals by their ability to displace others in reactions. Sodium is highly reactive, followed by magnesium, iron, and copper, which is less reactive. Thus, the correct order of decreasing reactivity is: Sodium, magnesium, iron, copper.

Torque is the product of the force and the perpendicular distance (lever arm) from the pivot. A longer lever arm increases this distance, resulting in greater torque, assuming the same force is applied.

The turning effect (moment or torque) is the product of the force and the perpendicular distance from the pivot. Using a longer wrench increases this distance, increasing the moment for the same force applied.