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Stuff

  • A coffee mug
  • A small piece of cardboard
  • A rubber ball

Do

1. Place the coffee mug on the table and cover it with the piece of cardboard and place the ball on it.

SLIPPERY CARDBOARD

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2. Slowly pull the sheet and observe what happens.

SLIPPERY CARDBOARD

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3. Now, again put the sheet back in place with the ball on it.

SLIPPERY CARDBOARD

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4. This time flick the sheet with your index finger very quickly. Observe what happens.

SLIPPERY CARDBOARD

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See

SLIPPERY CARDBOARD

When you slowly pull the sheet, the ball doesn’t fall off. When you flick the sheet with your finger, the sheet flies off but the ball doesn’t. It falls into the coffee mug.

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Think about

Why does the ball fall into the glass when you flick the sheet off the glass?

Newton’s First Law of Motion states that a body at rest stays at rest and a body in motion stays in motion unless acted upon by an outside force. When you are in a moving car you are going at the same speed as the car. When brakes are applied, the brakes stop the car but not you. You move forward till your seat belt stops you. Here, when the paper sheet is slowly moved, the ball moves along with it because of friction between ball and sheet. Friction is the force between one surface and another. The friction between the ball and sheet causes the ball to move along with the sheet. When the sheet is flicked away quickly with your index finger, the ball does not move along with the sheet as the speed of the applied force is more than the friction between ball and sheet. The force of gravity on the ball makes it fall into the glass.