Question

You hold a thumbtack still between your index finger and thumb with a force of 10 N (Your finger is holding the needle end and your thumb is holding the flat end). The needle has an area of 2 x 10⁻⁷ m² whereas the flat end has an area of 3 x 10⁻⁴ m².

What is the force experienced by your finger?

Answer 1

The force experienced by your finger with this information is still the same, 10 N.

Here's why:

• The question states that you are holding the thumbtack with a force of 10 N. This force represents the total force you are exerting on the thumbtack, regardless of how it's distributed between the needle and flat end.
• While the areas of the two ends differ, these areas only affect the pressure (force per unit area) experienced by each end, not the total force you exert.
• Assuming you don't apply any additional force from your wrist or other fingers, the action-reaction principle dictates that the thumbtack pushes back on you with the same total force of 10 N. This force is also divided between the two ends based on their respective areas.

Therefore, even though the pressure felt by your finger and thumb might be different due to the varying areas, the total force you apply and experience from the thumbtack remains 10 N.