the threads of a screw used to fasten two pieces of wood are pitched at an angle. how does this increase the static friction force?
A) by increasing the surface area between the screw surface and the wood
B) by increasing the ease of inserting the screw into the wood
C) by increasing the coefficient of static friction between the screw and the wood
D) by increasing the normal force exerted by the wood on the screw thread
D) by increasing the normal force exerted by the wood on the screw thread
The correct answer is D) by increasing the normal force exerted by the wood on the screw thread.
When threads are pitched at an angle, the screw requires a rotational force to be inserted into the wood. As the screw is turned, the inclined plane of the threads pushes against the wood, increasing the normal force exerted by the wood on the screw. According to the equation of static friction (Fstatic = µs * N), the static friction force is directly proportional to the normal force. Therefore, increasing the normal force increases the static friction force between the screw and the wood, making it more difficult to loosen or remove the screw.
To determine the correct answer, let's analyze the options one by one:
A) Increasing the surface area between the screw surface and the wood doesn't directly affect the static friction force. A larger contact area might distribute the force over a larger area, but it doesn't necessarily increase the friction force.
B) Increasing the ease of inserting the screw into the wood doesn't directly affect the static friction force either. This option doesn't relate to the increase in friction force.
C) Increasing the coefficient of static friction between the screw and the wood is a possibility. By increasing the friction coefficient, the resistance to motion (friction force) is also increased. However, this option assumes that the coefficient of static friction can be directly influenced by the threads' angle, which may not be accurate.
D) Increasing the normal force exerted by the wood on the screw thread is the most likely correct answer. The normal force is the force exerted perpendicular to the surface of contact. By angling the screw threads, the force required to insert the screw will exert a component of force parallel to the surface. This inward force component can increase the normal force, thereby increasing the static friction force.
Therefore, the correct answer is D) by increasing the normal force exerted by the wood on the screw thread.