Question 9
A wooden crate is standing on a concrete floor. Which of these changes would leave the force needed to start it sliding unchanged?
The right answer is the one marked above.
Stand it on its narrow end instead of its wide face. That changes only the contact area, and area does not affect friction.
The other three all change one of the two factors that do matter: bricks raise the normal reaction, while oil and matting change the nature of the surfaces — oil reducing the friction, matting increasing it.