Final answer:
To find the force of friction without the coefficient, you can use various methods such as doubling the mass, increasing the surface area, or decreasing the normal force. Another method is using a spring balance to directly measure the force.
Step-by-step explanation:
To find the force of friction without the coefficient, you can use the equation f = μN, where f is the force of friction and N is the normal force. If the coefficient is unknown, you can find the force of friction by using other known quantities. For example, you can find the force of friction by doubling the mass of the object, increasing the surface area, or decreasing the normal force. Another way to find the force of friction is by using a spring balance to measure the force directly.