Question

A 2.0-N force acts horizontally on a 10-N block that is initially at rest on a horizontal surface. The coefficient of static friction between the block and the surface is 0.50. Suppose that the block now moves across the surface with constant speed under the action of a horizontal 3.0-N force. Which one of the following statements concerning this situation is false? The net force on the block is zero newtons. The coefficient of kinetic friction between the block and the surface is 0.30. The frictional force on the block has magnitude 3.0 N. The direction of the total force that the surface exerts on the block is vertically upward. The block is not accelerated.

Answer 1

Answer:

None of the statement is false - all of them are true

Explanation:

Let's analyze each statement:

- The net force on the block is zero newtons. --> TRUE. In fact, the block is moving across the surface at constant speed: constant speed means zero acceleration, and according to Newton's second law,

F = ma

this also means a net force of zero newtons.

- The frictional force on the block has magnitude 3.0 N. --> TRUE. The horizontal acceleration of the block is zero, so the resultant of the horizontal forces must be zero:

$F-F_f = 0$

where F = 3.0 N is the horizontal push and $F_f$ is the frictional force. From the equation, we find

$F_f = F = 3.0 N$

- The coefficient of kinetic friction between the block and the surface is 0.30. --> TRUE. The frictional force is 3.0 N, and its expression is

$F_f = \mu_k (mg)$

where $\mu_k$ is the coefficient of kinetic friction and (mg)=10 N is the weight of the block. Solving for $\mu_k$, we find

$\mu_k = \frac{F_f}{mg}=\frac{3 N}{10 N}=0.30$

- The direction of the total force that the surface exerts on the block is vertically upward. --> TRUE. Since gravity is acting downward, and the block is not accelerating on the vertical direction neither, there must be an equal and opposite force acting upward on the block: and this force is the force exerted by the surface on the block.

- The block is not accelerated. --> TRUE: the block is moving at constant speed, so its acceleration is zero.