Worksheet: Static Friction

In this worksheet, we will practice analyzing the forces acting on objects on frictional surfaces that are required to induce motion in the objects.

Q1:

A brick is initially at rest on top of a rough horizontal surface, as shown in the diagram. The brick is pushed by a force F=25N parallel to the surface and starts to move. The weight of the brick 𝑊=28N. What must the coefficient of static friction of the brick with the surface be less than?

Q2:

The figure shows two blocks, block A and block B. Block A is on a rough surface, and block B is on block A. A force F acts on block A, parallel to the surface. Initially, block A does not move, and F is steadily increased. When F has increased enough to make block A start to move, both blocks move in the direction of F, but block A has a greater acceleration than block B. Which of the following is a correct statement about the relation of 𝜇, the coefficient of static friction between block A and the surface, to 𝜇, the coefficient of static friction between the blocks?

  • A𝜇<𝜇
  • B𝜇=𝜇
  • C𝜇>𝜇

Q3:

A brick is initially at rest on top of a rough horizontal surface, as shown in the diagram. The brick is pushed by a force F=22N parallel to the surface and starts to move. The coefficient of static friction of the brick with the surface is 0.75. What must the weight of the brick, 𝑊, be less than?

Q4:

A brick with a mass of 2.2 kg is initially at rest on a rough surface, as shown in the diagram. The coefficient of static friction of the brick with the surface is 0.84. The lengths of the sides of the face of the brick in contact with the surface are 18 cm and 8.0 cm. A force F can act parallel to the surface and perpendicularly to the 18 cm side length face of the brick. A force F can act parallel to the surface and perpendicularly to the 8.0 cm side length face of the brick. Both forces are just great enough to start the brick moving over the surface. What is the ratio of the magnitudes of F and F?

Q5:

A box that is at rest on a rough surface has a weight of 165 N. The coefficient of static friction of the box with the surface is 0.815. The box is pulled by a cord that is attached to its top surface, as shown in the diagram. The cord makes an angle of 55 with a line parallel to the horizontal. Assume that the box is in rotational equilibrium at all times.

What magnitude force F must be exceeded to make the box start to move along the surface? Give your answer to three significant figures.

When the box starts to move along the surface, what is the ratio of the force that it applies to the surface to the weight of the box?

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