# Lesson Worksheet: Static Friction Physics • 9th Grade

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:

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

Q2:

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 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, , be less than? 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 parallel to the surface and starts to move. The weight of the brick . What must the coefficient of static friction of the brick with the surface 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 cm. A force can act parallel to the surface and perpendicularly to the 18 cm side length face of the brick. A force can act parallel to the surface and perpendicularly to the 8 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 and ? 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 with a line parallel to the horizontal. Assume that the box is in rotational equilibrium at all times. What magnitude force 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?

Q6:

A box that is at rest on a rough surface has a weight of 150 N. 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 with a line parallel to the horizontal. Assume that the box is in rotational equilibrium at all times as a force is applied to the cord. When the force applied to the cord exceeds 120 N, the box starts to move along the surface. What is the coefficient of static friction of the box with the surface? Q7:

A brick with a mass of 2.3 kg is at rest on a rough surface that is at a slope that makes an angle above the horizontal, as shown in the diagram. If the slope becomes any steeper, the brick will start to slide downward along the slope. The coefficient of static friction of the brick with the surface is 0.67. What value must be less than for the brick to remain at rest? Q8:

A brick with a mass of 2.1 kg is at rest on a rough surface that is on a slope that makes an angle of above the horizontal, as shown in the diagram. If the slope becomes any steeper, the brick will start to slide downward along the slope. What is the coefficient of static friction of the brick with the surface? Q9:

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 parallel to the surface. The weight of the brick . The coefficient of static friction of the brick with the surface is 0.75. What must the magnitude of exceed to start the brick moving across the surface? Q10:

The figure shows two blocks, block A and block B. The blocks have equal masses. Block A is on a rough surface, and block B is on block A. A force acts on block B, parallel to the surface. Initially, block B does not move, and is steadily increased. When has increased enough to make block B start to move, both blocks move in the direction of at the same speed. Which of the following is a correct statement about the relation of , the coefficient of static friction between block A and the surface, to , the coefficient of static friction between the blocks? • A
• B
• C