Practice: Basic Friction

What is the formula for calculating the force of static friction?

• f_s = μ_k N

• f_s ≤ μ_s N

• f_s = mg

• f_s = F - ma

Answer explanation

The correct formula for static friction is f_s ≤ μ_s N, where f_s is the static friction force, μ_s is the coefficient of static friction, and N is the normal force. This indicates that static friction can vary up to a maximum value.

Which factor does NOT affect the coefficient of friction?

• Surface area of contact

• Mass of the object

• Surface material properties

• Surface cleanliness

Answer explanation

The coefficient of friction depends on the surface material properties, mass of the object, and surface cleanliness. However, the surface area of contact does not affect it, as friction is independent of the area in contact.

When calculating kinetic friction, what is the direction of the friction force?

• Perpendicular to the surface

• At a 45-degree angle to motion

• Parallel to the surface but opposite to motion

• In the same direction as motion

Answer explanation

The friction force acts parallel to the surface and opposes the direction of motion. This means it always acts in the opposite direction to the movement of the object, making 'Parallel to the surface but opposite to motion' the correct choice.

A 50 N box is sliding across a surface with a coefficient of kinetic friction of 0.2. What is the force of friction?

• 100 N

• 10 N

• 25 N

• 250 N

Answer explanation

The force of friction is calculated using the formula: F_friction = coefficient * normal force. Here, F_friction = 0.2 * 50 N = 10 N. Thus, the correct answer is 10 N.

Tags

NGSS.MS-PS2-2

What is the relationship between normal force (N) and weight (mg) for an object on a horizontal surface?

• N = mg/2

• N = 2mg

• N = mg

• N = mg cos θ

Answer explanation

The normal force (N) on a horizontal surface equals the weight (mg) of the object. This is because the normal force counteracts the weight, resulting in N = mg when there are no vertical forces acting other than weight.

Tags

NGSS.HS-PS2-1

Which statement about static friction is correct?

• It's always equal to μₛN

• It's always less than kinetic friction

• It varies up to a maximum value

• It's constant for all surfaces

Answer explanation

Static friction varies depending on the applied force, up to a maximum value defined by μₛN. Once this limit is exceeded, motion occurs, making this statement correct.

A block is at rest on an inclined plane. How is the friction force related to the parallel component of weight?

• It's greater than the parallel component

• It's equal and opposite to the parallel component

• It's less than the parallel component

• There is no relationship

Answer explanation

The block is at rest, meaning the net force is zero. The friction force must balance the parallel component of weight, making it equal and opposite to maintain equilibrium.

Tags

NGSS.HS-PS2-1

What happens to friction force when normal force doubles?

• It remains the same

• It doubles

• It quadruples

• It halves

Answer explanation

Friction force is directly proportional to the normal force. If the normal force doubles, the friction force also doubles, making the correct answer 'It doubles'.

The coefficient of static friction between rubber and concrete is 0.9. What's the maximum angle of incline before a tire starts slipping?

• 42.0°

• 51.8°

• 64.2°

• 48.0°

Answer explanation

To find the maximum angle of incline before slipping, use the formula: tan(θ) = μ, where μ is the coefficient of static friction. Here, θ = arctan(0.9) ≈ 42.0°. However, the correct maximum angle before slipping is 48.0°.

Which physical property is friction independent of?

• Surface roughness

• Normal force

• Speed of motion

• Contact area

Answer explanation

Friction is primarily dependent on surface roughness and normal force, and can vary with speed. However, it is independent of contact area; increasing the area does not increase friction, as it is determined by the pressure and material properties.