AP Physics 1: Kinetic Friction Questions and Answers

📚 Quick Study Guide

• ⚖️ Kinetic friction arises when two surfaces slide against each other.
• 📐 The force of kinetic friction ($F_k$) is proportional to the normal force ($F_N$) between the surfaces.
• 🔢 The formula for kinetic friction is: $F_k = \mu_k F_N$, where $\mu_k$ is the coefficient of kinetic friction.
• 📝 The coefficient of kinetic friction ($\mu_k$) is a dimensionless quantity that depends on the nature of the two surfaces in contact. It's usually less than the coefficient of static friction.
• 💡 Kinetic friction always opposes the motion of the object.
• 📏 Remember to use consistent units (SI units preferred: Newtons for force, meters for distance, kilograms for mass).
• 🧪 Kinetic friction converts kinetic energy into thermal energy (heat).

Practice Quiz

1. A 5 kg block is sliding across a horizontal surface with a coefficient of kinetic friction of 0.2. What is the magnitude of the kinetic friction force acting on the block?
   1. 9.8 N
   2. 4.9 N
   3. 0 N
   4. 2.5 N
2. An object is pulled across a surface with a constant velocity. The applied force is 20 N, and the coefficient of kinetic friction is 0.25. What is the normal force acting on the object?
   1. 5 N
   2. 80 N
   3. 20 N
   4. 100 N
3. A 10 kg box is sliding down an inclined plane with an angle of 30 degrees. The coefficient of kinetic friction is 0.3. What is the force of kinetic friction acting on the box?
   1. 25.5 N
   2. 49 N
   3. 30 N
   4. 0 N
4. A wooden block is sliding on a steel surface. Which of the following would increase the kinetic friction force?
   1. Decreasing the normal force
   2. Increasing the surface area of contact
   3. Increasing the normal force
   4. Decreasing the velocity of the block
5. A box with a weight of 50 N is pulled horizontally across a floor at a constant speed with a force of 20 N. What is the coefficient of kinetic friction between the box and the floor?
   1. 0.4
   2. 2.5
   3. 0.2
   4. 4.0
6. A hockey puck is sliding across the ice. The coefficient of kinetic friction is 0.05, and the puck's mass is 0.1 kg. What is the magnitude of the kinetic friction force?
   1. 0.49 N
   2. 0.049 N
   3. 0.98 N
   4. 0.098 N
7. If the kinetic friction acting on a sliding object is 15 N, and the normal force is 75 N, calculate the coefficient of kinetic friction.
   1. 0.2
   2. 5
   3. 0.5
   4. 0.1