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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 3.5 Contact Forces

Contact forces are a class of forces that involve contact between the two systems that are acting on each other. Most forces you will encounter in your study of everyday physics are contact forces, including the list below.
  • A normal force is a force between two objects that acts perpendicular to the surface of contact between them. 1  Normal forces always push away from the surface of contact. There is no model for the magnitude of a normal force: if you do not know the magnitude of a normal force from some external source, you can probably determine it by exploring the context.
  • A friction force is a force between two objects that acts parallel to the surface of contact between them, in contrast to the normal force above. You will learn more about the force of friction starting in Section 3.11.
  • A tension force is a force between a rope or a string and another system. Tension forces always pull along the direction of the rope or string at the surface of contact. You will learn more about the force of tension in Section 3.13.

Exercises Activities: Practice with Contact Forces

1. Mountain Climbing.

A mountain climber is connected to a vertical rope anchored at the top of a cliff. The climber decides to take a short break at a point where the surface of the cliff makes a \(75^o\) angle to the horizontal. Draw a free-body diagram for the climber showing all of the following forces: normal, friction, tension, and gravity 2 . Your diagram should be qualitatively correct, but you do not need to solve for anything.

2. Explaining the Forces.

Is the horizontal component of the normal force greater than, less than, or equal to the horizontal component of the friction force? If your original free-body diagram does not agree with your answer, draw a new one that does!
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