Dynamics: (d) the terms tension, normal contact force, upthrust and friction

The idea

Tension is the pulling force transmitted through a string, rope or cable when it is stretched. Normal contact force acts perpendicular to a surface where two objects touch. Upthrust is the upward force exerted by a fluid on an object immersed in it, due to pressure differences. Friction opposes relative motion or the tendency of relative motion between surfaces in contact. In MCQs, you may identify which force acts in a given situation, or match descriptions to terms. For example, a book on a table experiences a normal contact force upwards; a submerged ball experiences upthrust; a box sliding on a floor experiences friction; a hanging mass experiences tension in the string. Recognise the direction and origin of each force.

What earns the marks

• Tension is a pulling force in a string, rope or cable.
• Normal contact force acts perpendicular to a surface at the point of contact.
• Upthrust is an upward force on an object in a fluid due to pressure difference.
• Friction opposes relative motion or the tendency of relative motion between surfaces.
• Each force has a specific direction and origin; identify them from the physical situation.

Common mistakes and how to put them right

• Calling the normal contact force 'normal force' without specifying it is perpendicular to the surface.

  Put it right by stating it acts at right angles to the contact surface.

• Thinking upthrust depends on the object's weight rather than the fluid displaced.

  Put it right by linking upthrust to fluid pressure difference and Archimedes' principle.

• Believing friction always opposes motion.

  Put it right by stating it opposes relative motion or its tendency.

• Confusing tension with compression.

  Put it right by noting tension is a pulling force in a stretched string.

Exam tips

• Read the scenario carefully and identify the contact surfaces or fluid involved.
• Match the direction of the force to the term: upthrust is always upward, normal contact force is perpendicular to the surface.
• For friction, ask whether it is opposing motion or the tendency of motion.