Non-Contact Forces (DP IB Physics)

A child drags a sledge behind them as they climb up a hill.

Describe the contact and non-contact forces acting on the child and the sledge.

Answer:

Step 1: Identify the contact forces acting on the child and the sledge

• The child pulls on one end of the rope and the sledge pulls on the other end of the rope

  • This force is tension

• The ground pushes against the child and the sledge

  • This is the normal contact force

• The surface of the sledge moves over the the surface of the ground opposing the motion of the sledge

  • This force is surface friction

• The surfaces of the child's shoes move over the surface of the ground (enabling the child to walk)

  • This force is also surface friction

• The child and the sledge move through the air 

  • This force is fluid resistance or drag

Step 2: Identify the non-contact forces acting on the child and the sledge

• The gravitational pull of the Earth acts on the child and the sledge

  • This force is weight

You will often see weight as W rather than F_g, even on the IB exam papers. It is always best to stick with whichever symbols you have been given in the question. However, if no symbols are given in the question, use the correct symbols from the syllabus (F_g).