02 Who is Pulling?

02 Who is Pulling?#

Aim#

To show the reality of Action=-Reaction.

Subjects#

  • 1H10 (Action and Reaction)

Diagram#

../../../../../_images/figure_028.png

Fig. 80 .#

Equipment#

  • Light cart that rolls easily.

  • Rope, \(l=10\mathrm{~m}\).

  • An assistant to the demonstrator.

Presentation#

The demonstrator and his assistant are standing each at one side of the lecture hall. Each of them holding one end of the rope. The demonstrator steps on the cart, and then his assistant hauls in the rope. The cart with the demonstrator is approaching the assistant. The demonstrator returns to the point of departure, but now the demonstrator, on his cart, hauls in the rope. Again, the cart with the demonstrator is approaching the assistant.

(A variation to the last demonstration can be shown when instead of the assistant holding the rope, it is tied to the wall.)

Explanation#

The demonstration shows that the effect of pulling by either the assistant or the demonstrator is the same: the cart + demonstrator moves towards the assistant. The resultant force has to be directed towards the assistant in both situations. So in both cases, the rope pulls the demonstrator toward the assistant (see Diagram; red arrows: first demonstration; yellow arrows: second demonstration). The yellow arrows show Newton’s third law. (And all arrows apply in both demonstrations!)

Sources#

  • Mansfield, M and O’Sullivan, C., Understanding physics, pag. 119

Contents