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Friday, July 13, 2018

A uniform solid block has weight 500 N, width 0.4 m and height 0.6 m. The block rests on the edge of a step of depth 0.8 m, as shown.


Question 8
A uniform solid block has weight 500 N, width 0.4 m and height 0.6 m. The block rests on the edge of a step of depth 0.8 m, as shown.


The block is knocked over the edge of the step and rotates through 90° before coming to rest with the 0.6 m edge horizontal.

What is the change in gravitational potential energy of the block?
A 300 J                        B 400 J                        C 450 J                       D 550 J





Reference: Past Exam Paper – June 2015 Paper 12 Q15





Solution:
Answer: C.

Since the block is said to be uniform, the centre of mass can be considered to be at the centre.


At the top of the step,
Height of position of centre of mass above ground = 0.8 + (0.6/2) = 0.8 + 0.3 = 1.1 m


At the bottom of the step,
Height of position of centre of mass above ground = 0.4/2 = 0.2 m


Change in GPE = mgΔh = 500 × (1.1 – 0.2) = 450 J

4 comments:

  1. http://physics-ref.blogspot.com/2014/11/physics-9702-doubts-help-page-13.html
    At this link^ question 73 why do you use the highest height achieved by the block BEFORE it topples but not here?

    ReplyDelete
    Replies
    1. here, we are dealing with the change in GPE.

      change in GPE = final GPE - initial GPE

      At the other link, we want to know how much energy is required to change the position of the object. we first need to raise it to the highest position.

      Delete
  2. why do we use the centre of mass

    ReplyDelete
    Replies
    1. The centre of mass is the point where the force of gravity is taken to act. We need to consider the centre of mass, else each point on the box would have its own GPE as they have different heights.

      Delete

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