FORCES AND SHAPE
4 marks · HardThe diagram shows a metal block at rest on a horizontal table. Several forces act on the block. A downward force of 35 N acts on the block (its weight). The table exerts an upward force of 35 N (normal reaction).
State and explain whether the forces on the block are balanced or unbalanced. Justify your answer using vector ideas.
Describe what will happen to the block if an extra upward force of 10 N is applied to it.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) The forces are balanced because the upward force (35 N) and downward force (35 N) are equal in size and opposite in direction, so their vector sum is zero. · must show States forces are balanced, Justifies using vector sum (zero resultant force)
(b) The forces are now unbalanced (upward 45 N vs downward 35 N), so there is a resultant force of 10 N upwards. The block will accelerate upwards (start to move up). · must show States forces are unbalanced, Calculates resultant (10 N up), Describes upward acceleration/movement
Common mistakes: Ignoring direction of forces; Not adding the extra force in calculation
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Marking points
B1 (a) · 1 mark · States the forces are balanced
B2 (a) · 1 mark · Gives a correct explanation using vector/addition ideas (equal size, opposite direction, sum is zero)
B3 (b) · 1 mark · Identifies the new forces are unbalanced or resultant force exists
B4 (b) · 1 mark · States the block will accelerate upwards or start to move up
Worked solution
a) The block has an upward force (normal reaction) of 35 N and a downward force (weight) of 35 N. These two forces are equal in size and act in opposite directions, so their vector sum is zero. Therefore, the forces are balanced.
b) With an extra upward force of 10 N, total upward force is 35 N + 10 N = 45 N. The downward force remains 35 N. There is now a net upward force of 10 N, so the block will accelerate upwards.