Forces, Movement, Shape, and Momentum
4 marks · NormalA tennis ball of mass 60 g is dropped from rest. It eventually reaches a constant terminal velocity as it falls through the air.
Calculate the weight of the tennis ball. (Take .)
Describe what happens to the forces acting on the ball as it falls until it reaches terminal velocity.
Explain why the ball continues to move at terminal velocity instead of speeding up.
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.
Show answer
(a) 0.6 N
(b) At first, only gravity (weight) acts. As the ball speeds up, air resistance increases. Eventually, air resistance equals weight; net force becomes zero.
(c) Because the forces are balanced; no net (unbalanced) force acts.
Common mistakes: Not converting g to kg; Forgetting air resistance increases as speed increases
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Weight calculated correctly (0.6 N).
M1 (b) · 1 mark · Describes change in forces as ball falls (increasing air resistance).
A2 (b) · 1 mark · States balance: air resistance = weight at terminal velocity.
A3 (c) · 1 mark · Explains motion at constant speed/no acceleration due to balanced forces.
Worked solution
a) Mass = 60 g = 0.060 kg Weight = m × g = 0.060 × 10 = 0.6 N
b) At first, only the weight acts downwards, causing acceleration. As the ball falls, it speeds up, and air resistance acting upward increases. Eventually, air resistance grows large enough to equal the weight.
c) Here, forces are balanced (no resultant force), so the ball continues at constant terminal velocity.