An object of mass 5.0 kg falls vertically through still air. The air resistance acting on the object has a constant value of 50 N. Which of the following statements about the object is/are correct?
1 The acceleration of the object is 10 m s –2.
2 There is no resultant force on the object.
3 The object is travelling at its terminal velocity. (gravitational field strength = 10 N kg –1)/

Answer:

3) G

1 – wrong. Let us calculate the weight of the object. Please remember, weight = mass x gravitational field strength = 5 kg x 10 = 50 N. The air resistance is 50 N. We can then conclude that the upward force (air resistance = 50 N) is equal to downward force (weight = 50 N). Since both forces are equal, the object in free fall would be at terminal velocity, i.e. its velocity is not changing. When velocity does not change (neither decreases nor increases), its acceleration should be zero. So it cannot be 10 ms^2.
2 – correct. We deduced above that there is no net resultant force on the object as upward force and downward forces are equal.
3- correct. We deduced that when upward force and downward forces are equal, and there is no net resultant force on the object, the object’s velocity does not change. It has reached its terminal velocity.