Question

In which of the following cases the net force is not equal to zero?
A. A kite skillfully held stationary in the sky.
B. A ball freely falling from a height.
C. An airplane rising upwards at an angle of 45 degree with the horizontal with a constant speed.
D. A cork floating on the surface of water.

Answer 1

Hint: From the following cases, we have to find the case when the net force is not zero. When the body experiences a push and pull effect or changes its position, then we can say that the body experiences a force. A force can be zero if the body is in stable condition and does not change its shape and size. Also, when the body possesses uniform velocity, so the force becomes zero.

Complete step by step answer:In the first case, a kite is flying in the sky and held stationary in the sky. At first, it requires some efforts to move in the sky and after it reaches a certain height where the kite gets a sufficient amount of air. After a kite skillfully held stationary in the sky, then the kite having the net force is zero.

In the second case, a ball is freely falling from a height. In this condition, a ball freely falling from a height is not experiencing zero net force. When the ball is falling from some height, it experiences a gravitational force from the Earth and also accelerates under the influence of gravitational force.
In the third case, an airplane rises upwards at an angle of 45 degrees with constant speed, so some amount of force acting on the airplane is zero.
In the fourth case, a cork floats on the surface of water, so the cork is in stable condition. Therefore, no force would act on the cork.

Note: In this question, the student has the knowledge of force, that is how the force is acting on a body. Mathematically the force is directly related to the acceleration and mass, if acceleration is constant then the force becomes zero.