Question

What happens to the speed of an object when the applied force is in the opposite direction to the direction of motion?
A. Speed increases
B. Speed decreases
C. Speed remains the same
D. None of the above

Answer 1

Here, we can apply Newton's second law of motion to relate the applied force and acceleration of the object. Then, from the same equation we can also relate the applied force and velocity (speed) of the object.

Complete step-by-step solution:
If the applied force is in the opposite direction to the direction of motion of an object then the speed of that object will decrease. It is just because the applied force will slow down or oppose the movement (motion) of the object.
In other words, this concept is clearly understood by Newton's second law of motion which states that the force is calculated by the product of mass and acceleration.
As we know that mathematically, $F = m \times a$
Where, F is the applied force on an object, m is the mass of the object and a is the acceleration.
To calculate the velocity (speed), the above equation can also be written as,

F = m \times a \\\ = m \times \dfrac{{dv}}{{dt}} \\\ \end{gathered} $$ From Newton’s law of motion it proves that the force is directly proportional to the acceleration or the velocity (speed) of an object. Therefore, if we apply a force in the same direction of motion then acceleration and velocity of the object will increase. Similarly, if the applied force is in the opposite direction to the direction of motion then acceleration will decrease. **Hence, the correct option for this problem is, B.** **Note:-** The concept of opposite force to the direction of motion of an object will become clearer if we take an example from our daily life. When we throw a ball in the upward direction then the velocity of the ball will decrease due to gravitational force acting opposite to the direction of movement of the ball and after some interval of time the ball will return back to earth.