If a force is applied on a moving object in a direction opposite to the direction of its motion, what happens to the speed of the object?

Introduction / Context:

This question checks understanding of how force and motion are related through Newton second law. When a force acts in the same direction as motion, it speeds an object up, and when it acts opposite to the direction of motion, it tends to slow the object down. Recognising this basic relationship is essential for analysing braking, friction, and many everyday motion situations.

Given Data / Assumptions:

• An object is already moving in some direction with a certain speed.
• A force is applied in the direction opposite to the direction of motion.
• No specific numerical values are given for mass, speed, or force.
• We assume that the applied force has a non zero magnitude and acts for some time.

Concept / Approach:

According to Newton second law, a net force on an object produces acceleration in the direction of the net force. If the net force is opposite to the direction of motion, the acceleration is negative relative to the velocity and is often called retardation or deceleration. This reduces the magnitude of the velocity vector, meaning the object slows down. If such a force acts long enough and is strong enough, the object can eventually come to rest, but the immediate effect is that speed decreases. Therefore, the correct qualitative answer is that speed decreases.

Step-by-Step Solution:

Verification / Alternative check:

Consider a car moving forward and the driver applying brakes. The frictional force from the brakes acts in the direction opposite to motion, causing the car to slow down. Eventually, if braking continues, the car may stop, but the first effect is a reduction in speed. Similarly, a ball rolling on the ground slows down due to friction, which acts opposite to its motion. These real life examples match the theoretical conclusion from Newton second law.

Why Other Options Are Wrong:

• The speed of the object increases: This would occur if the force were in the same direction as motion, not in the opposite direction.
• The object stops instantly in all cases: Stopping requires time and depends on the magnitude and duration of the force. It is not instant in the general case described.
• There is no effect on the speed of the object: Any non zero net force must change the state of motion, so no effect is not possible under Newton laws if a real opposite force is applied.

Common Pitfalls:

Some learners think only friction can slow objects down, but any net force opposite to motion has the same effect. Others interpret questions simplistically and think opposite force always means immediate stopping, which is only true if the force is extremely large or acts for a long enough time. Remember to distinguish between the immediate trend (slowing down) and the final outcome (possibly stopping) when a retarding force acts.

Final Answer:

When a force is applied opposite to the direction of motion, the speed of the object decreases.