Problem 71 - A Complete Resource Book in Physics for JEE Main

Question

A hockey player is moving northward and suddenly turns westward with the same speed to avoid an opponent. The force that acts on the player is (A) frictional force along westward. (B) muscle force along southward. (C) frictional force along south-west. (D) muscle force along south-west.

Step-by-Step Solution

Answer

The short answer to the question is: (B) muscle force along southward. This is because the frictional force acts westward, while the muscle force acts southward to allow the hockey player to change direction from northward to westward while maintaining the same speed.

Step 1: Identify the motion and draw a free body diagram

The player initially moves northward and then turns to the west. The player changes direction, which means there must be a force acting on them. To understand this situation, let's draw a free body diagram, representing the forces acting on the player at the moment they change direction.

Step 2: Identify the forces in the diagram

In the free-body diagram, we can identify the following forces acting on the player: 1. Gravitational force (Earth pulling the player downwards) 2. Normal force (the ground supporting the player, equal and opposite to the gravitational force) 3. Frictional force (between the player's shoes and the ground) 4. Muscle force (generated by player's muscles to make the turn) Since the player is moving horizontally (not up or down), we know the gravitational force and the normal force cancel each other out.

Step 3: Analyze Newton's laws for the given motion

According to Newton's first law, an object at rest or moving with a constant velocity will continue to do so unless acted upon by an external force. When the player changes direction, we can conclude that an external force must be acting on them. Now, let's consider Newton's second law, which states that the net force acting on an object is equal to the mass of the object multiplied by its acceleration (\(F = ma\)). As the player changes direction, they experience a change in velocity, which implies that there is some acceleration perpendicular to their initial motion (northward).

Step 4: Identify the force responsible for the change in direction

When the player changes direction from northward to westward, there must be a force acting on them perpendicularly to keep them in a curved path. The only forces that could be responsible are the frictional force and the muscle force. We need to identify which force is acting along which direction to understand the role of these forces in the player's motion. Frictional force generally opposes motion and acts parallel to the contact surface. Since the player is turning westward while moving, friction can't oppose the motion in northward direction. Instead, the friction acts in the opposite direction, which is westward. So, the frictional force is along the westward direction. The muscle force generated by the player's muscles allows them to change direction while maintaining the same speed. In this case, the muscle force must be acting southward to create a change in direction from northward to westward.

Step 5: Choose the correct answer

From our analysis of the motion and forces involved, we can conclude that: - The frictional force is acting westward - The muscle force is acting southward Therefore, the correct answer is: (B) muscle force along southward.