When the brakes of a moving vehicle are applied and it comes to a stop, which type of force between the tyres and the road surface is primarily responsible for stopping the vehicle?

Introduction / Context:
This question links everyday experience of braking in vehicles with basic physics of forces. When you press the brake pedal in a car or motorcycle, the vehicle slows down and eventually stops. Many forces act on the vehicle, but one particular contact force between the tyres and the road actually provides the horizontal opposition to motion. Understanding which force is responsible is important for road safety, vehicle design and basic mechanics.

Given Data / Assumptions:

• A vehicle is initially moving along a road.
• The driver applies the brakes, and the vehicle comes to rest.
• We want to identify the force that directly causes the vehicle to stop.
• Options include gravitational force, van der Waals force, frictional force and covalent force.

Concept / Approach:
When the brakes are applied, brake pads create friction on the wheel or disc, reducing its rotation. The tyres then resist sliding on the road because of friction between rubber and the road surface. This frictional force acts opposite to the direction of motion and provides the net horizontal force that decelerates the vehicle. Gravity acts vertically downward and does not directly oppose the horizontal motion. Van der Waals and covalent forces are microscopic forces at the molecular level and are not used to describe macroscopic braking. Therefore, frictional force is the correct answer.

Step-by-Step Solution:
Step 1: Identify that applying brakes reduces the rotation of the wheels using friction inside the brake system. Step 2: As the wheels slow down, the tyres try to slide relative to the road. Step 3: Friction between the tyre surface and the road opposes this relative motion and exerts a horizontal force on the tyres. Step 4: According to Newton second law, this horizontal frictional force acts opposite to motion and produces a deceleration that reduces the vehicle speed. Step 5: Over time, the vehicle speed drops to zero and it comes to rest. Step 6: Gravitational force acts vertically downward and is balanced by the normal reaction from the road; these vertical forces do not directly change the horizontal speed. Step 7: Conclude that frictional force between tyre and road is the main force responsible for stopping a vehicle when brakes are applied.

Verification / Alternative check:
If the road is icy or very slippery, the friction between tyres and road is greatly reduced. In such conditions, even when brakes are applied, the vehicle may skid and take a much longer distance to stop or may not stop in time. This clearly shows that stopping ability depends on friction. On rough dry roads, friction is higher, and braking is more effective. These real world observations confirm that frictional force is the key factor in stopping vehicles.

Why Other Options Are Wrong:
Gravitational force acts vertically and is always present, but it does not directly oppose the horizontal motion of the vehicle. Van der Waals force is a weak intermolecular force important in chemistry and surface phenomena, not in macroscopic vehicle braking descriptions. Covalent force refers to the strong bonds within molecules, which maintain the integrity of materials but are not used to describe the macroscopic braking process.

Common Pitfalls:
Some learners think the brake pads themselves stop the vehicle, forgetting that it is the friction between tyres and road that transmits the braking force to the vehicle. If tyres leave the ground or the road is very slippery, braking becomes ineffective even if brake pads are working. Remember that frictional force at the tyre road contact patch is the actual horizontal force that slows down and stops the vehicle.

Final Answer:
The force that makes a vehicle stop when the brakes are applied is the frictional force between the tyres and the road.