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When an object moves in a circular path at constant speed, it is said to be in circular motion. Despite constant speed, the object is accelerating because its direction is continuously changing.
The acceleration that acts towards the center of the circle is called centripetal acceleration, and the force that causes this acceleration is the centripetal force.
For example, in the bicycle problem, the cyclist is moving in a circle of radius 25 meters at a speed of 9 meters per second. The centripetal force required to keep the cyclist moving in a circular path can be calculated using the formula \( F_c = \frac{m \times v^2}{r} \). This force is primarily provided by the friction between the tires and the road surface. In this particular case, the centripetal force turned out to be 275.4 Newtons.

Friction is the force that resists the motion of objects sliding against each other. There are different types of friction, but when it comes to circular motion, especially when dealing with vehicles, the most relevant is kinetic friction.
Kinetic friction comes into play when an object is moving. It provides the necessary force to keep a vehicle or object in motion along a path, in this case: a circular path.
In the bicycle exercise, the friction force exactly equals the centripetal force needed to keep the bicycle on its circular path. Therefore, the force of friction on the bicycle from the road is 275.4 Newtons. This is calculated using the centripetal force formula we discussed previously.

The normal force is the force exerted perpendicularly by a surface against an object resting upon it.
It balances the gravitational force acting on the object to keep it stationary in the vertical direction. The magnitude of the normal force is equal to the weight of the object as long as the surface itself is horizontal.
Using the exercise again, the gravitational force acting on the bicycle and rider is given by \( F_N = m \times g \), where \( g \) is the acceleration due to gravity (9.81 m/s²). For a mass of 85 kg, the normal force works out to be 833.85 Newtons.

Newton's laws of motion are fundamental principles that describe the relationship between the movement of an object and the forces acting on it:

• **First Law (Inertia)**: An object at rest remains at rest, and an object in motion remains in motion with the same speed and direction, unless acted upon by an unbalanced force.
• **Second Law (Acceleration)**: The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This is often written as \( F = m \times a \).
• **Third Law (Action and Reaction)**: For every action, there is an equal and opposite reaction.

In the context of the bicycle exercise, the second law is particularly relevant. It explains how the centripetal force (friction between the tires and the road) causes the cyclist to follow a circular path. The normal force is an example of the third law, counteracting the gravitational force to keep the bicycle in equilibrium vertically.