An unbalanced force must be acting when an object a. accelerates. b. changes direction but not speed. c. changes speed but not direction. d. changes speed and direction. e. all of the above

Newton's laws of motion are fundamental principles that describe the relationship between a body and the forces acting upon it. These laws help us understand how objects move and interact with each other.

• The first law, often called the Law of Inertia, states that an object at rest will stay at rest, and an object in motion will stay in motion unless acted upon by an unbalanced force. This explains why you need to apply a force to change an object's state of motion.
• The second law explains how the velocity of an object changes when it is subjected to an external force. Mathematically, it is expressed as \( F = ma \), where \( F \) is the force applied, \( m \) is the mass, and \( a \) is the acceleration. This means that the acceleration of an object is directly proportional to the net force acting upon it and inversely proportional to its mass.
• The third law states that for every action, there is an equal and opposite reaction. This principle is why you feel a push back when you push against a wall.

In sum, these three laws help explain why objects accelerate, decelerate, or change direction when forces are applied.

Acceleration is the rate at which an object's velocity changes over time. It occurs whenever there is a change in speed, direction, or both.
Acceleration can be positive (an increase in speed) or negative (a decrease in speed, often called deceleration).
According to Newton's second law, we can calculate acceleration by rearranging the formula \( a = \frac{F}{m} \). If an unbalanced force is applied to an object, it will cause the object to accelerate in the direction of the force. Consider a car at a stoplight:

• If the driver steps on the gas, the car accelerates forward as an unbalanced force is applied.
• If the driver brakes, the car decelerates because an unbalanced force is applied in the opposite direction of motion.

Acceleration is a key concept for understanding changes in motion and is directly impacted by the forces acting on an object.

Change in velocity refers to any alteration in the speed or direction of an object's motion. This could involve speeding up, slowing down, or changing direction.
Velocity is a vector quantity, which means it has both magnitude (speed) and direction. So, a change in either speed or direction, or both, means a change in velocity:

• A car accelerating from a stoplight experiences a change in velocity because its speed increases.
• A car turning a corner also experiences a change in velocity because its direction is changing, even if its speed remains constant.

Changes in velocity are always caused by unbalanced forces. For example, a ball rolling on the ground eventually stops due to the unbalanced force of friction acting opposite to its motion. Understanding changes in velocity is crucial for predicting and explaining the motion of objects.

Directional change occurs when an object alters its path of motion, even if the speed remains constant.
For instance, when a car takes a turn, it is changing its direction but may still maintain the same speed before and after the turn. This change in direction is due to unbalanced forces acting perpendicular to the object's initial motion.
The force required for a directional change is often referred to as a centripetal force, which always points towards the center of the curved path the object is following. During a turn, this force is responsible for continuously pulling the object towards this center path:

• Think of a satellite orbiting Earth. It changes direction constantly while maintaining a roughly constant speed due to the gravitational pull exerting a centripetal force.
• Similarly, a cyclist maneuvering a bend relies on the frictional force between the tires and the ground to change direction.

Understanding directional changes is vital for grasping more complex motions, such as circular and oscillatory movements.