### Acceleration

Topic: Translational Motion

Acceleration is a vector representing the rate at which the velocity of a body changes with time.

In physics, acceleration is the rate at which the velocity of a body changes with time. It is a vector quantity with both magnitude and direction. Acceleration is accompanied by a force, as described by Newton’s Second Law; the force, as a vector, is the product of the mass of the object being accelerated and the acceleration (vector), or . The SI unit of acceleration is the meter per second squared: m/s

Acceleration is a vector that points in the same direction as the change in velocity, though it may not always be in the direction of motion. For example, when an object slows down, or decelerating, its acceleration is in the opposite direction of its motion.

The motion of an object can be depicted graphically by plotting the position of an object over time. This distance-time graph can be used to create another graph that shows changes in velocity over time. Because acceleration is velocity in m/s divided by time in s, we can further derive a graph of acceleration from a graph of an object’s speed or position.

Below is a graph of an object’s position over time. This graph is similar to the motion of a car. In the beginning, the object’s position changes slowly as it gains speed. In the middle, the speed is constant and the position changes at a constant rate. As it slows down toward the end, the position changes more slowly. From this graph, we can derive a velocity vs time graph. This shows the velocity of the object over time. The object’s velocity increases in the beginning as it accelerates at the beginning, then remains constant in the middle before it slows down toward the end. Notice that this graph is a representation of the slope of the previous position vs time graph. From this graph, we can further derive an acceleration vs time graph. To do this, we would also plot the slope of the velocity vs time graph. In this graph, the acceleration is constant in the three different stages of motion. As we noted earlier, the object is increasing speed and changing positions slowly in the beginning. The acceleration graph shows that the object was increasing at a positive constant acceleration during this time. In the middle, when the object was changing position at a constant velocity, the acceleration was 0. This is because the object is no longer changing its velocity and is moving at a constant rate. Towards the end of the motion, the object slows down. This is depicted as a negative value on the acceleration graph. Note that in this example, the motion of the object is still forward (positive), but since it is decelerating, the acceleration is negative. Practice Questions

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Key Points

• Acceleration is the rate at which the velocity of a body changes with time.

• Acceleration is a vector that points in the same direction as the change in velocity, though it may not always be in the direction of motion.

• Because acceleration is velocity in m/s divided by time in s, we can derive a graph of acceleration from a graph of an object’s speed or position.

Key Terms

Acceleration: The amount by which a speed or velocity increases (and so a scalar quantity or a vector quantity).

Velocity: A vector quantity that denotes the rate of change of position with respect to time, or a speed with a directional component.

Position: A place or location.

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