While studying the behavior of a body in motion, we usually encounter that it does not maintain a constant velocity. The fact that a body can increase the magnitude of its velocity (also known as speed) while in motion, is what is normally known as acceleration. When the magnitude of its velocity decreases, we talk about braking. In physics, both types of motion are studied as the same magnitude: acceleration.

For example, we say that a car is accelerating when, in a straight line, it goes from 90 km/h to 120 km/h, or when you drop a rubber ball and allow it to fall freely, we can see how its speed increases as time passes (increases because of gravity).

Experiment and Learn
Concept of acceleration

The graph represents the speed (magnitude of the velocity) of a body over time.

Drag the value of the acceleration and increase it to observe how the speed increases with time, or decrease it to see how the speed lowers. The greater the acceleration the steeper the increase of the speed v, and vice versa.

You need to realize that when the value of the acceleration is zero, the speed remains constant (horizontal line), it does not increase nor decrease.

We have marked the speed at the point t=2.5 s, so you can verify the increase or decrease numerically.

However, in physics, we also call acceleration when a body modifies the direction of its velocity. For example, a train that takes a curve modifies the direction of its velocity.

We say that a body has acceleration when it varies it velocity with time either in:

  • magnitude or
  • direction

Experiment and Learn
Uniform circular motion (u.c.m.)

The graph shows a body doing uniform circular motion.

Drag the value of the speed (magnitude of the velocity vector) to observe how the body moves faster or slower.

Observe the different kinematic magnitudes. Additionally, verify that the velocity vector, in green, is tangent at each point to the trajectory and, on the other hand, the normal acceleration, in red, is responsible for the change in direction of the speed. Its direction always points to the center the circular trajectory and its value (magnitude) depends on the speed of the body.

Kinds of acceleration

In physics, we usually talk about different kinds of acceleration, depending on different factors. The following is a classification, as well as the links, so that you can understand what we mean in each case

Solved exercises worksheet

Here you can test what you have learned in this section.

Acceleration graphs


Given the following graphs that show the variation of the speed with time of three different bodies, answer the following questions:

a) Which case has the fastest variation of the speed?
b) What does the slope represent in each one of the 3 cases?
c) Why is the line in case I horizontal?
d) In case I, does the possibility exist that the body is experiencing acceleration?

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