Distance covered per unit time is known as speed; it is a scalar quantity because it has magnitude but no direction. Speed is measured in metres per second.
The concept of the speed-time graph is all about understanding and analysing the motion of a body, in which the speed and time are mainly taken into consideration. The speed-time graph is divided into four segments, or we can say that there are four types of speed-time graphs. They are:
- Speed-time graph with constant speed
- Speed-time graph with constant acceleration
- Speed-time graph with increasing acceleration
- Speed-time graph when the object is decelerating
The speed-time graph looks similar to the distance-time graph, but in this graph, the speed is plotted on the Y-axis, and the time is plotted on the X-axis.
Let’s dig deep into the topic and take a quick review of each type of graph:
- Speed-time graph with constant speed: Here, the straight line does not mean that the object is not moving, rather it means that the object is moving at a constant speed with changing time intervals.
- Speed-time graph with constant acceleration: This graph shows the increase in speed with constant acceleration.
- Speed-time graph with increasing acceleration: The magnitude of increasing acceleration in the graph can be determined by using the slope of the speed-time graph at any instant in time.
- Speed-time graph when the object is decelerating: The downward slope shows that the object is slowing down, and this is the reason why it is called a decelerating graph.
Important Questions on Speed Time Graph
1) What does a straight horizontal line represent in a speed-time graph?
a) The body is at rest
b) At a constant speed, the body is moving
c) The body is moving at a maximum acceleration
d) None of the above
Correct Option: (a)
Explanation: In a speed-time graph, the speed is plotted at Y-axis, and the time is plotted at X-axis. The straight horizontal line in a speed-time graph shows that the body is moving at a constant speed.
2) If a ball is falling from a height, then what will be its velocity-time graph before hitting the ground?
a) A positive slope straight line
b) A negative slope straight line
c) A zero slope straight line
d) A parabola
Correct Option: (a)
Explanation: The velocity of the ball increases as it falls down because of the acceleration due to gravity. Hence, the graph will be like a straight line with a positive slope. This can be verified by the first equation of motion.
3) What is speed?
Distance covered per unit time is known as speed, and it is a scalar quantity because it has magnitude but no direction. Speed is measured in metres per second.
4) At what position the instantaneous speed of a ball thrown up in the sky will be minimum?
a) The starting position
b) On the top most position
c) Halfway through the whole trajectory
d) None of the above
Correct Option: (c)
Explanation: Speed is maximum at the initial and final points because when the ball rises up, there will be a point where the speed of the ball will be 0, and it will be in the state of instantaneous rest at that position.
5) What can be said about the instantaneous velocity of a car moving in a spiral with uniform angular velocity, and it started from the origin?
a) Instantaneous velocity increases with time
b) Instantaneous velocity decreases with time
c) Instantaneous velocity remains constant
d) Instantaneous velocity does not depend on the time
Correct Option: (a)
Explanation: The motion of the car can be called a circular motion with an increasing radius. As the radius increases, the tangential velocity also increases. Now, the speed will be equal to the magnitude of the tangential velocity because there is only one velocity present.
6) In a non-uniformly accelerated motion, what will happen to the instantaneous velocity?
a) Instantaneous velocity increases
b) Instantaneous velocity decreases
c) Instantaneous velocity varies as the acceleration
d) Instantaneous velocity remains constant
Correct Option: (a)
Explanation: If the motion is accelerated, the instantaneous velocity will increase with time. It doesn’t matter if the acceleration is variable or constant; the instantaneous velocity will still increase. The change in velocity can be analysed by using the variations in acceleration.
7) When a body falls under gravity with terminal velocity, what will happen to the average velocity?
a) Average velocity increases
b) Average velocity decreases
c) Average velocity remains constant
d) Average velocity changes exponentially
Correct Option: (c)
Explanation: There is no change in the velocity of the body when it moves with terminal velocity. It means that in equal time intervals, equal displacement is being covered. Therefore, the average velocity remains constant.
8) What is velocity?
The speed of a body in a particular direction is known as velocity. Velocity deals with the displacement of the body as it is the straight and shortest path between the initial and final point. So, we can conclude that velocity is known as the rate of change in displacement with respect to time. With both magnitude and direction, velocity is considered as a vector quantity.
9) What is instantaneous speed?
The speed of an object moving with a variable speed at any instant of time is known as instantaneous speed.
10) Define average velocity.
When the change in displacement or position is divided by the time intervals in which the displacement occurs, then the quantity obtained is known as average velocity. Average velocity is a vector quantity. Metre per second is the SI unit of average velocity. The average velocity can be positive or negative, but it depends on the displacement sign.
Practice Questions
1) What is the average speed?
2) What is the average velocity?
3) Define motion.
4) State Newton’s laws of motion.
5) What is the difference between displacement and distance?
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