Punjab 9th Physics Ch 2 Kinematics Short Questions

Scalar: A scalar is a physical quantity that can be described completely by its magnitude only. Magnitude includes a number and an appropriate unit.
Examples: Mass, length, time and speed, etc.

Vector: A vector is a physical quantity that requires both magnitude and direction to be described completely.
Examples: Velocity, displacement and force etc.

Scalar quantities: volume, work, energy, pressure, power

Vector quantities: force, momentum, torque, acceleration, weight

The head-to-tail rule is a graphical method used to add vectors. It states that to add two or more vectors, redraw their representative lines such that the head of one vector coincides with the tail of the other. The resultant vector is the single vector directed from the tail of the first vector to the head of the last vector.

Distance-Time Graph: Shows how distance changes over time. The slope of this graph represents speed.

Speed-Time Graph: Shows how speed changes over time. The slope of this graph represents acceleration, and the area under the curve represents distance traveled.

No, a heavier object does not fall faster than a lighter object when dropped from the same height near the Earth’s surface, assuming air resistance is negligible. Both objects will fall at the same rate. Near the Earth’s surface, all objects fall with a constant acceleration of approximately 10 ms⁻², regardless of their mass. This is because gravity acts on all objects uniformly.

When vector quantities are written in scalar notation (without boldface), their direction is indicated through various means:
(i) Positive and Negative Signs
(ii) Directional Words (North, south, east, west)
(iii) Angles
(iv) Unit Vectors

No, a body moving with uniform speed may not have uniform velocity. Velocity includes both magnitude and direction. If the direction changes, the velocity changes even if the speed remains constant.

(i) Constant velocity:
No, a body moving with constant velocity (both speed and direction remain unchanged) has zero acceleration. Acceleration is the rate of change of velocity, so if the velocity does not change, the acceleration is zero.

(ii) Constant speed:
Yes, a body moving with constant speed can have acceleration if its direction is changing. For example, an object moving in a circular path with constant speed is continuously changing direction, which results in centripetal acceleration directed toward the center of the circle, even though the speed remains constant.

The car’s velocity changes at every instant on a circular road because velocity is a vector quantity. While the speed remains constant, the direction of motion changes continuously along the circular path, causing a change in velocity.

The equations of motion for freely falling bodies are:
• Vf = Vi + gt
• h = Vit + ½gt²
• 2gh = Vf² – Vi²

(i) Acceleration in the direction of motion: A freely falling object has direction of motion downward and acceleration due to gravity is also downward. An apple falling from a tree is an example of acceleration in the direction of motion.

(ii) Acceleration against the direction of motion: A body thrown upward has direction of motion upward and direction of acceleration downward. An upward throwing cricket ball is an example of acceleration against the direction of motion.

(iii) Acceleration is zero and body is in motion: A uniformly moving body in a fixed direction has zero acceleration. A uniformly moving car in the right direction is an example of zero acceleration and the body is in motion.

(a) Each line on the graph represents the motion of a cyclist (A, B, and C) in terms of the distance traveled over time at a constant speed.

(b) Cyclist A traveled the most distance as its line is the highest on the graph.

(c)
• The greatest speed: Cyclist A (steepest slope).
• The lowest speed: Cyclist C (least steep slope).
• All cyclists traveled at constant speeds as their lines are straight.

The gradient of a distance-time graph represents the speed of the object. A steeper gradient indicates a higher speed, while a flatter gradient indicates a lower speed.

The gradient of a speed-time graph represents the acceleration of the object. A positive gradient indicates increasing speed (positive acceleration), while a negative gradient indicates decreasing speed (negative acceleration).

The area under a speed-time graph represents the distance traveled by an object.

If a body starts motion and finally reaches its initial position, then in such a case displacement is zero but not the distance.

Displacement and distance are equal when the body goes straight from initial point to the final point. For example, a car moves straight to the school. Here distance and displacement are equal.

We can add two or more vectors to get a single vector. This is called a resultant vector. It has the same effects as the combined effect of all the vectors to be added.

A vector can be represented graphically by drawing a straight line with an arrow head at one end. The length of line represents the magnitude of the vector quantity according to a suitable scale while the direction of arrow indicates the direction of the vector.

In 1905, famous scientist Albert Einstein proposed his revolutionary theory of special relativity which modified many of the basic concepts of physics. According to this theory, speed of light is a universal constant. Its value is approximately 3×10⁸ ms⁻¹. Speed of light remains the same for all motions. Any object with mass cannot achieve speeds equal to or greater than that of light. This is known as universal speed limit.