krit.club logo

Motion - Motion in a Plane

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Scalars and Vectors: Scalar quantities have only magnitude (e.g., distance, speed), while vector quantities have both magnitude and direction (e.g., displacement, velocity, acceleration). Motion in a plane involves two-dimensional vectors.

•

Definition of Motion in a Plane: It refers to motion in two dimensions, where the position of an object is defined by two coordinates, typically (x,y)(x, y). Examples include an object moving in a circular path or a projectile.

•

Uniform Circular Motion: When an object moves in a circular path with a constant speed, its motion is called Uniform Circular Motion. Although the speed is constant, the velocity is variable because the direction of motion changes continuously at every point.

•

Acceleration in Circular Motion: Since the velocity vector changes direction at every point along the circular path, circular motion is always an accelerated motion. The acceleration is directed towards the center of the circle.

•

Direction of Velocity: At any point on the circular path, the direction of the velocity vector is along the tangent to the circle at that point.

•

Centripetal Force: For an object to move in a circle, a force must act upon it towards the center of the path. This center-seeking force is called centripetal force.

📐Formulae

v=2πrtv = \frac{2\pi r}{t}

Distance (s)=Number of revolutions×2πr\text{Distance (s)} = \text{Number of revolutions} \times 2\pi r

Displacement (after half a revolution)=2r\text{Displacement (after half a revolution)} = 2r

Displacement (after one full revolution)=0\text{Displacement (after one full revolution)} = 0

ac=v2ra_c = \frac{v^2}{r}

💡Examples

Problem 1:

An artificial satellite is moving in a circular orbit of radius 42250 km42250\text{ km}. Calculate its speed if it takes 24 hours24\text{ hours} to revolve around the earth.

Solution:

Given radius r=42250 kmr = 42250\text{ km} and time t=24 hourst = 24\text{ hours}. Speed is calculated as: v=2πrtv = \frac{2\pi r}{t} v=2×3.14×4225024v = \frac{2 \times 3.14 \times 42250}{24} v=26533024≈11055.41 km/hv = \frac{265330}{24} \approx 11055.41\text{ km/h} To convert to km/s\text{km/s}: v=11055.413600≈3.07 km/sv = \frac{11055.41}{3600} \approx 3.07\text{ km/s}

Explanation:

Since the satellite moves in a circular path, we use the circumference of the orbit as the distance covered in one revolution.

Problem 2:

A cyclist goes around a circular track once every 2 minutes2\text{ minutes}. If the radius of the track is 105 m105\text{ m}, calculate his speed. (Take π=227\pi = \frac{22}{7})

Solution:

Radius r=105 mr = 105\text{ m}, Time t=2 min=120 st = 2\text{ min} = 120\text{ s}. Speed vv is: v=2πrtv = \frac{2\pi r}{t} v=2×227×105120v = \frac{2 \times \frac{22}{7} \times 105}{120} v=2×22×15120v = \frac{2 \times 22 \times 15}{120} v=660120=5.5 m/sv = \frac{660}{120} = 5.5\text{ m/s}

Explanation:

The speed is found by dividing the total distance of one lap (circumference) by the time taken to complete that lap.