Motion in a Plane Class 11 Notes Physics Chapter 4
• Motion in a plane is called as motion in two dimensions e.g., projectile motion, circular motion etc. For
the analysis of such motion our reference will be made of an origin and two co-ordinate axes X and Y.
• Scalar and Vector Quantities
Scalar Quantities. The physical quantities which are completely specified by their magnitude or size alone
are called scalar quantities.
Examples. Length, mass, density, speed, work, etc.
Vector Quantities. Vector quantities are those physical quantities which are characterised by both
magnitude and direction.
Examples. Velocity, displacement, acceleration, force, momentum, torque etc.
• Characteristics of Vectors
Following are the characteristics of vectors:
(i) These possess both magnitude and direction.
(ii) These do not obey the ordinary laws of Algebra.
(iii) These change if either magnitude or direction or both change.
(iv) These are represented by bold-faced letters or letters having arrow over them.
• Unit Vector
A unit vector is a vector of unit magnitude and points in a particular direction. It is used to specify the
direction only. Unit vector is represented by putting a cap (^) over the quantity.
• Equal Vectors
• Zero Vector
• Negative of a Vector
• Parallel Vectors
• Coplanar Vectors
Vectors are said to be coplanar if they lie in the same plane or they are parallel to the same plane,
otherwise they are said to be non-coplanar vectors.
• Displacement Vector
The displacement vector is a vector which gives the position of a point with reference to a point other than
, the origin of the co-ordinate system.
• Parallelogram Law of Vector Addition
If two vectors, acting simultaneously at a point, can be represented both in magnitude and direction by the
two adjacent sides of a parallelogram drawn from a point, then the resultant is represented completely
both in magnitude and direction by the diagonal of the parallelogram passing through that point.
• Triangle Law of Vector Addition
If two vectors are represented both in magnitude and direction by the two sides of a triangle taken in the
same order, then the resultant of these vectors is represented both in magnitude and direction by the third
side of the triangle taken in the opposite order.
• Motion in a plane is called as motion in two dimensions e.g., projectile motion, circular motion etc. For
the analysis of such motion our reference will be made of an origin and two co-ordinate axes X and Y.
• Scalar and Vector Quantities
Scalar Quantities. The physical quantities which are completely specified by their magnitude or size alone
are called scalar quantities.
Examples. Length, mass, density, speed, work, etc.
Vector Quantities. Vector quantities are those physical quantities which are characterised by both
magnitude and direction.
Examples. Velocity, displacement, acceleration, force, momentum, torque etc.
• Characteristics of Vectors
Following are the characteristics of vectors:
(i) These possess both magnitude and direction.
(ii) These do not obey the ordinary laws of Algebra.
(iii) These change if either magnitude or direction or both change.
(iv) These are represented by bold-faced letters or letters having arrow over them.
• Unit Vector
A unit vector is a vector of unit magnitude and points in a particular direction. It is used to specify the
direction only. Unit vector is represented by putting a cap (^) over the quantity.
• Equal Vectors
• Zero Vector
• Negative of a Vector
• Parallel Vectors
• Coplanar Vectors
Vectors are said to be coplanar if they lie in the same plane or they are parallel to the same plane,
otherwise they are said to be non-coplanar vectors.
• Displacement Vector
The displacement vector is a vector which gives the position of a point with reference to a point other than
, the origin of the co-ordinate system.
• Parallelogram Law of Vector Addition
If two vectors, acting simultaneously at a point, can be represented both in magnitude and direction by the
two adjacent sides of a parallelogram drawn from a point, then the resultant is represented completely
both in magnitude and direction by the diagonal of the parallelogram passing through that point.
• Triangle Law of Vector Addition
If two vectors are represented both in magnitude and direction by the two sides of a triangle taken in the
same order, then the resultant of these vectors is represented both in magnitude and direction by the third
side of the triangle taken in the opposite order.
