scalars and vectors
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scalars and vectors:
This topic is covered in Physics as well as in maths (in calculus). I'm covering in Physics as most of it's application is in Physics.
Scalars: Things which don't have any sense of direction, are called scalars. As an example of scalar, mass of a thing, is a scalar quantity. It doesn't have any sense of direction, whether it's going north, south, etc, the mass is the same. Similarly for time, temp, volume, etc.
Vectors: Things which have direction along with a magnitude, are called vectors. As an example of vector, force is a vector quantity. It depends on which direction you apply force, so it has a sense of direction. If the force is applied in same direction as the movement of the body, then the body will accelerate, while if it's applied in opposite dir, then the body slows down.So, direction of force matters. Similarly for velocity, acceleration, etc.
Vector: A vector is rep by an arrow, whose length is the magnitude of the vector, and whose dir is the angle that that vector makes wrt x and y axis (for 2D vector), or wrt x, y and z axis (for 3D vector). Vectors can be moved anywhere in space and will remain same, as long as mag and dir remain same (i.e starting point and ending pt of vector can be anywhere).
Vector is rep by i, j, k for x,y and z dir or by coords <x,y,z>, (x,y,z) or in matrix form as x, y, z written vertically in 1st column. An arrow is put on top of the line segment to indicate it's vector. Magnitude or norm of vector is denoted by ||PQ||, while dirn is denoted by an angle (tan(θ)=slope). If ||v||=1, or v/||v|| it's called a unit vector. i, j, k are called std unit vectors in dir of x, y and z axis.
For simplicity, initial point of vector is usually chosen as origin, so only the end pt need to be specified in such a case. This rep of vector is called std position. or component form of vector. End point Coords <v1,v2> are called components of vector v.
Direction cosine: The 3 angles (α, β, γ) that a vector makes with x, y and z axis are called direction angles of v, and the 3 cosine values are called Direction cosine. For v=<v1,v2,v3>, v1=||v||*cos(α), v2=||v||*cos(β), v3=||v||*cos(γ). This yields cos2(α) + cos2(β) + cos2(γ) = 1
We'll study some operations (algebra) on vector quantities.
- Addition/subtraction: Addition/subtraction of vectors is done by placing end point of a vector to the start point of next vector and so on, until all vectors are connected. Then you connect the start point of 1st vector to end point of last vector, and that's the sum of all the vectors. For subtraction, you still treat it as addition, except that the direction of vector is reversed for the subtracting vectors. You still place them the same way as in addition, except that -ve vector is pointing in other dir.
- For vector <x1,y1> and <x2,y2>, the sum is <x1+x2,y1+y2>. The magnitude of vector is sq rt of [(x1+x2)^2 + (y1+y2)^2]. Mag may also be calc by using cosine thm, , if you know the mag of 2 vectors and angle b/w them.
- Many properties as Commutative, associative, identity, distributive properties hold true for vector and can be proved easily by using above formula. Any set of vectors that satisfy these 8 properties is called "vector space"
- Multiplication: There's no multiplication of vectors. We call it the product of vectors. Product of vectors may give a scalar or a vector depending on how we define it. There are 2 kinds of product. Link => https://math.libretexts.org/Bookshelves/Calculus/Supplemental_Modules_(Calculus)/Vector_Calculus/1%3A_Vector_Basics/1.5%3A_The_Dot_and_Cross_Product
- Dot product (scalar product): It's a scalar, and is determined by multiplying coefficients as shown in link above.
- Dot product is simply multiplying the length of one vector, with the length of the shadow (or projection) of the other vector on the 1st vector. So, u.v = ||u||.||v cos θ|| = ||u||.||v|| cos θ. It can also be calculated by using formula u.v=u1.v1 + u2.v2 + u3.v3.
- Cosine Thm: By using cosine thm, we may write ||u - v||^2 = ||u||^2 + ||v||^2 - 2 ||u||.||v||. cos θ => (u-v).(u-v) = ||u||^2 + ||v||^2 - 2 ||u||.||v||. cos θ . Cancelling terms, we get u.v = ||u||*||v|| * cos(θ). This is how we get this formula for u.v. This implies that dot product of orthogonal vectors is 0.
- Projection of vectors: Many times, we decompose a vector into orthogonal components, once pointing towards X axis and other towards Y axis, which is called the projection of vector on x and y axis. These are the i,j coeff of vector. Some times, we want to find component of a vector in the dir of another vector (not necessarily in dir of i,j), then we need to find projection onto that vector.
- Proj of u vector in dir of v vector is given by w1 = projv(u) = (u.v)/[||v||^2] * v. This is easy to prove, as dot product gives the product of projected vector component with the other vector. But then we need to divide by ||v|| since it includes that in dot product. So, (u.v)/||v|| gives the mag of such projection. We need to put dir in it (in dir of v vector), so we take unit vector in that dir, which is v/||v||. That gives above formula. ||u||cos(θ) is the component of u in dir of v. The component perpendicular to w1 is w2=u-w1.
- Angle θ b/w the 2 vectors is taken to be b/w 0 to π. So, cos θ value goes from +1 to -1. Since ||u|| and ||v|| are +ve, u.v and cos θ will have same sign.
- Dot product has many of the same properties as Commutative, distributive properties and can be proved easily by using above formula.
- Dot product is simply multiplying the length of one vector, with the length of the shadow (or projection) of the other vector on the 1st vector. So, u.v = ||u||.||v cos θ|| = ||u||.||v|| cos θ. It can also be calculated by using formula u.v=u1.v1 + u2.v2 + u3.v3.
- Cross product (vector product): It's a vector, and is determined by multiplying coefficients as shown in link above. It also has dir (as the product is a vector) which is orthogonal to both the vectors whose cross product is being determined.
- Cross product is found by getting determinant of the matrix with alternating +ve and -ve sign. It also has dir which is perpendicular to the plane of the 2 vectors u, v (i.e u X v is perpendicular to both u and v).
- Mag of u X v = ||u X v|| = |u|| * ||v|| * Sin(θ) (Note dot product was similar except that dot product had cos(θ)). This can be proved by using dot product.
- |u|| * ||v|| * Sin(θ) = |u|| * ||v|| * √ (1-Cos2(θ)) = |u|| * ||v|| * √ (1 - [u.v/(||u||*||v||)]^2) = √ (||u||^2*||v||^2 - [u1v1 + u2v2 + u3v3]^2) = √ [(u1^2 + u2^2 + u3^2)*(v1^2+v2^2+v3^2) - (u1v1 + u2v2 + u3v3)^2] which gives the reqd answer
- ||u X v|| gives area of parallelogram with sides u and v. Easy to see as ||v|| sin(θ) is the height of the parallelogram.
- u X v forma a right handed system. curl your fingers in dir from u to v, then the thumb points in dir of u X v. k = i X j.
- Mag of u X v = ||u X v|| = |u|| * ||v|| * Sin(θ) (Note dot product was similar except that dot product had cos(θ)). This can be proved by using dot product.
- Cross product has properties as distributive and few others.
- u X v = -(v X u).
- u X (v+w) = u X v + u X w.
- Triple scalar product: u.(v X w) = v.(w X u) = w.(u X v) => dot here reps the dot product
- Triple Scalar product gives the vol of parallelopiped (a polyhedron, whose all sides are parallelogram) only if u, v, w don't lie in same plane.
- 3 vectors u, v, w which have same starting pt are coplanar if and only if their triple scalar product is 0.
- u X v =0 if u and v are scalar multiples of each other (i.e u and v have same dir, i.e parallel to each other)
- Cross product is found by getting determinant of the matrix with alternating +ve and -ve sign. It also has dir which is perpendicular to the plane of the 2 vectors u, v (i.e u X v is perpendicular to both u and v).
- Scalar multiplication of vector v by a constant c, yields cv, i. mag of new vector is multiplied by c, with no change in dir (for c=-ve, dir is reversed). Parallel vectors are such that u=cv, where c is a scalar and u.v are 2 vectors.
- Dot product (scalar product): It's a scalar, and is determined by multiplying coefficients as shown in link above.
Matrix: Matrices are useful when evaluating dot and cross product of matrices. Matrices are studied in Maths and provided in Advanced Maths section
3D geometry as vectors:
3D figures in space can be expressed as vectors, which sometimes yields eaiser way to manipulate them.
Lines in space: A line in space can be rep as vector. Consider a line "L" which is parallel to vector v=<a,b,c>, and passes thru pt P (x1,y1,z1). Vector v is dir vector for line L, and a,b,c are dir numbers. We are looking for eqn of vector PQ, where Q is any pt <x,y,z>. Since PQ is parallel to v, we get PQ=t.v, where t is a scaling factor. Vec PQ = (x-x1)i + (y-y1)j + (z-z1)k = t(ai + bj = ck) => x-x1=ta, y-y1=tb, z-z1=tc. This is called parametric eqn where "t" is an independent parameter which can take any real value, x=x1+at, y=y1+bt, z=z1+ct. This parametric eqn can also be written as symmetric eqn by eliminating t => (x-x1)/a = (y-y1)/b = (z-z1)/c
NOTE: neither parametric eqn nor symmetric eqn are unique. By choosing some t, we can get other point on that line, and use that pt to get parametric and symmetric eqn which would be different.
Planes in space: A plane in space can be rep as an eqn by having a pt that lies on that plane, and having vector "n" which is perpendicular to that plane. Let's consider pt (x1,y1,z1) which lies on a plane and a normal vector n=<a,b,c>. Then plane consists of all points Q=(x,y,z) such that vec PQ is perpendicular to n. i.e PQ.n=0 => [(x-x1)i + (y-y1)j + (z-z1)k].<a.b.c>=0 => (x-x1)a + (y-y1)b +(z-z1)c = 0 => This is called the std form of plane. We can rearrange it to write general form of plane which is ax + by +cz + d =0. Given the general form of plane, it's easy to find a vector normal to plane as it's coeff a,b,c, so n=<a,b,c>
Find eqn of plane given 3 points in plane: First we form 2 vec from the 3 points, and find their cross product to get a vector normal to plane. Then we use above eqn to plug in a.b.c and then find d by using any of the points.
Find angle b/w 2 planes: The angle b/w 2 planes can be found by finding normal vector of the 2 planes, and then finding angle b/w the 2 normal vectors using dot product.
Finding line of intersecton of 2 planes: Given 2 planes, they may either be parallel or intersect in a line. A line will only involve 2 var (x,y or y,z or x,z), so solve the 2 simultaneous eqn for 2 planes to eliminate a var, and we get the eqn for line intersecting them. We can also write it as parametric eqn by introducig var "t". Other way to get line of intersection, is to get normal vec n1=<a1,b1,c1> and n2=<a2,b2,c2> and then find cross product n1 X n2, The vec obtained is perpendicular to both vec, and hence has to be a intersecting line, since it's common to both of the planes.
Trace of the plane: Plane in space intersects the coord plane (xy, yz, xz planes). The line of intersection is called the trace of the given plane in the coord plane. To find the xy trace, we set z=0, and so on for other planes. Any eqn of a straight line in 2 var is also a plane in 3D space, as it extends indefinitely in the plane of missing var (i.e 2x+y=5 is a line in 2D space, but a plane in 3D space).
- If 1 var is missing in eqn of plane, then it's parallel to the x axis. i.e 2x+y=5 is a plane parallel to the z axis.
- If 2 var are missing in eqn of plane, then it's parallel to the coord plane. ex: x=2 is a plane parallel to y-z coord plane
Distance b/w point and plane: To find the distance of a plane to a pt Q, we take a arbitrary pt P (choose x=0, y=0 to get a simple pt z) on plane, and form a vector PQ, then project this vector onto normal vector. So D = ||projn(PQ)|| = (PQ.n)/||n||. The choice of pt P on plane doesn't change the answer.
- For plane ax+by+cz+d=0, pt P on plane <x1,y1,z1> and pt Q = (x0,y0,z0), then D = PQ.n/||n|| = [(x0-x1)i + (y0-y1)j + (z0-z1)k].<a,b,c> / ||n|| = a(x0-x1) + b(y0-y1) + c(z0-z1) / √ (a^2+b^2+c^2) = | a*x0+b*y0+c*z0+d | / √ (a^2+b^2+c^2) ( we use the fact that a*x1+b*y1+c*z1+d=0, since pt (x1,y1,z1) lies on plane)
Distance b/w 2 parallel planes: To find the distance b/w 2 parallel planes, use a pt (x0,y0,z0) on 1 plane and then use the normal vector of other plane to find distance as shown above. So, formula is same as above
Distance b/w point and a line in space: To find the distance b/w pt and a line, we just do cross product instead of dot product. So, D= ||PQ X u|| / ||u||, where P is the pt o line, and Q is the other point, and u is direction vector of the line. Proof is simple PQ*Sin(θ) is the distance, which is what the cross product gives.
Spheres in space: The eqn for sphere is just an extension of circles in 2D, (x-x0)^2 + (y-y0)^2+ (z-z0)^2 = r^2
Cylinders in space: Cylinders are any general curve, which is extended by copying and placing that curve on top of each other indefinitely in the 3D space. Such a curve is called the generating curve, since the 3D figure is obtained by repeating this curve by parallel lines, which are called rulings.
Simplest ex of cylinder is a cylinder obtained by repeating a circle: x^2+y^2=r^2. This is a circle in 2D, but in3D it becomes a cyclinder since z can take on any value.
Other ex: z = y^2, Z=sin(x), etc
Quadric Surfaces: These are 3D analog of conic sections. It's 2nd degree eqn of form ax^2 + by^2 + cz^2 + Dxy + Exz + Fyz + Gx + Hy + Iz + J = 0
There are 6 types of Quadric surfaces:
- Ellipsoid: 3D extension of ellipse in 2D: x^2/a^2 + y^2/b^2 + z^2/c^2 = 1. The surface is a sphere if a=b=c≠0.
- Hyperboloid: Here all var are raised to 2nd power, and one or two of the var have -ve coeff.
- Hyperboloid of 1 sheet (only one coeff is -ve): x^2/a^2 + y^2/b^2 - z^2/c^2 = 1. It's hyperbola rotated around one of the axis. The axis of the Hyperboloid corresponds to the var whose coeff is -ve (in his case, that axis is "z" axis)
- Hyperboloid of 2 sheets (two coeff are -ve): x^2/a^2 - y^2/b^2 - z^2/c^2 = 1. It's hyperbola rotated and separated in 2 parts. The axis of the Hyperboloid corresponds to the var whose coeff is +ve (in his case, that axis is "x" axis)
- Elliptic cone: It's a cone shape opening in top and bottom. Eqn is x^2/a^2 + y^2/b^2 - z^2/c^2 = 0 (NOT 1 as in Ellipsoid). The axis of the cone corresponds to the var whose coeff is -ve (in his case, that axis is "z" axis)
- Paraboloid: One of the var is raised to only 1st power.
- Elliptic paraboloid: It's a parabola in shape of ellipse (similar to a cone pointing upwards, but with gradual edges, i.e parabola rotated around). Eqn is z= x^2/a^2 + y^2/b^2. The axis of the paraboloid corresponds to the var raised to 1st power (in his case, that axis is "z" axis)
- Hyperbolic paraboloid: It's a parabola in shape of hyperbola. Eqn is z= y^2/b^2 - x^2/a^2(NOTE the -ve sign). The axis of the paraboloid corresponds to the var raised to 1st power (in his case, that axis is "z" axis)
NOTE: some 2nd degree eqn in x,y,z don't represent any quadric surface. Ex: ^2=y^2+z^2 is a single point (0,0,0). x^2+y^2=1 is a right circular cylinder.
The general eqn ax^2 + by^2 + cz^2 + Dxy + Exz + Fyz + Gx + Hy + Iz + J = 0 shows a quadric surface not centered at origin.
Surface of revolution: This is 5th special kind of surface that employs calculus to find surface area, vol, etc.
- Consider eqn y=r(z) in yz plane.
- If we revolve this around z axis, we'll get a surface of revolution around z axis. The trace will be a circle, so x^2+y^2=[r(z)]^2.
- If we revolve this around y axis, we'll get a surface of revolution around z axis. The trace will be a circle, so x^2+z^2=[r(y)]^2.
- If we revolve this around x axis, we'll get a surface of revolution around z axis. The trace will be a circle, so y^2+z^2=[r(x)]^2.
NOTE: The generating curve for a surface of revolution is not unique.
Cylindrical and spherical coordinates:
Representing coords in (x,y,z) in rectangular coord is how we represent surfaces in space. However, sometimes, some other coord system are easier to represent some particular surfaces.