# Problems in General Physics → Physical Fundamentals of Mechanics → Kinematics

 1.1. A motorboat going downstream overcame a raft at a point A; τ = 60 min later it turned back and after some time passed the raft at a distance l = 6.0 km from the point A. Find the flow velocity assuming the duty of the engine to be constant. Free solution >> 1.2. A point traversed half the distance with a velocity v0. The remaining part of the distance was covered with velocity v1 for half the time, and with velocity v2 for the other half of the time. Find the mean velocity of the point averaged over the whole time of motion. Free solution >> 1.3. A car starts moving rectilinearly, first with acceleration w = 5.0 m/s2 (the initial velocity is equal to zero), then uniformly, and finally, decelerating at the same rate w, comes to a stop. The total time of motion equals τ = 25 s. The average velocity during that time is equal to = 72 km per hour. How long does the car move uniformly? Free solution >> 1.4. A point moves rectilinearly in one direction. Fig. 1.1 shows the distance s traversed by the point as a function of the time t. Using the plot find: (a) the average velocity of the point during the time of motion; (b) the maximum velocity; (c) the time moment t0 at which the instantaneous velocity is equal to the mean velocity averaged over the first t0 seconds. Free solution >> 1.5. Two particles, 1 and 2, move with constant velocities v1 and v2. At the initial moment their radius vectors are equal to r1 and r2. How must these four vectors be interrelated for the particles to collide? Free solution >> 1.6. A ship moves along the equator to the east with velocity v0 = 30 km/hour. The southeastern wind blows at an angle φ = 60° to the equator with velocity v = 15 km/hour. Find the wind velocity v' relative to the ship and the angle φ' between the equator and the wind direction in the reference frame fixed to the ship. Free solution >> 1.7. Two swimmers leave point A on one bank of the river to reach point B lying right across on the other bank. One of them crosses the river along the straight line AB while the other swims at right angles to the stream and then walks the distance that he has been carried away by the stream to get to point B. What was the velocity u of his walking if both swimmers reached the destination simultaneously? The stream velocity v0 = 2.0 km/hour and the velocity v' of each swimmer with respect to water equals 2.5 km per hour. Free solution >> 1.8. Two boats, A and B, move away from a buoy anchored at the middle of a river along the mutually perpendicular straight lines: the boat A along the river, and the boat B across the river. Having moved off an equal distance from the buoy the boats returned. Find the ratio of times of motion of boats τA/τB if the velocity of each boat with respect to water is η = 1.2 times greater than the stream velocity. Free solution >> 1.9. A boat moves relative to water with a velocity which is n = 2.0 times less than the river flow velocity. At what angle to the stream direction must the boat move to minimize drifting? Free solution >> 1.10. Two bodies were thrown simultaneously from the same point: one, straight up, and the other, at an angle of θ = 60° to the horizontal. The initial velocity of each body is equal to v0 = 25 m/s. Neglecting the air drag, find the distance between the bodies t = 1.70 s later. Free solution >> 1.11. Two particles move in a uniform gravitational field with an acceleration g. At the initial moment the particles were located at one point and moved with velocities v1 = 3.0 m/s and v2 = 4.0 m/s horizontally in opposite directions. Find the distance between the particles at the moment when their velocity vectors become mutually perpendicular. Free solution >> 1.12. Three points are located at the vertices of an equilateral triangle whose side equals a. They all start moving simultaneously with velocity v constant in modulus, with the first point heading continually for the second, the second for the third, and the third for the first. How soon will the points converge? Free solution >> 1.14. A train of length l = 350 m starts moving rectilinearly with constant acceleration w = 3.0*10-2 m/s2; t = 30 s after the start the locomotive headlight is switched on (event 1), and τ = 60 s after that event the tail signal light is switched on (event 2). Find the distance between these events in the reference frames fixed to the train and to the Earth. How and at what constant velocity V relative to the Earth must a certain reference frame K move for the two events to occur in it at the same point? Free solution >> 1.15. An elevator car whose floor-to-ceiling distance is equal to 2.7 m starts ascending with constant acceleration 1.2 m/s2; 2.0 s after the start a bolt begins falling from the ceiling of the car. Find: (a) the bolt's free fall time; (b) the displacement and the distance covered by the bolt during the free fall in the reference frame fixed to the elevator shaft. Free solution >> 1.16. Two particles, 1 and 2, move with constant velocities v1 and v2 along two mutually perpendicular straight lines toward the intersection point O. At the moment t = 0 the particles were located at the distances l1 and l2 from the point O. How soon will the distance between the particles become the smallest? What is it equal to? Free solution >> 1.17. From point A located on a highway (Fig. 1.2) one has to get by car as soon as possible to point B located in the field at a distance l from the highway. It is known that the car moves in the field η times slower than on the highway. At what distance from point D one must turn off the highway? Free solution >> 1.19. A point traversed half a circle of radius R = 160 cm during time interval τ = 10.0 s. Calculate the following quantities averaged over that time: (a) the mean velocity ; (b) the modulus of the mean velocity vector ||; (c) the modulus of the mean vector of the total acceleration || if the point moved with constant tangent acceleration. Free solution >> 1.20. A radius vector of a particle varies with time t as r = at (1 - αt), where a is a constant vector and α is a positive factor. Find: (a) the velocity v and the acceleration w of the particle as functions of time; (b) the time interval Δt taken by the particle to return to the initial points, and the distance s covered during that time. Free solution >> 1.21. At the moment t = 0 a particle leaves the origin and moves in the positive direction of the x axis. Its velocity varies with time as v = v0 (1 - t/τ), where v0 is the initial velocity vector whose modulus equals v0 = 10.0 cm/s; τ = 5.0 s. Find: (a) the x coordinate of the particle at the moments of time 6.0, 10, and 20 s; (b) the moments of time when the particle is at the distance 10.0 cm from the origin; (c) the distance s covered by the particle during the first 4.0 and 8.0 s; draw the approximate plot s(t). Free solution >> 1.22. The velocity of a particle moving in the positive direction of the x axis varies as v = α sqrt(x), where α is a positive constant. Assuming that at the moment t = 0 the particle was located at the point x = 0, find: (a) the time dependence of the velocity and the acceleration of the particle; (b) the mean velocity of the particle averaged over the time that the particle takes to cover the first s metres of the path. Free solution >> 1.23. A point moves rectilinearly with deceleration whose modulus depends on the velocity v of the particle as w = a sqrt(v), where a is a positive constant. At the initial moment the velocity of the point is equal to v0. What distance will it traverse before it stops? What time will it take to cover that distance? Free solution >> 1.24. A radius vector of a point A relative to the origin varies with time t as r = ati - bt2j, where a and b are positive constants, and i and j are the unit vectors of the x and y axes. Find: (a) the equation of the point's trajectory y(x); plot this function; (b) the time dependence of the velocity v and acceleration w vectors, as well as of the moduli of these quantities; (c) the time dependence of the angle α between the vectors w and v; (d) the mean velocity vector averaged over the first t seconds of motion, and the modulus of this vector. Free solution >> 1.25. A point moves in the plane xy according to the law x = at, y = at (1 - αt), where a and α are positive constants, and t is time. Find: (a) the equation of the point's trajectory y(x); plot this function; (b) the velocity v and the acceleration w of the point as functions of time; (c) the moment t0 at which the velocity vector forms an angle π/4 with the acceleration vector. Free solution >> 1.26. A point moves in the plane xy according to the law x = a sin ωt, y = a(1 - cos ωt), where a and ω are positive constants. Find: (a) the distance s traversed by the point during the time τ; (b) the angle between the point's velocity and acceleration vectors. Free solution >> 1.27. A particle moves in the plane xy with constant acceleration w directed along the negative y axis. The equation of motion of the particle has the form y = ax - bx2, where a and b are positive constants. Find the velocity of the particle at the origin of coordinates. Free solution >> 1.29. A body is thrown from the surface of the Earth at an angle α to the horizontal with the initial velocity v0. Assuming the air drag to be negligible, find: (a) the time of motion; (b) the maximum height of ascent and the horizontal range; at what value of the angle α they will be equal to each other; (c) the equation of trajectory y(x), where y and x are displacements of the body along the vertical and the horizontal respectively; (d) the curvature radii of trajectory at its initial point and at its peak. Free solution >> 1.31. A ball starts falling with zero initial velocity on a smooth inclined plane forming an angle α with the horizontal. Having fallen the distance h, the ball rebounds elastically off the inclined plane. At what distance from the impact point will the ball rebound for the second time? Free solution >> 1.32. A cannon and a target are 5.10 km apart and located at the same level. How soon will the shell launched with the initial velocity 240 m/s reach the target in the absence of air drag? Free solution >> 1.33. A cannon fires successively two shells with velocity v0 = 250 m/s; the first at the angle θ1 = 60° and the second at the angle θ2 = 45° to the horizontal, the azimuth being the same. Neglecting the air drag, find the time interval between firings leading to the collision of the shells. Free solution >> 1.34. A balloon starts rising from the surface of the Earth. The ascension rate is constant and equal to v0. Due to the wind the balloon gathers the horizontal velocity component vx = ay, where a is a constant and y is the height of ascent. Find how the following quantities depend on the height of ascent: (a) the horizontal drift of the balloon x(y); (b) the total, tangential, and normal accelerations of the balloon. Free solution >> 1.35. A particle moves in the plane xy with velocity v = ai + bxj, where i and j are the unit vectors of the x and y axes, and a and b are constants. At the initial moment of time the particle was located at the point x = y = 0. Find: (a) the equation of the particle's trajectory y(x); (b) the curvature radius of trajectory as a function of x. Free solution >> 1.36. A particle A moves in one direction along a given trajectory with a tangential acceleration wτ = aτ, where a is a constant vector coinciding in direction with the x axis (Fig. 1.4), and τ is a unit vector coinciding in direction with the velocity vector at a given point. Find how the velocity of the particle depends on x provided that its velocity is negligible at the point x = 0. Free solution >> 1.37. A point moves along a circle with a velocity v = at, where a = 0.50 m/s2. Find the total acceleration of the point at the moment when it covered the n-th (n = 0.10) fraction of the circle after the beginning of motion. Free solution >> 1.38. A point moves with deceleration along the circle of radius R so that at any moment of time its tangential and normal accelerations are equal in moduli. At the initial moment t = 0 the velocity of the point equals v0. Find: (a) the velocity of the point as a function of time and as a function of the distance covered s; (b) the total acceleration of the point as a function of velocity and the distance covered. Free solution >> 1.39. A point moves along an arc of a circle of radius R. Its velocity depends on the distance covered s as v = a sqrt(s), where a is a constant. Find the angle α between the vector of the total acceleration and the vector of velocity as a function of s. Free solution >> 1.40. A particle moves along an arc of a circle of radius R according to the law l = a sin ωt, where l is the displacement from the initial position measured along the arc, and a and ω are constants. Assuming R = 1.00 m, a = 0.80 m, and ω = 2.00 rad/s, find: (a) the magnitude of the total acceleration of the particle at the points l = 0 and l = ±a; (b) the minimum value of the total acceleration wmin and the corresponding displacement lm. Free solution >> 1.41. A point moves in the plane so that its tangential acceleration wτ = a, and its normal acceleration wn = bt4, where a and b are positive constants, and t is time. At the moment t = 0 the point was at rest. Find how the curvature radius R of the point's trajectory and the total acceleration w depend on the distance covered s. Free solution >> 1.42. A particle moves along the plane trajectory y(x) with velocity v whose modulus is constant. Find the acceleration of the particle at the point x = 0 and the curvature radius of the trajectory at that point if the trajectory has the form (a) of a parabola y = ax2; (b) of an ellipse (x/a)2 + (y/b)2 = 1; a and b are constants here. Free solution >> 1.43. A particle A moves along a circle of radius R = 50 cm so that its radius vector r relative to the point O (Fig. 1.5) rotates with the constant angular velocity ω = 0.40 rad/s. Find the modulus of the velocity of the particle, and the modulus and direction of its total acceleration. Free solution >> 1.44. A wheel rotates around a stationary axis so that the rotation angle φ varies with time as φ = at2, where a = 0.20 rad/s2. Find the total acceleration w of the point A at the rim at the moment t = 2.5 s if the linear velocity of the point A at this moment v = 0.65 m/s. Free solution >> 1.45. A shell acquires the initial velocity v = 320 m/s, having made n = 2.0 turns inside the barrel whose length is equal to l = 2.0 m. Assuming that the shell moves inside the barrel with a uniform acceleration, find the angular velocity of its axial rotation at the moment when the shell escapes the barrel. Free solution >> 1.46. A solid body rotates about a stationary axis according to the law φ = at - bt3, where a = 6.0 rad/s and b = 2.0 rad/s3. Find: (a) the mean values of the angular velocity and angular acceleration averaged over the time interval between t = 0 and the complete stop; (b) the angular acceleration at the moment when the body stops. Free solution >> 1.47. A solid body starts rotating about a stationary axis with an angular acceleration β = at, where a = 2.0*10-2 rad/s3. How soon after the beginning of rotation will the total acceleration vector of an arbitrary point of the body form an angle α = 60° with its velocity vector? Free solution >> 1.48. A solid body rotates with deceleration about a stationary axis with an angular deceleration β ∼ sqrt(ω) where ω is its angular velocity. Find the mean angular velocity of the body averaged over the whole time of rotation if at the initial moment of time its angular velocity was equal to ω0. Free solution >> 1.49. A solid body rotates about a stationary axis so that its angular velocity depends on the rotation angle φ as ω = ω0 - aφ, where ω0 and a are positive constants. At the moment t = 0 the angle φ = 0. Find the time dependence of (a) the rotation angle; (b) the angular velocity. Free solution >> 1.50. A solid body starts rotating about a stationary axis with an angular acceleration β = β0 cos φ, where β0 is a constant vector and φ is an angle of rotation from the initial position. Find the angular velocity of the body as a function of the angle φ. Draw the plot of this dependence. Free solution >> 1.51. A rotating disc (Fig. 1.6) moves in the positive direction of the x axis. Find the equation y(x) describing the position of the instantaneous axis of rotation, if at the initial moment the axis C of the disc was located at the point O after which it moved (a) with a constant velocity v, while the disc started rotating counterclockwise with a constant angular acceleration β (the initial angular velocity is equal to zero); (b) with a constant acceleration w (and the zero initial velocity), while the disc rotates counterclockwise with a constant angular velocity ω. Free solution >> 1.52. A point A is located on the rim of a wheel of radius R = 0.50 m which rolls without slipping along a horizontal surface with velocity v = 1.00 m/s. Find: (a) the modulus and the direction of the acceleration vector of the point A; (b) the total distance s traversed by the point A between the two successive moments at which it touches the surface. Free solution >> 1.53. A ball of radius R = 10.0 cm rolls without slipping down an inclined plane so that its center moves with constant acceleration w = 2.50 cm/s2; t = 2.00 s after the beginning of motion its position corresponds to that shown in Fig. 1.7. Find: (a) the velocities of the points A, B, and O; (b) the accelerations of these points. Free solution >> 1.54. A cylinder rolls without slipping over a horizontal plane. The radius of the cylinder is equal to r. Find the curvature radii of trajectories traced out by the points A and B (see Fig; 1.7). Free solution >> 1.55. Two solid bodies rotate about stationary mutually perpendicular intersecting axes with constant angular velocities ω1 = 3.0 rad/s and ω2 = 4.0 rad/s. Find the angular velocity and angular acceleration of one body relative to the other. Free solution >> 1.56. A solid body rotates with angular velocity ω = ati + bt2j, where a = 0.50 rad/s2, b = 0.060 rad/s3, and i and j are the unit vectors of the x and y axes. Find: (a) the moduli of the angular velocity and the angular acceleration at the moment t = 10.0 s; (b) the angle between the vectors of the angular velocity and the angular acceleration at that moment. Free solution >> 1.57. A round cone with half-angle α = 30° and the radius of the base R = 5.0 cm rolls uniformly and without slipping over a horizontal plane as shown in Fig. 1.8. The cone apex is hinged at the point O which is on the same level with the point C, the cone base centre. The velocity of point C is v = 10.0 cm/s. Find the moduli of (a) the vector of the angular velocity of the cone and the angle it forms with the vertical; (b) the vector of the angular acceleration of the cone. Free solution >> 1.58. A solid body rotates with a constant angular velocity ω0 = 0.50 rad/s about a horizontal axis AB. At the moment t = 0 the axis AB starts turning about the vertical with a constant angular acceleration β0 = 0.10 rad/s2. Find the angular velocity and angular acceleration of the body after t = 3.5 s. Free solution >>