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Question 1016: [Kinematics > Projectile motion]

(a) Explain what is meant by a scalar quantity and by a vector quantity.

(b) A ball leaves point P at the top of a cliff with a horizontal velocity of 15 m s^–1, as shown in Fig. 2.1.

The height of the cliff is 25 m. The ball hits the ground at point Q.

Air resistance is negligible.

(i) Calculate the vertical velocity of the ball just before it makes impact with the ground at Q.

(ii) Show that the time taken for the ball to fall to the ground is 2.3 s.

(iii) Calculate the magnitude of the displacement of the ball at point Q from point P.

(iv) Explain why the distance travelled by the ball is different from the magnitude of the displacement of the ball.

Reference: Past Exam Paper – June 2014 Paper 23 Q2

Solution 1016:

(a)

A scalar quantity has a magnitude only.

A vector quantity has both a magnitude and a direction.

(b)

(i)

{initial vertical velocity is zero. Using v² = u² + 2as,}

v² = 0 + 2(9.81)(25)    (or using ½mv² = mgh {considering vertical motion only})

Vertical speed at Q, v = 22(.1) ms^-1

{Note that this is the vertical component of the velocity, not the resultant velocity since the ball also has a horizontal component.}

(ii)

{Using v = u + at. For the OR case, s = ut + ½at² where u = 0}

22.1 = 0 + 9.81t          (or 25 = ½ × 9.81 × t²)

Time taken, t (= 22.1 / 9.81) = 2.26s               or t [= √(5.097)] = 2.26s

(iii)

Horizontal distance travelled = 15 × t = 15 × 2.257 = 33.86 (allow 15×2.3 = 34.5)

{From Pythagoras’ theorem,}

(displacement)² = (horizontal distance)² + (vertical distance)²

(displacement)² = 25² + 33.86²

Magnitude of displacement = 42m (42.08)

(iv) The distance travelled is the actual (curved) path followed by the ball while the displacement is the straight line / minimum distance from P to Q

Question 1017: [Work, Energy and Power + Momentum]

A conveyor belt is driven at velocity v by a motor. Sand drops vertically on to the belt at a rate of m kg s^–1.

What is the additional power needed to keep the conveyor belt moving at a steady speed when the sand starts to fall on it?

A ½ mv                                   B mv                           C ½ mv²                                 D mv²

Reference: Past Exam Paper – June 2015 Paper 11 Q19

Solution 1017:

Go to

A conveyor belt is driven at velocity v by a motor. Sand drops vertically on to the belt at a rate of m kg s-1.

Question 1018: [Dynamics > Moments > Equilibrium]

A rod PQ is attached at P to a vertical wall, as shown in Fig. 3.1.

The length of the rod is 1.60 m. The weight W of the rod acts 0.64 m from P. The rod is kept horizontal and in equilibrium by a wire attached to Q and to the wall at R. The wire provides a force F on the rod of 44 N at 30° to the horizontal.

(a) Determine

(i) the vertical component of F,

(ii) the horizontal component of F.

(b) By taking moments about P, determine the weight W of the rod.

(c) Explain why the wall must exert a force on the rod at P.

(d) On Fig. 3.1, draw an arrow to represent the force acting on the rod at P. Label your arrow with the letter S.

Reference: Past Exam Paper – June 2015 Paper 22 Q3

Solution 1018:

Go to
A rod PQ is attached at P to a vertical wall, as shown in Fig. 3.1. The length of the rod is 1.60 m. The weight W of the rod acts 0.64 m from P.

Question 1019: [Kinematics > Linear motion]

The variation with time t of the velocity v of a ball is shown in Fig. 2.1.

The ball moves in a straight line from a point P at t = 0. The mass of the ball is 400 g.

(a) Use Fig. 2.1 to describe, without calculation, the velocity of the ball from t = 0 to t = 16 s.

(b) Use Fig. 2.1 to calculate, for the ball,

(i) the displacement from P at t = 10 s,

(ii) the acceleration at t = 10 s,

(iii) the maximum kinetic energy.

(c) Use your answers in (b)(i) and (b)(ii) to determine the time from t = 0 for the ball to return to P.

Reference: Past Exam Paper – June 2015 Paper 23 Q2

Solution 1019:

(a) There is a constant rate of increase in velocity/acceleration from t = 0 to t = 8 s. There is a constant deceleration from t = 8 s to t = 16 s    OR constant rate of increase in velocity in the opposite direction from t = 10 s to t = 16 s

(b)

(i)

Displacement = area under lines to 10 s

Displacement = {(5.0 × 8.0) / 2} + {(5.0 × 2.0) / 2} = 25 m

OR Displacement = ½ (10.0 × 5.0) = 25 m

(ii)

Acceleration a = (v – u) / t      OR gradient of line

{The velocity changes from +5 to -15 in a time of (16 – 8) seconds.}

Acceleration a = (–15.0 –5.0) / 8.0

Acceleration a = (–) 2.5 m s^–2

(iii)

{The maximum KE depends on the velocity where the magnitude is highest.}

KE = ½ mv²   

KE = 0.5 × 0.4 × (15.0)² = 45 J

(c)

{Equation for uniformly accelerated motion: s = ut + ½ at².

The distance s moved from time t = 0 until the ball starts moving towards P again is 25m. So, the ball should again travel a distance of 25m to return to P.

We now need to consider the motion during the deceleration. Initial velocity u = 0. Deceleration a = 2.5ms^-2. t is the time from the instant it starts moving towards P, until it reaches P.}

distance = 25 = ut + ½ at² = 0 + ½ × 2.5 × t²            

Time t = 4.5 (4.47) s,   therefore time to return = {10 + 4.5 =} 14.5 s

{The backwards motion starts at t = 10s, so we need to include these 10s too.}