WAEC Physics 2010 Theory Past Questions

Model answer

The stone is projected vertically upwards, so its initial speed is entirely vertical: uy=30m/s. Height of tower, H1=50m. Maximum height reached from the point of release, H2 = uy²/(2g) = 30²/(2x10) = 900/20 = 45m. Maximum height reached from the ground = H1+H2 = (50+45)m = 95m.

Model answer

Given: Force (F)=40N, extension (e)=2.4x10⁻⁴m, original length=3m, diameter (d)=2.0mm=2x10⁻³m. (a) Area = πd²/4 = π(2x10⁻³)²/4. Stress on the wire = Force/Area of wire = 40/(3.14x(2x10⁻³)²/4) = 40/(3.14x4x10⁻⁶/4) = 1.27x10⁷ N/m². (b) Strain in the wire = Extension/original length = 2.4x10⁻⁴/3 = 8x10⁻⁵.

Model answer

When a lead-acid accumulator is fully charged, the gases given off are: (i) Hydrogen (at the anode, which is lead peroxide) (ii) Oxygen (at the cathode, which is lead plate). Factors that affect the mass of elements deposited during electrolysis: (i) quantity of electricity, (ii) nature/e.c.e of electrochemical equipment.

Model answer

Non-electrolytes include: water, kerosene, petrol, ethanol (any two).

Model answer

Factors that affect the angle of deviation of a ray of light through a triangular glass prism are: (i) angle of incidence, (ii) refracting angle (A) of the prism, (iii) refractive index of the material of the glass prism (any two).

Model answer

The number of images formed when two plane mirrors are inclined at angle θ is given by: N = 360/θ - 1. Given N=7: 7+1 = 360/θ => θ = 360/8 = 45°.

Model answer

Speed of waves generated = v = 0.8/0.2 = 4 m/s. Using v=fλ, λ=v/f, so for each frequency f, wavelength λ=v/f and λ⁻¹=f/v. F(Hz): 2.0, 4.0, 6.0, 8.0, 10.0. λ(m): 2.0, 1.0, 0.667, 0.5, 0.4. λ⁻¹(m⁻¹): 0.5, 1.0, 1.5, 2.0, 2.5. The slope of the graph of f against λ⁻¹ represents the speed/velocity of the waves.

Model answer

When a positively charged conductor is placed near a candle flame, the flame spreads out because: (i) the candle flame ionizes the air around it; (ii) the pathway charged conductor attracts the negative charges in the air and repels the positive charges. This makes the flame spread out.

Model answer

Velocity of proton = 5.0x10⁵ m/s; magnetic flux density, B=0.2T; angle to field, θ=30°. F=qvBsinθ = 1.6x10⁻¹⁹ x 5.0x10⁵ x 0.2 x sin30° = 8x10⁻¹⁵N.

Model answer

Conclusions from Rutherford's model: (i) The electrons are revolving around the nucleus at high speed. (ii) The volume of the nucleus is negligibly small compared to that of the volume of the atom. (iii) Most of the space in an atom is empty. (iv) The number of electrons in an atom is equal to the number of protons, hence it is electrically neutral. (v) The atom of an element consists of a small, positively charged nucleus in the centre, which centres almost the entire mass of the atom (any three).

Model answer

Energy level of n2=-2.0eV; Energy level of n0=-12.0eV. Energy change as it moves from n2 to n0 = E20 = E2-E0 = -2-(-12) = 10eV. (i) E=hf. f=E/h = (10x1.602x10⁻¹⁹)/(6.63x10⁻³⁴) = 2.42x10¹⁵ Hz. (ii) c=λf; λ=c/f = (3x10⁸)/(2.42x10¹⁵) = 1.24x10⁻⁷ m.

Model answer

²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α represents a natural radioactivity decay (alpha decay occurring spontaneously). ¹⁴₇N + ⁴₂α → ¹⁷₈O + ¹₁P represents an artificial radioactivity decay (induced by bombarding nitrogen with an alpha particle). Difference between Natural and Artificial Radioactivity: (1) Natural radioactivity is a spontaneous disintegration of atomic nuclei of an element, while artificial radioactivity is achieved by bombarding a non-radioactive element with a particle e.g. neutron, proton, alpha particle etc. (2) In natural radioactivity, the alpha particle is a by-product, while it is used as a bombarding particle in artificial radioactivity. (3) Heavy nuclei are involved in natural radioactivity, but in artificial radioactivity, light atoms are also involved.

Model answer

Perfectly Elastic Collision: (i) Directions of the bodies are reversed (in a head-on collision). (ii) Kinetic energy is conserved. (iii) Colliding bodies do not stick together after collision. Perfectly Inelastic Collision: (i) Directions of bodies are not necessarily reversed. (ii) Kinetic energy is not conserved (it decreases). (iii) Colliding bodies stick together and move as a unit with the same velocity after collision.

Model answer

(i) For uniform speed, the distance-time graph is a straight line with a constant (positive) slope. (ii) For variable (non-uniform) speed, the distance-time graph is a curve, with the slope changing continuously over time.

Model answer

For the first motion: distance covered=120m, time interval=12s, initial velocity=0. Using s=ut+½at²: 120=½xax12²; a=(120x2)/144=1.67ms⁻². Using v=u+at: v=(1.67x12)=20m/s. For the second motion, the final velocity of the first motion becomes the initial velocity of the second (20m/s); distance covered=300m, time interval=12s. Using s=ut+½at²: 300=20(12)+½xax12²; 300-240=½xax144; a=(60x2)/144=0.83ms⁻². v=u+at=20+(0.83x12)=20+10=30m/s. Also, for the third motion, initial velocity=final velocity of the second motion=30m/s; distance covered=180cm... [as given, 180m], time interval=12s. s=ut+½at²: 180-30(12)=½a(144); 180-360=½a(144); a=(-180x2)/144=-2.5ms⁻² (deceleration). v=u+at=30+(-2.5x12)=30-30=0ms⁻¹, meaning the body decreases to rest at the end of the third interval.

Model answer

(i) Inertia: the tendency of a body or object to continue in its state of rest or uniform motion unless acted upon by an external force. (ii) Inertia mass: represents resistance to any type of force whatever. The more the mass of a body, the more the force that is required to give it acceleration, in the formula f=ma, where m is the inertia mass of the body.

Model answer

Rate of evaporation depends on: (i) temperature, (ii) pressure, (iii) surface area, (iv) the nature of liquid, (v) wind, (vi) humidity (any three).

Model answer

When two ice cubes are pressed together for some time, they stick together after releasing them, showing that the melting point of ice is lowered by an increase in pressure. When the ice cubes are pressed together, the parts of ice with high pressure melt. When the pressure is released, the water refreezes and therefore joins the two ice cubes together (regelation).

Model answer

Density of A, PA=9.2x10²kg/m³; density of B, PB=1.3x10³kg/m³; height of A, HA=26cm; height of B, hB=? Since pressure=ρhg, and pressure at same level is equal, then: ρAhAg=ρBhBg; hB=(PA x hA)/PB = (9.2x10² x 26)/(1.3x10³) = 18.4 cm.

Model answer

Factors that affect the angle of direction of a ray of light through a triangular glass prism are: (i) angle of incidence, (ii) refracting angle (A) of the prism, (iii) refractive index of the material of the glass prism.

Model answer

The number of images formed when two plane mirrors are inclined at an angle θ is given by N = 360/θ - 1. Given N=7: 7+1=360/θ; θ=360/8=45°.

Model answer

Speed, v = distance/time = 0.8/0.2 = 4 m/s.