WJEC Physics for AS Level: Revision Workbook

Answers Practice questions: Unit 1: Motion, Energy and Matter Section 1: Basic physics Q1 metre (m) kilogram (kg) second (s) mole (mol) ampère (A) kelvin (K) Q2 (a) newton (N) (b) N = kg m s −2 (c) Work = Force × distance moved in direction of force So J = N m = (kg m s −2 ) m = kg m 2 s −2 Q3 (a) k = v , so [ k ] = m s N m s kg m s −1 −2 −1 = = kg s −1 (b) K = Av F 2 , so [ K ] = A v F m m s kg m s m s kg m s 2 2 1 2 −2 4 2 2 − − − = = ^ h 6 6 6 @ @ @ = kg m −3 This is the unit of density, so K might represent the density of the fluid through which the object is moving, perhaps with a numerical (unit-less) multiplier. Q4 [ ] [ ] [ ] r A m m 2 2 2 Ɏ = = = 1, that is, no units. Alternatively, we could show it using the equation: circumference = 2 Ɏ” Q5 Scalars: energy, time, density, temperature, pressure Vectors: acceleration, velocity, momentum Note : pressure is a scalar because only the magnitude of a force is involved in its definition: Pressure = area of a surface magnitude of force normal to a surface Q6 [ v ] = m s −1 ; [ u ] = m s −1 ; [ at ] = m s −2 × s = m s −1 So the two terms on the right-hand side have the same units, so can be added together and the unit of the left-hand side is the same as the unit of the right-hand side. Q7 (a) 28 N 45 N resultant magnitude 73 N Direction = that of the 45 N force (b) 45 N 28 N Resultant magnitude 17 N Direction = that of the 45 N force (c) 45 N 28 N Resultant magnitude = 45 28 2 2 + = 53 N Direction = 45N tan –1 28 N c m = 32° to 45 N force Q8 (a) F sin 25°= 53 so F = sin 25 53 c = 125 N or F cos (90°− 25 ) = 53 leading to F = 125 N (b) Horizontal component = F cos 25° = 125 N × cos 25° = 114 N 53 N 184 | WJEC Physics for AS Level: Revision Workbook | Answers