Convert units: nm, mm and cm to m, and check whether angles are measured from the normal.
Check signs and plausibility: a virtual image has dᵢ < 0, an approaching source sounds higher, a closed pipe has only odd harmonics.
2Practice problems
1.KinematicsMedium
A car driving at 72 km/h brakes with a constant deceleration of 7 m/s² until it stops. Find the braking distance s.
v₀ = 72 km/ha = −7 m/s²v = 0
s = m
2.KinematicsHard
A tram starts from rest and accelerates at 2 m/s² for 12 s, then runs at constant speed for 24 s, and finally brakes at 5 m/s² until it stops. Find the total distance s between the two stops.
a1 = 2 m/s²t1 = 12 st2 = 24 sa3 = −5 m/s²
s = m
3.Newton’s lawsMedium
A 15 kg sled is pulled across level snow by a rope at 35° above the horizontal with a force of 60 N. The coefficient of kinetic friction is 0.3. Use g = 9.81 m/s². Find the acceleration a of the sled.
m = 15 kgF = 60 Nθ = 35°μ = 0.3g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
A 4.5 kg block on a ramp at 30° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 9.5 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.25. Use g = 9.81 m/s². Find the acceleration a of the system.
A ball is launched from level ground at 40° above the horizontal and lands 95 m away. Ignore air resistance and use g = 9.81 m/s². Find the launch speed v0.
θ = 40°R = 95 mg = 9.81 m/s²
v0 = m/s
6.Projectile motionHard
A ball is launched at 20° above horizontal from the top of a 60 m cliff. Ignore air resistance. Find the time of flight t until the ball hits the ground.
v0 = 14 m/sθ = 20°h = 60 mg = 9.81 m/s²
t = s
7.Circular motionMedium
A car takes a flat curve of radius 85 m. The coefficient of static friction between tyres and road is 0.4. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 85 mμs = 0.4g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A curve of radius 210 m is banked at 18°, and the coefficient of static friction between tyres and road is 0.2. Use g = 9.81 m/s². Find the highest speed vmax at which a car can take the curve without sliding up and out.
r = 210 mθ = 18°μs = 0.2g = 9.81 m/s²
vmax = m/s
9.Work & energyMedium
A block slides from rest down a slope at 30° to the horizontal. The coefficient of kinetic friction is 0.1. Find its speed v after it has slid 19 m along the slope.
θ = 30°μ = 0.1d = 19 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
A 4 kg block is released from rest at height 3.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 2.5 m long (μ = 0.2) and then runs onto a spring (k = 5000 N/m) on a smooth surface. Find the speed v of the block when it reaches the spring.
m = 4 kgh = 3.5 mL = 2.5 mμ = 0.2k = 5000 N/mg = 9.81 m/s²
v = m/s
11.Momentum & collisionsMedium
A 20 g bullet is fired horizontally into a 2.5 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 19 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 20 gM = 2.5 kgh = 19 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
At a crossing a 1200 kg car travelling east at 17 m/s collides with a 1000 kg car travelling north at 10 m/s. The cars lock together. Find the direction θ of the wreck just after the collision, measured north of east.
m1 = 1200 kgv1 = 17 m/s (E)m2 = 1000 kgv2 = 10 m/s (N)
θ = °
13.Springs & pendulumsMedium
A 3.5 kg mass on a spring with k = 360 N/m is pulled 10 cm in the positive direction and released from rest at t = 0. Find its displacement x from equilibrium at t = 0.1 s (with sign).
m = 3.5 kgk = 360 N/mA = 10 cmt = 0.1 s
x = cm
14.Springs & pendulumsHard
A mass on a spring with k = 240 N/m oscillates on a frictionless surface with an amplitude of 5 cm. Find the energy U stored in the spring when the mass is 1 cm from equilibrium.
k = 240 N/mA = 5 cmx = 1 cm
U = J
15.Waves, sound & opticsMedium
An object stands 8 cm in front of a thin converging lens with a focal length of 10 cm. Find the image distance di. Use the sign convention 1/f = 1/do + 1/di (f < 0 for a diverging lens, di < 0 for a virtual image).
do = 8 cmf = 10 cm
di = cm
16.Waves, sound & opticsHard
A diffraction grating has 1200 lines per millimetre. Light of wavelength 486 nm falls on it at right angles. What is the highest order mmax of bright maximum that can be seen?
N = 1200 mm⁻¹λ = 486 nm
mmax =
3Solutions
1.Kinematics
Answer: s = 28.57 m
Motion with constant acceleration: v = v₀ + a·t, s = v₀·t + ½·a·t² and v² = v₀² + 2·a·s. Pick the equation that contains the three things you know and the one you want, and keep the signs consistent with one positive direction.
Units
v₀ = 72/3.6 = 20 m/s
Braking distance
0 = v₀² − 2·a·s → s = v₀²/(2a) = 20²/(2·7) = 28.57 m
Answer
s = 28.57 m
2.Kinematics
Answer: s = 777.6 m
Motion with constant acceleration: v = v₀ + a·t, s = v₀·t + ½·a·t² and v² = v₀² + 2·a·s. Pick the equation that contains the three things you know and the one you want, and keep the signs consistent with one positive direction.
Top speed
v = a1·t1 = 2·12 = 24 m/s
Distance of each phase
s1 = ½·2·12² = 144 m
s2 = 24·24 = 576 m
s3 = 24²/(2·5) = 57.6 m
Total distance
s = 144 + 576 + 57.6 = 777.6 m
Answer
s = 777.6 m
3.Newton’s laws
Answer: a = 1.022 m/s²
Newton’s second law: the net force on a body equals its mass times its acceleration, ΣF = m·a. Draw a free-body diagram, choose a positive direction along the motion, and add every force in that direction: applied forces, friction μ·N against the motion, and the components of gravity.
Normal force
N = m·g − F·sin θ = 147.2 − 60·sin 35° = 112.7 N
Horizontal forces
m·a = F·cos θ − μ·N = 49.15 − 33.82
a = 15.33/15 = 1.022 m/s²
Answer
a = 1.022 m/s²
4.Newton’s laws
Answer: a = 4.397 m/s²
Newton’s second law: the net force on a body equals its mass times its acceleration, ΣF = m·a. Draw a free-body diagram, choose a positive direction along the motion, and add every force in that direction: applied forces, friction μ·N against the motion, and the components of gravity.
Forces along the string
m₂·g = 93.2 N
m₁·g·sin θ = 22.07 N
μ·m₁·g·cos θ = 9.558 N
Acceleration of the system
a = (93.2 − 22.07 − 9.558)/14 = 4.397 m/s²
Answer
a = 4.397 m/s²
5.Projectile motion
Answer: v0 = 30.76 m/s
Projectile motion: with no air resistance, x = vxt and y = h + vyt − ½gt². The two directions are independent except that they share the same time.
Solve the range formula for v₀
v₀ = √(g·R/sin 2θ) = √(9.81·95/sin 80°) = 30.76 m/s
Answer
v0 = 30.76 m/s
6.Projectile motion
Answer: t = 4.019 s
Projectile motion: with no air resistance, x = vxt and y = h + vyt − ½gt². The two directions are independent except that they share the same time.
Split the launch velocity
Horizontal velocity stays constant; vertical motion has a constant acceleration g = 9.81 m/s² downward.
vx = v0·cos θ = 14·cos 20° = 13.16 m/s
vy = v0·sin θ = 14·sin 20° = 4.788 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 60 + 4.788·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (4.788 + √(22.93 + 1177))/9.81 = 4.019 s
Answer
t = 4.019 s
7.Circular motion
Answer: vmax = 18.26 m/s
Uniform circular motion: an object moving at speed v on a circle of radius r accelerates towards the centre with a = v²/r, so it needs a net inward force F = m·v²/r. That force is supplied by something real: friction, tension, gravity or a normal force.
Friction provides the centripetal force
m·v²/r = μ·m·g
v = √(μ·g·r) = √(0.4·9.81·85) = 18.26 m/s
Answer
vmax = 18.26 m/s (65.75 km/h)
8.Circular motion
Answer: vmax = 34.01 m/s
Uniform circular motion: an object moving at speed v on a circle of radius r accelerates towards the centre with a = v²/r, so it needs a net inward force F = m·v²/r. That force is supplied by something real: friction, tension, gravity or a normal force.
Force balances
Friction points down the slope.
N·(sin θ + μ·cos θ) = m·v²/r
N·(cos θ − μ·sin θ) = m·g
Speed
v = √(210·9.81·0.4992/0.8893) = 34.01 m/s
Answer
vmax = 34.01 m/s
(122.4 km/h)
9.Work & energy
Answer: v = 12.41 m/s
Work–energy principle: the change in kinetic energy equals the work done by all forces. Writing gravity and springs as potential energy (mgh and ½kx²) turns this into a balance: initial energy = final energy + energy lost to friction (μN·d).
Energy balance along the slope
The block drops a height d·sin θ while friction removes μmg·cos θ·d. The mass cancels.
½v² = g·d·(sin θ − μ·cos θ)
v = √(2·9.81·19·(0.5 − 0.1·0.866)) = 12.41 m/s
Answer
v = 12.41 m/s
10.Work & energy
Answer: v = 7.672 m/s
Work–energy principle: the change in kinetic energy equals the work done by all forces. Writing gravity and springs as potential energy (mgh and ½kx²) turns this into a balance: initial energy = final energy + energy lost to friction (μN·d).
Energy left after the rough strip
The ramp is smooth, so only the strip removes energy.
mgh − μmgL = 39.24·(3.5 − 0.2·2.5) = 117.7 J
Speed at the spring
½mv² = mg(h − μL) → v = √(2g(h − μL)) = √(2·9.81·3) = 7.672 m/s
Answer
v = 7.672 m/s
11.Momentum & collisions
Answer: v = 243.3 m/s
Conservation of momentum: when no external force acts during a collision, the total momentum Σm·v before equals the total after. Velocities have a direction, so pick a positive direction and give velocities the other way a minus sign. In an elastic collision kinetic energy is conserved too; in a perfectly inelastic one the bodies stick together.
Swing after impact
V = √(2·g·h) = √(2·9.81·0.19) = 1.931 m/s
Collision
m·v = (m + M)·V → v = (0.02 + 2.5)·1.931/0.02 = 243.3 m/s
Answer
v = 243.3 m/s
12.Momentum & collisions
Answer: θ = 26.11 °
Conservation of momentum: when no external force acts during a collision, the total momentum Σm·v before equals the total after. Velocities have a direction, so pick a positive direction and give velocities the other way a minus sign. In an elastic collision kinetic energy is conserved too; in a perfectly inelastic one the bodies stick together.
Momentum components
px = 1200·17 = 20400 kg·m/s
py = 1000·10 = 10000 kg·m/s
Direction
tan θ = py/px = 0.4902 → θ = 26.11°
Answer
θ = 26.11°
13.Springs & pendulums
Answer: x = 5.283 cm
Simple harmonic motion: a mass m on a spring of stiffness k oscillates with angular frequency ω = √(k/m) and period T = 2π/ω = 2π·√(m/k). A simple pendulum of length L has T = 2π·√(L/g). With amplitude A, the largest speed is A·ω, the largest acceleration A·ω² and the energy ½·k·A².
Angular frequency
ω = √(360/3.5) = 10.14 rad/s
ω·t = 10.14·0.1 = 1.014 rad
Displacement
x = A·cos(ω·t) = 10·cos(1.014) = 5.283 cm
Answer
x = 5.283 cm
14.Springs & pendulums
Answer: U = 0.012 J
Simple harmonic motion: a mass m on a spring of stiffness k oscillates with angular frequency ω = √(k/m) and period T = 2π/ω = 2π·√(m/k). A simple pendulum of length L has T = 2π·√(L/g). With amplitude A, the largest speed is A·ω, the largest acceleration A·ω² and the energy ½·k·A².
Total energy
E = ½·k·A² = ½·240·0.05² = 0.3 J
Spring energy at x
U = ½·k·x² = ½·240·0.01² = 0.012 J
Answer
U = 0.012 J
15.Waves, sound & optics
Answer: di = −40 cm
Geometric optics: at a boundary, n₁·sin θ₁ = n₂·sin θ₂ (Snell's law), with angles measured from the normal and n = c/v. Going into a slower medium the ray bends towards the normal. From a denser medium (n₁ > n₂) beyond the critical angle θc = arcsin(n₂/n₁) all light is reflected: total internal reflection. Thin lenses and spherical mirrors obey 1/f = 1/do + 1/di with the magnification m = −di/do. Sign convention: f > 0 for a converging lens and a concave mirror (f = R/2), f < 0 for a diverging lens and a convex mirror; di > 0 for a real image, di < 0 for a virtual one, and m < 0 for an inverted image.
Thin-lens equation
1/di = 1/f − 1/do = 1/10 − 1/8
di = −40 cm
Ray diagram
virtual, on the same side as the object
Answer
di = −40 cm
16.Waves, sound & optics
Answer: mmax = 1
Interference and diffraction: light from two slits a distance d apart reaches a point on the screen with a path difference d·sin θ. Bright fringes appear where d·sin θ = mλ (m = 0, ±1, ±2, …). For a screen at distance L ≫ d, the m-th bright fringe is at y = mλL/d and the fringe spacing is Δy = λL/d. A grating with N lines per millimetre has d = 1/N and sharp maxima at d·sin θ = mλ; since sin θ ≤ 1, the highest order is the largest whole number m ≤ d/λ. A single slit of width a has dark fringes at a·sin θ = mλ, m = ±1, ±2, …