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 speeding at a constant 35 m/s passes a police car standing still. At that moment the police car starts and accelerates uniformly at 4 m/s². How far from its starting point does the police car catch up? Find d.
vcar = 35 m/sapolice = 4 m/s²v0,police = 0
d = m
2.KinematicsHard
A ball is thrown straight up at 20 m/s from the edge of a roof 20 m above the ground. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax above the ground.
v₀ = 20 m/sh₀ = 20 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
A 6 kg block slides down a frictionless slope at 30° to the horizontal. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.
m = 6 kgθ = 30°μ = 0g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
An 8.5 kg block on a ramp at 40° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 8 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.1. Use g = 9.81 m/s². Find the tension T in the string.
m1 = 8.5 kgm2 = 8 kgθ = 40°μ = 0.1g = 9.81 m/s²
T = N
5.Projectile motionMedium
A ball is launched from level ground at 38 m/s and must land 105 m away. Ignore air resistance and use g = 9.81 m/s². Find the smaller of the two possible launch angles θ.
v₀ = 38 m/sR = 105 mg = 9.81 m/s²
θ = °
6.Projectile motionHard
A ball is launched at 55° above horizontal from the top of a 45 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 23 m/sθ = 55°h = 45 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A car takes a flat curve of radius 95 m. The coefficient of static friction between tyres and road is 0.8. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 95 mμs = 0.8g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 1.7 kg ball on a string 0.8 m long is swung in a vertical circle. At the lowest point it moves at 7 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.
m = 1.7 kgr = 0.8 mv = 7 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
A block slides from rest down a slope at 45° to the horizontal. The coefficient of kinetic friction is 0.4. Find its speed v after it has slid 10 m along the slope.
θ = 45°μ = 0.4d = 10 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
A 0.5 kg cart is released from rest at a height of 9 m on a smooth track that leads into a vertical circular loop of radius 3 m. Find the normal force N the track exerts on the cart at the top of the loop.
m = 0.5 kgh = 9 mR = 3 mg = 9.81 m/s²
N = N
11.Momentum & collisionsMedium
A 13 kg projectile moving horizontally at 29 m/s explodes into two fragments that move along the same line. The 12 kg fragment continues forwards at 74 m/s. Find the velocity v′2 of the 1 kg fragment (positive in the original direction).
M = 13 kgu = 29 m/sm1 = 12 kgv′1 = 74 m/sm2 = 1 kg
v′2 = m/s
12.Momentum & collisionsHard
At a crossing a 1400 kg car travelling east at 11 m/s collides with an 800 kg car travelling north at 20 m/s. The cars lock together. Find the direction θ of the wreck just after the collision, measured north of east.
m1 = 1400 kgv1 = 11 m/s (E)m2 = 800 kgv2 = 20 m/s (N)
θ = °
13.Springs & pendulumsMedium
An astronaut on another planet finds that a simple pendulum 0.8 m long has a period of 1.89 s for small swings. Find the gravitational acceleration g on that planet.
L = 0.8 mT = 1.89 s
g = m/s²
14.Springs & pendulumsHard
A 12 g bullet moving at 400 m/s hits and stays in a 3.5 kg block resting on a frictionless surface. The block is attached to a horizontal spring with k = 1200 N/m. Find the amplitude A of the resulting oscillation in cm.
m = 12 gv = 400 m/sM = 3.5 kgk = 1200 N/m
A = cm
15.Waves, sound & opticsMedium
An object stands 10 cm in front of a thin converging lens with a focal length of 15 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 = 10 cmf = 15 cm
di = cm
16.Waves, sound & opticsHard
Light of wavelength 500 nm falls at right angles on a grating with 1200 lines per millimetre. Find the angle θ1 of the order-1 maximum.
N = 1200 mm⁻¹λ = 500 nmm = 1
θ1 = °
3Solutions
1.Kinematics
Answer: d = 612.5 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.
Equal distances
v·t = ½·a·t² → t = 2v/a = 2·35/4 = 17.5 s
Distance
d = v·t = 35·17.5 = 612.5 m
Answer
d = 612.5 m
At that moment the police car moves at 2v = 70 m/s.
2.Kinematics
Answer: hmax = 40.39 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.
Rise above the roof
0 = v₀² − 2·g·Δh → Δh = 20²/(2·9.81) = 20.39 m
Height above the ground
hmax = 20 + 20.39 = 40.39 m
Answer
hmax = 40.39 m
3.Newton’s laws
Answer: a = 4.905 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.
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 = 78.48 N
m₁·g·sin θ = 53.6 N
μ·m₁·g·cos θ = 6.388 N
Acceleration of the system
a = (78.48 − 53.6 − 6.388)/16.5 = 1.121 m/s²
Tension from the hanging mass
m2·g − T = m2·a → T = 8·(9.81 − 1.121) = 69.51 N
Answer
T = 69.51 N
5.Projectile motion
Answer: θ = 22.75 °
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.
Range formula
R = v₀²·sin 2θ/g → sin 2θ = 9.81·105/38² = 0.7133
Angle
2θ = 45.51° → θ = 22.75°
Answer
θ = 22.75°
The other solution is 90° − θ = 67.25°.
6.Projectile motion
Answer: x = 72.65 m
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 = 23·cos 55° = 13.19 m/s
vy = v0·sin θ = 23·sin 55° = 18.84 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 45 + 18.84·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (18.84 + √(355 + 882.9))/9.81 = 5.507 s
Horizontal distance
x = vx·t = 13.19·5.507 = 72.65 m
Answer
x = 72.65 m
7.Circular motion
Answer: vmax = 27.3 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) = √(0.8·9.81·95) = 27.3 m/s
Answer
vmax = 27.3 m/s (98.3 km/h)
8.Circular motion
Answer: T = 120.8 N
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.
Forces at the lowest point
Towards the centre (up) is positive.
T − m·g = m·v²/r
T = 1.7·9.81 + 1.7·7²/0.8 = 16.68 + 104.1 = 120.8 N
Answer
T = 120.8 N
9.Work & energy
Answer: v = 9.124 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 = √(2·9.81·10·(0.7071 − 0.4·0.7071)) = 9.124 m/s
Answer
v = 9.124 m/s
10.Work & energy
Answer: N = 4.905 N
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). On a circular path the net force towards the centre must equal mv²/r.
Speed at the top
v² = 2g(h − 2R) = 2·9.81·(9 − 6) = 58.86 m²/s²
Forces at the top
m·g = 0.5·9.81 = 4.905 N
N = mv²/R − mg = 0.5·58.86/3 − 4.905 = 4.905 N
Answer
N = 4.905 N
The cart just makes it round when h = 2.5R; then N = 0 at the top.
11.Momentum & collisions
Answer: v′2 = −511 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.
Momentum before
p = M·u = 13·29 = 377 kg·m/s
Momentum after
377 = 12·74 + 1·v′2
v′2 = (377 − 888)/1 = −511 m/s
Answer
v′2 = −511 m/s
The minus sign means the second fragment flies backwards.
12.Momentum & collisions
Answer: θ = 46.09 °
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 = 1400·11 = 15400 kg·m/s
py = 800·20 = 16000 kg·m/s
Direction
tan θ = py/px = 1.039 → θ = 46.09°
Answer
θ = 46.09°
13.Springs & pendulums
Answer: g = 8.842 m/s²
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.
Solve for g
g = 4π²·L/T² = 4π²·0.8/1.89² = 8.842 m/s²
Answer
g = 8.842 m/s²
14.Springs & pendulums
Answer: A = 7.394 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.
Speed after impact
V = m·v/(m + M) = 0.012·400/3.512 = 1.367 m/s
Angular frequency
ω = √(k/(m + M)) = √(1200/3.512) = 18.48 rad/s
Amplitude
A = V/ω = 1.367/18.48 = 0.07394 m = 7.394 cm
Answer
A = 7.394 cm
15.Waves, sound & optics
Answer: di = −30 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 = 1/15 − 1/10
di = −30 cm
Ray diagram
virtual, on the same side as the object
Answer
di = −30 cm
16.Waves, sound & optics
Answer: θ1 = 36.87 °
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, …