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 plane needs a speed of 240 km/h to take off and the usable runway is 2400 m long. Starting from rest, what constant acceleration a does it need?
v₀ = 0v = 240 km/hs = 2400 m
a = m/s²
2.KinematicsHard
A tram starts from rest and accelerates at 3 m/s² for 12 s, then runs at constant speed for 5 s, and finally brakes at 3 m/s² until it stops. Find the total distance s between the two stops.
a1 = 3 m/s²t1 = 12 st2 = 5 sa3 = −3 m/s²
s = m
3.Newton’s lawsMedium
A box slides across a level floor with an initial speed of 6 m/s and comes to rest after 7.3 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.
v₀ = 6 m/sd = 7.3 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
A 5.5 kg block on a ramp at 20° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 5.5 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 acceleration a of the system.
m1 = 5.5 kgm2 = 5.5 kgθ = 20°μ = 0.1g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
A ball is launched at 25° above horizontal from level ground. Ignore air resistance. Find the time of flight t until the ball hits the ground.
v0 = 16 m/sθ = 25°g = 9.81 m/s²
t = s
6.Projectile motionHard
A ball is launched at 23 m/s and 35° above the horizontal from the top of a 45 m cliff. Ignore air resistance and use g = 9.81 m/s². Find the greatest height H above the ground at the foot of the cliff.
v₀ = 23 m/sθ = 35°h = 45 mg = 9.81 m/s²
H = m
7.Circular motionMedium
An 800 kg car drives over the top of a hump in the road at 6 m/s. The top of the hump is a circular arc of radius 25 m. Use g = 9.81 m/s². Find the normal force N of the road on the car at the top.
m = 800 kgv = 6 m/sr = 25 mg = 9.81 m/s²
N = N
8.Circular motionHard
A 0.3 kg ball on a string 1.4 m long is swung in a vertical circle. At the lowest point it moves at 6 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.
m = 0.3 kgr = 1.4 mv = 6 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
A 10 kg block slides 5 m down a slope at 45° to the horizontal, with a coefficient of kinetic friction of 0.3. Find the work Wf done against friction.
m = 10 kgθ = 45°μ = 0.3d = 5 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
A 10 kg block is released from rest at height 3 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 4.5 m long (μ = 0.25) and then runs onto a spring (k = 1000 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 10 kgh = 3 mL = 4.5 mμ = 0.25k = 1000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 3.5 kg glider moving at 5 m/s makes a perfectly elastic head-on collision with a 1 kg glider at rest on a frictionless track. Find the velocity v′1 of the first glider after the collision (positive in its original direction).
m1 = 3.5 kgm2 = 1 kgv1 = 5 m/sv2 = 0
v′1 = m/s
12.Momentum & collisionsHard
A 0.1 kg ball hits a wall at 30 m/s and bounces straight back at 16 m/s. The contact lasts 20 ms. Find the magnitude of the impulse J the wall gives the ball.
m = 0.1 kgvin = 30 m/svout = 16 m/sΔt = 20 ms
J = N·s
13.Springs & pendulumsMedium
An astronaut on another planet finds that a simple pendulum 0.6 m long has a period of 2.52 s for small swings. Find the gravitational acceleration g on that planet.
L = 0.6 mT = 2.52 s
g = m/s²
14.Springs & pendulumsHard
A body oscillates in simple harmonic motion with period 2.8 s and amplitude 8 cm. What is the shortest time t it needs to move from equilibrium to a displacement of 5.6 cm?
T = 2.8 sA = 8 cmx = 5.6 cm
t = s
15.Waves, sound & opticsMedium
An object stands 10 cm in front of a thin converging lens with a focal length of 20 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 = 20 cm
di = cm
16.Waves, sound & opticsHard
Light of wavelength 532 nm falls at right angles on a grating with 300 lines per millimetre. Find the angle θ1 of the order-1 maximum.
N = 300 mm⁻¹λ = 532 nmm = 1
θ1 = °
3Solutions
1.Kinematics
Answer: a = 0.9259 m/s²
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
v = 240/3.6 = 66.67 m/s
Acceleration
a = v²/(2s) = 66.67²/(2·2400) = 0.9259 m/s²
Answer
a = 0.9259 m/s²
2.Kinematics
Answer: s = 612 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 = 3·12 = 36 m/s
Distance of each phase
s1 = ½·3·12² = 216 m
s2 = 36·5 = 180 m
s3 = 36²/(2·3) = 216 m
Total distance
s = 216 + 180 + 216 = 612 m
Answer
s = 612 m
3.Newton’s laws
Answer: μ = 0.2514
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.
Deceleration
a = v₀²/(2d) = 6²/(2·7.3) = 2.466 m/s²
Friction coefficient
μ·m·g = m·a → μ = a/g = 2.466/9.81 = 0.2514
Answer
μ = 0.2514
4.Newton’s laws
Answer: a = 2.766 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 = 53.96 N
m₁·g·sin θ = 18.45 N
μ·m₁·g·cos θ = 5.07 N
Acceleration of the system
a = (53.96 − 18.45 − 5.07)/11 = 2.766 m/s²
Answer
a = 2.766 m/s²
5.Projectile motion
Answer: t = 1.379 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 = 16·cos 25° = 14.5 m/s
vy = v0·sin θ = 16·sin 25° = 6.762 m/s
Time in the air
Starting and landing at the same height: t = 2vy/g.
t = 2·6.762/9.81 = 1.379 s
Answer
t = 1.379 s
6.Projectile motion
Answer: H = 53.87 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.
Vertical launch speed
vy = 23·sin 35° = 13.19 m/s
Rise above the cliff
Δh = vy²/(2g) = 13.19²/(2·9.81) = 8.87 m
Height above the ground
H = 45 + 8.87 = 53.87 m
Answer
H = 53.87 m
7.Circular motion
Answer: N = 6696 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 top
Towards the centre (down) is positive.
m·g − N = m·v²/r
N = 800·9.81 − 800·6²/25 = 7848 − 1152 = 6696 N
Answer
N = 6696 N
The car feels lighter: the road pushes up less than its weight.
8.Circular motion
Answer: T = 10.66 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 = 0.3·9.81 + 0.3·6²/1.4 = 2.943 + 7.714 = 10.66 N
Answer
T = 10.66 N
9.Work & energy
Answer: Wf = 104.1 J
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).
Friction force
N = mg·cos θ = 98.1·0.7071 = 69.37 N
F = μN = 0.3·69.37 = 20.81 N
Work against friction
Wf = F·d = 20.81·5 = 104.1 J
Answer
Wf = 104.1 J
10.Work & energy
Answer: x = 60.65 cm
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 = 98.1·(3 − 0.25·4.5) = 183.9 J
Maximum compression
½kx² = 183.9 → x = √(2·183.9/1000) = 0.6065 m = 60.65 cm
Answer
x = 60.65 cm
11.Momentum & collisions
Answer: v′1 = 2.778 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.
Elastic collision with a target at rest
Solving momentum and energy conservation together gives these two results.
v′1 = (3.5 − 1)/(3.5 + 1)·5 = 2.778 m/s
v′2 = 2·3.5/(3.5 + 1)·5 = 7.778 m/s
Answer
v′1 = 2.778 m/s
12.Momentum & collisions
Answer: J = 4.6 N·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.
Change in momentum
J = m·Δv = 0.1·(16 + 30) = 4.6 N·s
Answer
J = 4.6 N·s
13.Springs & pendulums
Answer: g = 3.73 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 = 4π²·0.6/2.52² = 3.73 m/s²
Answer
g = 3.73 m/s²
14.Springs & pendulums
Answer: t = 0.3455 s
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 = 2π/T = 2π/2.8 = 2.244 rad/s
Solve for the time
0.7 = sin(ω·t) → ω·t = arcsin(0.7) = 0.7754 rad
t = 0.7754/2.244 = 0.3455 s
Answer
t = 0.3455 s
15.Waves, sound & optics
Answer: di = −20 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 = 1/20 − 1/10
di = −20 cm
Ray diagram
virtual, on the same side as the object
Answer
di = −20 cm
16.Waves, sound & optics
Answer: θ1 = 9.184 °
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, …