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 90 km/h brakes with a constant deceleration of 4.5 m/s² until it stops. Find the braking distance s.
v₀ = 90 km/ha = −4.5 m/s²v = 0
s = m
2.KinematicsHard
A tram starts from rest and accelerates at 2.5 m/s² for 7 s, then runs at constant speed for 19 s, and finally brakes at 2.5 m/s² until it stops. Find the total travel time t.
a1 = 2.5 m/s²t1 = 7 st2 = 19 sa3 = −2.5 m/s²
t = s
3.Newton’s lawsMedium
A box slides across a level floor with an initial speed of 11.5 m/s and comes to rest after 130 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.
v₀ = 11.5 m/sd = 130 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
A 4.5 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 4 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.2. Use g = 9.81 m/s². Find the tension T in the string.
m1 = 4.5 kgm2 = 4 kgθ = 20°μ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
A ball is launched at 60° above horizontal from level ground. Ignore air resistance. Find the time of flight t until the ball hits the ground.
v0 = 16 m/sθ = 60°g = 9.81 m/s²
t = s
6.Projectile motionHard
A ball is launched at 45° 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 = 16 m/sθ = 45°h = 60 mg = 9.81 m/s²
t = s
7.Circular motionMedium
A curve of radius 200 m is banked at 25°. At what speed v can a car take it with no friction needed at all? Use g = 9.81 m/s².
r = 200 mθ = 25°g = 9.81 m/s²
v = m/s
8.Circular motionHard
A 1.9 kg ball on a string 1.2 m long is swung in a vertical circle. At the lowest point it moves at 3.5 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.
m = 1.9 kgr = 1.2 mv = 3.5 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
A car travelling at 110 km/h on a level road brakes hard and skids to a stop with its wheels locked. The coefficient of kinetic friction between the tyres and the road is 0.5. Find the stopping distance d.
v = 110 km/hμ = 0.5g = 9.81 m/s²
d = m
10.Work & energyHard
A 0.5 kg block is released from rest at height 2 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 2.5 m long (μ = 0.1) and then runs onto a spring (k = 5000 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 0.5 kgh = 2 mL = 2.5 mμ = 0.1k = 5000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 500 kg car moving at 10 m/s runs into a stationary 900 kg car and the two lock together. How much kinetic energy ΔK is lost in the collision?
m1 = 500 kgv1 = 10 m/sm2 = 900 kgv2 = 0
ΔK = J
12.Momentum & collisionsHard
An 8 g bullet is fired horizontally into a 4 kg block hanging from light strings 0.7 m long, and stays in it. The strings swing out to a largest angle of 35° from the vertical. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 8 gM = 4 kgL = 0.7 mθ = 35°g = 9.81 m/s²
v = m/s
13.Springs & pendulumsMedium
How long must a simple pendulum be to have a period of 1.1 s? Use g = 9.81 m/s². Find L.
T = 1.1 sg = 9.81 m/s²
L = m
14.Springs & pendulumsHard
A 2.9 kg mass is attached to the lower end of an unstretched vertical spring with k = 160 N/m and released from rest. Use g = 9.81 m/s². Find the greatest extension xmax of the spring in cm.
m = 2.9 kgk = 160 N/mg = 9.81 m/s²
xmax = cm
15.Waves, sound & opticsMedium
A pipe open at both ends must have its harmonic number 3 at 515 Hz. The speed of sound is 343 m/s. Find the length L of the pipe.
v = 343 m/sn = 3f3 = 515 Hz
L = m
16.Waves, sound & opticsHard
Light of wavelength 650 nm falls on two slits 0.25 mm apart. The screen is 2.5 m away. Find the spacing Δy between neighbouring bright fringes.
λ = 650 nmd = 0.25 mmL = 2.5 m
Δy = mm
3Solutions
1.Kinematics
Answer: s = 69.44 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 = 90/3.6 = 25 m/s
Braking distance
0 = v₀² − 2·a·s → s = v₀²/(2a) = 25²/(2·4.5) = 69.44 m
Answer
s = 69.44 m
2.Kinematics
Answer: t = 33 s
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 = 2.5·7 = 17.5 m/s
Braking time
t3 = v/a3 = 17.5/2.5 = 7 s
Total time
t = 7 + 19 + 7 = 33 s
Answer
t = 33 s
3.Newton’s laws
Answer: μ = 0.05185
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) = 11.5²/(2·130) = 0.5087 m/s²
Friction coefficient
μ·m·g = m·a → μ = a/g = 0.5087/9.81 = 0.05185
Answer
μ = 0.05185
4.Newton’s laws
Answer: T = 31.78 N
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 = 39.24 N
m₁·g·sin θ = 15.1 N
μ·m₁·g·cos θ = 8.297 N
Acceleration of the system
a = (39.24 − 15.1 − 8.297)/8.5 = 1.864 m/s²
Tension from the hanging mass
m2·g − T = m2·a → T = 4·(9.81 − 1.864) = 31.78 N
Answer
T = 31.78 N
5.Projectile motion
Answer: t = 2.825 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 60° = 8 m/s
vy = v0·sin θ = 16·sin 60° = 13.86 m/s
Time in the air
Starting and landing at the same height: t = 2vy/g.
t = 2·13.86/9.81 = 2.825 s
Answer
t = 2.825 s
6.Projectile motion
Answer: t = 4.836 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 45° = 11.31 m/s
vy = v0·sin θ = 16·sin 45° = 11.31 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 60 + 11.31·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (11.31 + √(128 + 1177))/9.81 = 4.836 s
Answer
t = 4.836 s
7.Circular motion
Answer: v = 30.25 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.
v = √(r·g·tan θ) = √(200·9.81·tan 25°) = 30.25 m/s
Answer
v = 30.25 m/s
(108.9 km/h)
8.Circular motion
Answer: T = 38.03 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 = 1.9·9.81 + 1.9·3.5²/1.2 = 18.64 + 19.4 = 38.03 N
Answer
T = 38.03 N
9.Work & energy
Answer: d = 95.17 m
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).
Speed in base units
v = 110/3.6 = 30.56 m/s
Energy balance
½mv² = μmg·d → d = v²/(2μg) = 30.56²/(2·0.5·9.81) = 95.17 m
Answer
d = 95.17 m
Doubling the speed makes the stopping distance four times longer.
10.Work & energy
Answer: x = 5.86 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 = 4.905·(2 − 0.1·2.5) = 8.584 J
Maximum compression
½kx² = 8.584 → x = √(2·8.584/5000) = 0.0586 m = 5.86 cm
Answer
x = 5.86 cm
11.Momentum & collisions
Answer: ΔK = 16071 J
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.
Common velocity
v′ = m1·v1/(m1 + m2) = 500·10/1400 = 3.571 m/s
Kinetic energy before and after
Kbefore = ½·500·10² = 25000 J
Kafter = ½·1400·3.571² = 8929 J
Energy lost
ΔK = 25000 − 8929 = 16071 J
Answer
ΔK = 16071 J
The lost energy goes into deformation, heat and sound.
12.Momentum & collisions
Answer: v = 789.6 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.
Rise of the block
h = L·(1 − cos θ) = 0.7·(1 − cos 35°) = 0.1266 m
Speed after impact
V = √(2·9.81·0.1266) = 1.576 m/s
Collision
v = (m + M)·V/m = 4.008·1.576/0.008 = 789.6 m/s
Answer
v = 789.6 m/s
13.Springs & pendulums
Answer: L = 0.3007 m
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 the length
L = g·(T/2π)² = 9.81·(1.1/2π)² = 0.3007 m
Answer
L = 0.3007 m
14.Springs & pendulums
Answer: xmax = 35.56 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.
Equilibrium and amplitude
x₀ = m·g/k = 2.9·9.81/160 = 0.1778 m = A
Lowest point
xmax = 2A = 0.3556 m = 35.56 cm
Answer
xmax = 35.56 cm
15.Waves, sound & optics
Answer: L = 0.999 m
Standing waves and the Doppler effect: a string fixed at both ends, or a pipe open at both ends, fits whole half-wavelengths: L = nλ/2, fn = n·v/(2L), n = 1, 2, 3, … A pipe closed at one end has a node at the closed end and an antinode at the open end: L = nλ/4 with odd n only, fn = n·v/(4L). When source or observer move along the line between them, f′ = f·(v ± vo)/(v ∓ vs): use the upper signs for motion towards each other. Two waves of nearly equal frequency beat at |f₁ − f₂|.
Wavelength
λ = v/f = 343/515 = 0.666 m
Length
L = nλ/2 = 3·0.666/2 = 0.999 m
Answer
L = 0.999 m
16.Waves, sound & optics
Answer: Δy = 6.5 mm
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, …