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 260 km/h to take off and the usable runway is 2200 m long. Starting from rest, what constant acceleration a does it need?
v₀ = 0v = 260 km/hs = 2200 m
a = m/s²
2.KinematicsHard
A ball is thrown straight up at 8 m/s from the edge of a roof 15 m above the ground. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax above the ground.
v₀ = 8 m/sh₀ = 15 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
A 19 kg block slides down a slope at 45° to the horizontal. The coefficient of kinetic friction is 0.2. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.
m = 19 kgθ = 45°μ = 0.2g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
A 7 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 1 kg mass. The table is frictionless. Use g = 9.81 m/s². Find the acceleration a of the system.
m1 = 7 kgm2 = 1 kgμ = 0g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
A ball is launched at 70° above horizontal from level ground. Ignore air resistance. Find the time of flight t until the ball hits the ground.
v0 = 22 m/sθ = 70°g = 9.81 m/s²
t = s
6.Projectile motionHard
A ball is launched at 13 m/s and 45° above the horizontal from the top of a 45 m cliff. Ignore air resistance and use g = 9.81 m/s². At what angle α below the horizontal does it hit the ground?
v₀ = 13 m/sθ = 45°h = 45 mg = 9.81 m/s²
α = °
7.Circular motionMedium
A car takes a flat curve of radius 65 m. The coefficient of static friction between tyres and road is 0.7. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 65 mμs = 0.7g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 1.2 kg ball on a string 0.9 m long is swung in a vertical circle. At the lowest point it moves at 5.5 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.
m = 1.2 kgr = 0.9 mv = 5.5 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
A block slides from rest down a slope at 25° to the horizontal. The coefficient of kinetic friction is 0.3. Find its speed v after it has slid 20 m along the slope.
θ = 25°μ = 0.3d = 20 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
A 0.5 kg block is released from rest at height 4.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 1 m long (μ = 0.1) and then runs onto a spring (k = 2500 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 0.5 kgh = 4.5 mL = 1 mμ = 0.1k = 2500 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 4 kg glider moving at 10 m/s makes a perfectly elastic head-on collision with a 3 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 = 4 kgm2 = 3 kgv1 = 10 m/sv2 = 0
v′1 = m/s
12.Momentum & collisionsHard
A 12 g bullet is fired horizontally into a 3 kg block hanging from light strings 1.4 m long, and stays in it. The strings swing out to a largest angle of 40° from the vertical. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 12 gM = 3 kgL = 1.4 mθ = 40°g = 9.81 m/s²
v = m/s
13.Springs & pendulumsMedium
A 2.8 kg mass on a spring with k = 390 N/m is pulled 11 cm in the positive direction and released from rest at t = 0. Find its displacement x from equilibrium at t = 1 s (with sign).
m = 2.8 kgk = 390 N/mA = 11 cmt = 1 s
x = cm
14.Springs & pendulumsHard
A 0.6 kg mass on a spring with k = 170 N/m is pulled 12 cm from equilibrium and released. Find its maximum acceleration amax.
m = 0.6 kgk = 170 N/mA = 12 cm
amax = m/s²
15.Waves, sound & opticsMedium
Light travels in flint glass (n = 1.62) towards a flat boundary with water (n = 1.33). Find the critical angle θc beyond which it is totally reflected.
n1 = 1.62n2 = 1.33
θc = °
16.Waves, sound & opticsHard
Two thin lenses share an axis, 15 cm apart: lens 1 with f₁ = 8 cm and lens 2 with f₂ = 5 cm. An object stands 15 cm in front of lens 1. Find the position di2 of the final image measured from lens 2 (positive beyond lens 2, real-is-positive convention).
f1 = 8 cmf2 = 5 cms = 15 cmdo = 15 cm
di2 = cm
3Solutions
1.Kinematics
Answer: a = 1.185 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 = 260/3.6 = 72.22 m/s
Acceleration
a = v²/(2s) = 72.22²/(2·2200) = 1.185 m/s²
Answer
a = 1.185 m/s²
2.Kinematics
Answer: hmax = 18.26 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 = 8²/(2·9.81) = 3.262 m
Height above the ground
hmax = 15 + 3.262 = 18.26 m
Answer
hmax = 18.26 m
3.Newton’s laws
Answer: a = 5.549 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.
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 = 22·cos 70° = 7.524 m/s
vy = v0·sin θ = 22·sin 70° = 20.67 m/s
Time in the air
Starting and landing at the same height: t = 2vy/g.
t = 2·20.67/9.81 = 4.215 s
Answer
t = 4.215 s
6.Projectile motion
Answer: α = 73.54 °
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
vx = 13·cos 45° = 9.192 m/s
vy0 = 13·sin 45° = 9.192 m/s
Vertical speed at impact
Energy or v² = v₀² + 2as in the vertical direction.
vy = √(9.192² + 2·9.81·45) = 31.1 m/s
Impact angle
tan α = vy/vx = 31.1/9.192 → α = 73.54°
Answer
α = 73.54°
7.Circular motion
Answer: vmax = 21.13 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) = √(0.7·9.81·65) = 21.13 m/s
Answer
vmax = 21.13 m/s (76.06 km/h)
8.Circular motion
Answer: T = 52.11 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 = 1.2·9.81 + 1.2·5.5²/0.9 = 11.77 + 40.33 = 52.11 N
Answer
T = 52.11 N
9.Work & energy
Answer: v = 7.691 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 = √(2·9.81·20·(0.4226 − 0.3·0.9063)) = 7.691 m/s
Answer
v = 7.691 m/s
10.Work & energy
Answer: x = 13.14 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·(4.5 − 0.1·1) = 21.58 J
Maximum compression
½kx² = 21.58 → x = √(2·21.58/2500) = 0.1314 m = 13.14 cm
Answer
x = 13.14 cm
11.Momentum & collisions
Answer: v′1 = 1.429 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 = (4 − 3)/(4 + 3)·10 = 1.429 m/s
v′2 = 2·4/(4 + 3)·10 = 11.43 m/s
Answer
v′1 = 1.429 m/s
12.Momentum & collisions
Answer: v = 636.3 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.
Rise of the block
h = L·(1 − cos θ) = 1.4·(1 − cos 40°) = 0.3275 m
Speed after impact
V = √(2·9.81·0.3275) = 2.535 m/s
Collision
v = (m + M)·V/m = 3.012·2.535/0.012 = 636.3 m/s
Answer
v = 636.3 m/s
13.Springs & pendulums
Answer: x = 7.94 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 = √(390/2.8) = 11.8 rad/s
ω·t = 11.8·1 = 11.8 rad
Displacement
x = A·cos(ω·t) = 11·cos(11.8) = 7.94 cm
Answer
x = 7.94 cm
14.Springs & pendulums
Answer: amax = 34 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.
Angular frequency
ω = √(170/0.6) = 16.83 rad/s
Maximum acceleration
amax = A·ω² = 0.12·16.83² = 34 m/s²
Answer
amax = 34 m/s²
15.Waves, sound & optics
Answer: θc = 55.18 °
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.
Refraction at 90°
sin θc = n₂/n₁ = 1.33/1.62 = 0.821
Critical angle
θc = arcsin(0.821) = 55.18°
Answer
θc = 55.18°
16.Waves, sound & optics
Answer: di2 = 1.5 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.