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 driver moving at 36 km/h sees an obstacle. Her reaction time is 0.5 s, after which the car brakes with a constant deceleration of 5.5 m/s². Find the total stopping distance d from the moment she sees the obstacle.
v₀ = 36 km/htr = 0.5 sa = −5.5 m/s²
d = m
2.KinematicsHard
A tram starts from rest and accelerates at 2 m/s² for 9 s, then runs at constant speed for 26 s, and finally brakes at 3.5 m/s² until it stops. Find the total distance s between the two stops.
a1 = 2 m/s²t1 = 9 st2 = 26 sa3 = −3.5 m/s²
s = m
3.Newton’s lawsMedium
A box slides across a level floor with an initial speed of 2.5 m/s and comes to rest after 0.8 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.
v₀ = 2.5 m/sd = 0.8 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
A 6 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 3.5 kg mass. The coefficient of kinetic friction between block and table is 0.2. Use g = 9.81 m/s². Find the tension T in the string.
m1 = 6 kgm2 = 3.5 kgμ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
A ball is launched at 45° above horizontal from level ground. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 40 m/sθ = 45°g = 9.81 m/s²
x = m
6.Projectile motionHard
A ball is launched at 24 m/s and 55° above the horizontal from the top of a 20 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 = 24 m/sθ = 55°h = 20 mg = 9.81 m/s²
H = m
7.Circular motionMedium
A car takes a flat curve of radius 40 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 = 40 mμs = 0.4g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A curve of radius 190 m is banked at 12°, and the coefficient of static friction between tyres and road is 0.3. 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 = 190 mθ = 12°μs = 0.3g = 9.81 m/s²
vmax = m/s
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 0.7. Find the stopping distance d.
v = 110 km/hμ = 0.7g = 9.81 m/s²
d = m
10.Work & energyHard
A 2.5 kg block is released from rest at height 4.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 5 m long (μ = 0.15) and then runs onto a spring (k = 5000 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 2.5 kgh = 4.5 mL = 5 mμ = 0.15k = 5000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
An 8 g bullet is fired horizontally into a 5 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 22 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 8 gM = 5 kgh = 22 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
A 0.5 kg ball hits a wall at 18 m/s and bounces straight back at 10 m/s. The contact lasts 40 ms. Find the magnitude of the impulse J the wall gives the ball.
m = 0.5 kgvin = 18 m/svout = 10 m/sΔt = 40 ms
J = N·s
13.Springs & pendulumsMedium
A simple pendulum has a length of 0.6 m. Use g = 9.81 m/s². Find its period T for small swings.
L = 0.6 mg = 9.81 m/s²
T = s
14.Springs & pendulumsHard
A 1.9 kg mass is attached to the lower end of an unstretched vertical spring with k = 220 N/m and released from rest. Use g = 9.81 m/s². Find the greatest speed vmax of the mass.
m = 1.9 kgk = 220 N/mg = 9.81 m/s²
vmax = m/s
15.Waves, sound & opticsMedium
Light travels in diamond (n = 2.42) towards a flat boundary with air (n = 1.00). Find the critical angle θc beyond which it is totally reflected.
n1 = 2.42n2 = 1.00
θc = °
16.Waves, sound & opticsHard
Two thin lenses share an axis, 10 cm apart: lens 1 with f₁ = 10 cm and lens 2 with f₂ = 20 cm. An object stands 25 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 = 10 cmf2 = 20 cms = 10 cmdo = 25 cm
di2 = cm
3Solutions
1.Kinematics
Answer: d = 14.09 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 = 36/3.6 = 10 m/s
Reaction distance
d1 = v₀·tr = 10·0.5 = 5 m
Braking distance
d2 = v₀²/(2a) = 10²/(2·5.5) = 9.091 m
Total
d = 5 + 9.091 = 14.09 m
Answer
d = 14.09 m
2.Kinematics
Answer: s = 595.3 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 = 2·9 = 18 m/s
Distance of each phase
s1 = ½·2·9² = 81 m
s2 = 18·26 = 468 m
s3 = 18²/(2·3.5) = 46.29 m
Total distance
s = 81 + 468 + 46.29 = 595.3 m
Answer
s = 595.3 m
3.Newton’s laws
Answer: μ = 0.3982
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) = 2.5²/(2·0.8) = 3.906 m/s²
Friction coefficient
μ·m·g = m·a → μ = a/g = 3.906/9.81 = 0.3982
Answer
μ = 0.3982
4.Newton’s laws
Answer: T = 26.02 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.
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 = 40·cos 45° = 28.28 m/s
vy = v0·sin θ = 40·sin 45° = 28.28 m/s
Time in the air
Starting and landing at the same height: t = 2vy/g.
t = 2·28.28/9.81 = 5.766 s
Horizontal distance
x = vx·t = 28.28·5.766 = 163.1 m
Answer
x = 163.1 m
6.Projectile motion
Answer: H = 39.7 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 = 24·sin 55° = 19.66 m/s
Rise above the cliff
Δh = vy²/(2g) = 19.66²/(2·9.81) = 19.7 m
Height above the ground
H = 20 + 19.7 = 39.7 m
Answer
H = 39.7 m
7.Circular motion
Answer: vmax = 12.53 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) = √(0.4·9.81·40) = 12.53 m/s
Answer
vmax = 12.53 m/s (45.1 km/h)
8.Circular motion
Answer: vmax = 31.94 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 = √(190·9.81·0.5014/0.9158) = 31.94 m/s
Answer
vmax = 31.94 m/s
(115 km/h)
9.Work & energy
Answer: d = 67.98 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/(2·0.7·9.81) = 67.98 m
Answer
d = 67.98 m
Doubling the speed makes the stopping distance four times longer.
10.Work & energy
Answer: x = 19.18 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 = 24.53·(4.5 − 0.15·5) = 91.97 J
Maximum compression
½kx² = 91.97 → x = √(2·91.97/5000) = 0.1918 m = 19.18 cm
Answer
x = 19.18 cm
11.Momentum & collisions
Answer: v = 1301 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) = √(2·9.81·0.22) = 2.078 m/s
Collision
m·v = (m + M)·V → v = (0.008 + 5)·2.078/0.008 = 1301 m/s
Answer
v = 1301 m/s
12.Momentum & collisions
Answer: J = 14 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 = 0.5·(10 + 18) = 14 N·s
Answer
J = 14 N·s
13.Springs & pendulums
Answer: T = 1.554 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.
Pendulum period
T = 2π·√(0.6/9.81) = 1.554 s
Answer
T = 1.554 s
14.Springs & pendulums
Answer: vmax = 0.9117 m/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.
Equilibrium and amplitude
x₀ = m·g/k = 1.9·9.81/220 = 0.08472 m = A
Greatest speed
The speed is largest at equilibrium.
ω = √(220/1.9) = 10.76 rad/s
vmax = A·ω = 0.08472·10.76 = 0.9117 m/s
Answer
vmax = 0.9117 m/s
15.Waves, sound & optics
Answer: θc = 24.41 °
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 = 1.00/2.42 = 0.4132
Critical angle
θc = arcsin(0.4132) = 24.41°
Answer
θc = 24.41°
16.Waves, sound & optics
Answer: di2 = 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.