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 17 m/s passes a police car standing still. At that moment the police car starts and accelerates uniformly at 5 m/s². How long does it take the police car to catch up? Find t.
vcar = 17 m/sapolice = 5 m/s²v0,police = 0
t = s
2.KinematicsHard
A tram starts from rest and accelerates at 1.5 m/s² for 5 s, then runs at constant speed for 20 s, and finally brakes at 2 m/s² until it stops. Find the total travel time t.
a1 = 1.5 m/s²t1 = 5 st2 = 20 sa3 = −2 m/s²
t = s
3.Newton’s lawsMedium
A 40 kg sled is pulled across level snow by a rope at 35° above the horizontal with a force of 130 N. The coefficient of kinetic friction is 0.2. Use g = 9.81 m/s². Find the acceleration a of the sled.
m = 40 kgF = 130 Nθ = 35°μ = 0.2g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
A 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 10 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 = 5 kgm2 = 10 kgθ = 20°μ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
A ball is kicked from level ground at 24 m/s and 50° above the horizontal towards a wall 23 m away. Ignore air resistance and use g = 9.81 m/s². At what height y does the ball pass the wall?
v₀ = 24 m/sθ = 50°x = 23 mg = 9.81 m/s²
y = m
6.Projectile motionHard
A ball is launched at 15° above horizontal from the top of a 60 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 12 m/sθ = 15°h = 60 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A curve of radius 350 m is banked at 12°. At what speed v can a car take it with no friction needed at all? Use g = 9.81 m/s².
r = 350 mθ = 12°g = 9.81 m/s²
v = m/s
8.Circular motionHard
A 1.9 kg ball on a string 1.5 m long is swung in a vertical circle. At the lowest point it moves at 9 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.
m = 1.9 kgr = 1.5 mv = 9 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 0.1. Find its speed v after it has slid 7 m along the slope.
θ = 25°μ = 0.1d = 7 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
A 4 kg block is released from rest at height 4 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 2.5 m long (μ = 0.25) and then runs onto a spring (k = 1500 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 4 kgh = 4 mL = 2.5 mμ = 0.25k = 1500 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 12 g bullet is fired horizontally into a 2 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 18 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 12 gM = 2 kgh = 18 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
A 0.45 kg ball hits a wall at 16 m/s and bounces straight back at 20 m/s. The contact lasts 10 ms. Find the average force Favg of the wall on the ball.
m = 0.45 kgvin = 16 m/svout = 20 m/sΔt = 10 ms
Favg = N
13.Springs & pendulumsMedium
A simple pendulum has a length of 2.9 m. Use g = 9.81 m/s². Find its period T for small swings.
L = 2.9 mg = 9.81 m/s²
T = s
14.Springs & pendulumsHard
A 3.1 kg mass on a spring with k = 120 N/m is pulled 18 cm from equilibrium and released. Find its maximum acceleration amax.
m = 3.1 kgk = 120 N/mA = 18 cm
amax = m/s²
15.Waves, sound & opticsMedium
An object stands 4 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 = 4 cmf = 20 cm
di = cm
16.Waves, sound & opticsHard
Two thin lenses share an axis, 10 cm apart: lens 1 with f₁ = 8 cm and lens 2 with f₂ = 5 cm. An object stands 20 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 = 10 cmdo = 20 cm
di2 = cm
3Solutions
1.Kinematics
Answer: t = 6.8 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.
Equal distances
v·t = ½·a·t² → t = 2v/a = 2·17/5 = 6.8 s
Answer
t = 6.8 s
At that moment the police car moves at 2v = 34 m/s.
2.Kinematics
Answer: t = 28.75 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 = 1.5·5 = 7.5 m/s
Braking time
t3 = v/a3 = 7.5/2 = 3.75 s
Total time
t = 5 + 20 + 3.75 = 28.75 s
Answer
t = 28.75 s
3.Newton’s laws
Answer: a = 1.073 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.
Normal force
N = m·g − F·sin θ = 392.4 − 130·sin 35° = 317.8 N
Horizontal forces
m·a = F·cos θ − μ·N = 106.5 − 63.57
a = 42.92/40 = 1.073 m/s²
Answer
a = 1.073 m/s²
4.Newton’s laws
Answer: T = 50.03 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 = 98.1 N
m₁·g·sin θ = 16.78 N
μ·m₁·g·cos θ = 9.218 N
Acceleration of the system
a = (98.1 − 16.78 − 9.218)/15 = 4.807 m/s²
Tension from the hanging mass
m2·g − T = m2·a → T = 10·(9.81 − 4.807) = 50.03 N
Answer
T = 50.03 N
5.Projectile motion
Answer: y = 16.51 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.
Time to the wall
vx = 24·cos 50° = 15.43 m/s
t = x/vx = 23/15.43 = 1.491 s
Height at that time
y = 18.39·1.491 − ½·9.81·1.491² = 16.51 m
Answer
y = 16.51 m
The ball is still rising at the wall.
6.Projectile motion
Answer: x = 44.38 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 = 12·cos 15° = 11.59 m/s
vy = v0·sin θ = 12·sin 15° = 3.106 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 60 + 3.106·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (3.106 + √(9.646 + 1177))/9.81 = 3.828 s
Horizontal distance
x = vx·t = 11.59·3.828 = 44.38 m
Answer
x = 44.38 m
7.Circular motion
Answer: v = 27.02 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) = √(350·9.81·tan 12°) = 27.02 m/s
Answer
v = 27.02 m/s
(97.25 km/h)
8.Circular motion
Answer: T = 121.2 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 + 1.9·9²/1.5 = 18.64 + 102.6 = 121.2 N
Answer
T = 121.2 N
9.Work & energy
Answer: v = 6.752 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 = √(2·9.81·7·(0.4226 − 0.1·0.9063)) = 6.752 m/s
Answer
v = 6.752 m/s
10.Work & energy
Answer: x = 42.02 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 = 39.24·(4 − 0.25·2.5) = 132.4 J
Maximum compression
½kx² = 132.4 → x = √(2·132.4/1500) = 0.4202 m = 42.02 cm
Answer
x = 42.02 cm
11.Momentum & collisions
Answer: v = 315.1 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) = √(2·9.81·0.18) = 1.879 m/s
Collision
m·v = (m + M)·V → v = (0.012 + 2)·1.879/0.012 = 315.1 m/s
Answer
v = 315.1 m/s
12.Momentum & collisions
Answer: Favg = 1620 N
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 = 0.45·(20 + 16) = 16.2 N·s
Average force
Favg = J/Δt = 16.2/0.01 = 1620 N
Answer
Favg = 1620 N
13.Springs & pendulums
Answer: T = 3.416 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·√(2.9/9.81) = 3.416 s
Answer
T = 3.416 s
14.Springs & pendulums
Answer: amax = 6.968 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 = √(120/3.1) = 6.222 rad/s
Maximum acceleration
amax = A·ω² = 0.18·6.222² = 6.968 m/s²
Answer
amax = 6.968 m/s²
15.Waves, sound & optics
Answer: di = −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.
Thin-lens equation
1/di = 1/f − 1/do = 1/20 − 1/4
di = −5 cm
Ray diagram
virtual, on the same side as the object
Answer
di = −5 cm
16.Waves, sound & optics
Answer: di2 = 2 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.