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 36 km/h brakes with a constant deceleration of 4.5 m/s² until it stops. Find the braking distance s.
v₀ = 36 km/ha = −4.5 m/s²v = 0
s = m
2.KinematicsHard
A ball is thrown straight up at 9 m/s from the edge of a roof 30 m above the ground. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax above the ground.
v₀ = 9 m/sh₀ = 30 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
A 9 kg block slides down a frictionless slope at 35° to the horizontal. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.
m = 9 kgθ = 35°μ = 0g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
A 4.5 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 3 kg mass. The coefficient of kinetic friction between block and table is 0.1. Use g = 9.81 m/s². Find the acceleration a of the system.
m1 = 4.5 kgm2 = 3 kgμ = 0.1g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
A ball is kicked from level ground at 20 m/s and 25° above the horizontal towards a wall 9 m away. Ignore air resistance and use g = 9.81 m/s². At what height y does the ball pass the wall?
v₀ = 20 m/sθ = 25°x = 9 mg = 9.81 m/s²
y = m
6.Projectile motionHard
A ball is launched at 35° above horizontal from the top of a 15 m cliff. Ignore air resistance. Find the time of flight t until the ball hits the ground.
v0 = 20 m/sθ = 35°h = 15 mg = 9.81 m/s²
t = s
7.Circular motionMedium
A car takes a flat curve of radius 50 m. The coefficient of static friction between tyres and road is 0.5. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 50 mμs = 0.5g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 3.5 kg cart is released from rest at a height of 20.15 m on a frictionless track and runs through a vertical loop of radius 6.5 m. Use g = 9.81 m/s². Find the normal force Ntop of the track on the cart at the top of the loop.
m = 3.5 kgh = 20.15 mR = 6.5 mg = 9.81 m/s²
Ntop = N
9.Work & energyMedium
A car travelling at 40 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 0.6. Find the stopping distance d.
v = 40 km/hμ = 0.6g = 9.81 m/s²
d = m
10.Work & energyHard
A 2.5 kg block is released from rest at height 3 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 4 m long (μ = 0.1) and then runs onto a spring (k = 1000 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 2.5 kgh = 3 mL = 4 mμ = 0.1k = 1000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 12 kg projectile moving horizontally at 40 m/s explodes into two fragments that move along the same line. The 6 kg fragment continues forwards at 86 m/s. Find the velocity v′2 of the 6 kg fragment (positive in the original direction).
M = 12 kgu = 40 m/sm1 = 6 kgv′1 = 86 m/sm2 = 6 kg
v′2 = m/s
12.Momentum & collisionsHard
A 0.1 kg ball hits a wall at 21 m/s and bounces straight back at 9 m/s. The contact lasts 10 ms. Find the average force Favg of the wall on the ball.
m = 0.1 kgvin = 21 m/svout = 9 m/sΔt = 10 ms
Favg = N
13.Springs & pendulumsMedium
A 1.2 kg mass on a spring with k = 260 N/m oscillates on a frictionless surface with an amplitude of 7 cm. Find its speed v when it is 1 cm from equilibrium.
m = 1.2 kgk = 260 N/mA = 7 cmx = 1 cm
v = m/s
14.Springs & pendulumsHard
A 2.4 kg mass on a spring with k = 40 N/m is pulled 11 cm from equilibrium and released. Find its maximum speed vmax.
m = 2.4 kgk = 40 N/mA = 11 cm
vmax = m/s
15.Waves, sound & opticsMedium
An object stands 6 cm in front of a thin diverging lens with a focal length of 15 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 = 6 cmf = −15 cm
di = cm
16.Waves, sound & opticsHard
An object is 4 cm in front of a concave mirror with a radius of curvature of 60 cm. Find the image distance di, positive for an image in front of the mirror (real) and negative for one behind it (virtual).
do = 4 cmR = 60 cm
di = cm
3Solutions
1.Kinematics
Answer: s = 11.11 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
Braking distance
0 = v₀² − 2·a·s → s = v₀²/(2a) = 10²/(2·4.5) = 11.11 m
Answer
s = 11.11 m
2.Kinematics
Answer: hmax = 34.13 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 = 9²/(2·9.81) = 4.128 m
Height above the ground
hmax = 30 + 4.128 = 34.13 m
Answer
hmax = 34.13 m
3.Newton’s laws
Answer: a = 5.627 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.
Time to the wall
vx = 20·cos 25° = 18.13 m/s
t = x/vx = 9/18.13 = 0.4965 s
Height at that time
y = 8.452·0.4965 − ½·9.81·0.4965² = 2.988 m
Answer
y = 2.988 m
The ball is still rising at the wall.
6.Projectile motion
Answer: t = 3.273 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 = 20·cos 35° = 16.38 m/s
vy = v0·sin θ = 20·sin 35° = 11.47 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 15 + 11.47·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (11.47 + √(131.6 + 294.3))/9.81 = 3.273 s
Answer
t = 3.273 s
7.Circular motion
Answer: vmax = 15.66 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) = √(0.5·9.81·50) = 15.66 m/s
Answer
vmax = 15.66 m/s (56.38 km/h)
8.Circular motion
Answer: Ntop = 41.2 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.
Speed from energy conservation
v² = 2·9.81·(20.15 − 2·6.5) = 140.3 m²/s²
Forces
Towards the centre is positive.
N = m·v²/R − m·g = 3.5·140.3/6.5 − 34.34 = 41.2 N
Answer
Ntop = 41.2 N
9.Work & energy
Answer: d = 10.49 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 = 40/3.6 = 11.11 m/s
Energy balance
½mv² = μmg·d → d = v²/(2μg) = 11.11²/(2·0.6·9.81) = 10.49 m
Answer
d = 10.49 m
Doubling the speed makes the stopping distance four times longer.
10.Work & energy
Answer: x = 35.71 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·(3 − 0.1·4) = 63.77 J
Maximum compression
½kx² = 63.77 → x = √(2·63.77/1000) = 0.3571 m = 35.71 cm
Answer
x = 35.71 cm
11.Momentum & collisions
Answer: v′2 = −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.
Momentum before
p = M·u = 12·40 = 480 kg·m/s
Momentum after
480 = 6·86 + 6·v′2
v′2 = (480 − 516)/6 = −6 m/s
Answer
v′2 = −6 m/s
The minus sign means the second fragment flies backwards.
12.Momentum & collisions
Answer: Favg = 300 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 = 0.1·(9 + 21) = 3 N·s
Average force
Favg = J/Δt = 3/0.01 = 300 N
Answer
Favg = 300 N
13.Springs & pendulums
Answer: v = 1.02 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.
Angular frequency
ω = √(260/1.2) = 14.72 rad/s
Speed at x
v = ω·√(A² − x²) = 14.72·√(0.07² − 0.01²) = 1.02 m/s
Answer
v = 1.02 m/s
14.Springs & pendulums
Answer: vmax = 0.4491 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.
Angular frequency
ω = √(40/2.4) = 4.082 rad/s
Maximum speed
vmax = A·ω = 0.11·4.082 = 0.4491 m/s
Answer
vmax = 0.4491 m/s
15.Waves, sound & optics
Answer: di = −4.286 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/(−15) − 1/6
di = −4.286 cm
Ray diagram
virtual, on the same side as the object
Answer
di = −4.286 cm
16.Waves, sound & optics
Answer: di = −4.615 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.