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 50 km/h brakes with a constant deceleration of 5.5 m/s² until it stops. Find the braking distance s.
v₀ = 50 km/ha = −5.5 m/s²v = 0
s = m
2.KinematicsHard
Ball A is dropped from rest from a height of 20 m. At the same instant ball B is thrown straight up from the ground directly below at 24 m/s. Ignore air resistance and use g = 9.81 m/s². At what height y above the ground do they meet?
H = 20 mv0B = 24 m/sg = 9.81 m/s²
y = m
3.Newton’s lawsMedium
A box slides across a level floor with an initial speed of 6.5 m/s and comes to rest after 22 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.
v₀ = 6.5 m/sd = 22 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
An 8.5 kg block on a ramp at 25° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 9 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.15. Use g = 9.81 m/s². Find the tension T in the string.
m1 = 8.5 kgm2 = 9 kgθ = 25°μ = 0.15g = 9.81 m/s²
T = N
5.Projectile motionMedium
A ball is launched at 65° above horizontal from level ground. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 32 m/sθ = 65°g = 9.81 m/s²
x = m
6.Projectile motionHard
A ball is launched at 50° above horizontal from the top of a 55 m cliff. Ignore air resistance. Find the time of flight t until the ball hits the ground.
v0 = 20 m/sθ = 50°h = 55 mg = 9.81 m/s²
t = s
7.Circular motionMedium
A car takes a flat curve of radius 115 m. 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 the car can take the curve without skidding.
r = 115 mμs = 0.3g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 1.5 kg ball on a light string 0.7 m long is swung in a vertical circle. At the lowest point its speed is 6.5 m/s. Ignore air resistance and use g = 9.81 m/s². Find the tension Ttop in the string at the highest point.
m = 1.5 kgr = 0.7 mvbottom = 6.5 m/sg = 9.81 m/s²
Ttop = N
9.Work & energyMedium
A 2.5 kg block slides 6 m down a slope at 35° to the horizontal, with a coefficient of kinetic friction of 0.4. Find the work Wf done against friction.
m = 2.5 kgθ = 35°μ = 0.4d = 6 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
A 2 kg block starts from rest at the bottom of a 20° slope and is pulled up the slope by a constant force of 25 N parallel to the slope. The coefficient of kinetic friction is 0.3. Find the speed v of the block after it has moved 2 m up the slope.
A 1400 kg car moving at 17 m/s runs into a stationary 2000 kg car and the two lock together. How much kinetic energy ΔK is lost in the collision?
m1 = 1400 kgv1 = 17 m/sm2 = 2000 kgv2 = 0
ΔK = J
12.Momentum & collisionsHard
An 8 g bullet is fired horizontally into a 4 kg block hanging from light strings 0.5 m long, and stays in it. The strings swing out to a largest angle of 25° from the vertical. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 8 gM = 4 kgL = 0.5 mθ = 25°g = 9.81 m/s²
v = m/s
13.Springs & pendulumsMedium
A 3.5 kg mass on a spring with k = 170 N/m oscillates on a frictionless surface with an amplitude of 13 cm. Find its speed v when it is 7 cm from equilibrium.
m = 3.5 kgk = 170 N/mA = 13 cmx = 7 cm
v = m/s
14.Springs & pendulumsHard
A 5 g bullet moving at 400 m/s hits and stays in a 2 kg block resting on a frictionless surface. The block is attached to a horizontal spring with k = 400 N/m. Find the amplitude A of the resulting oscillation in cm.
m = 5 gv = 400 m/sM = 2 kgk = 400 N/m
A = cm
15.Waves, sound & opticsMedium
An object 2 cm tall stands 12 cm in front of a thin converging lens with a focal length of 30 cm. Find the image height hi (negative for an inverted image).
ho = 2 cmdo = 12 cmf = 30 cm
hi = cm
16.Waves, sound & opticsHard
Two thin lenses share an axis, 30 cm apart: lens 1 with f₁ = 20 cm and lens 2 with f₂ = 5 cm. An object stands 30 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 = 20 cmf2 = 5 cms = 30 cmdo = 30 cm
di2 = cm
3Solutions
1.Kinematics
Answer: s = 17.54 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 = 50/3.6 = 13.89 m/s
Braking distance
0 = v₀² − 2·a·s → s = v₀²/(2a) = 13.89²/(2·5.5) = 17.54 m
Answer
s = 17.54 m
2.Kinematics
Answer: y = 16.59 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.
Meeting time
Up is positive, the ground is y = 0.
20 − ½g·t² = 24·t − ½g·t² → t = 20/24 = 0.8333 s
Meeting height
y = 24·0.8333 − ½·9.81·0.8333² = 16.59 m
Answer
y = 16.59 m
3.Newton’s laws
Answer: μ = 0.09788
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) = 6.5²/(2·22) = 0.9602 m/s²
Friction coefficient
μ·m·g = m·a → μ = a/g = 0.9602/9.81 = 0.09788
Answer
μ = 0.09788
4.Newton’s laws
Answer: T = 66.84 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 = 88.29 N
m₁·g·sin θ = 35.24 N
μ·m₁·g·cos θ = 11.34 N
Acceleration of the system
a = (88.29 − 35.24 − 11.34)/17.5 = 2.384 m/s²
Tension from the hanging mass
m2·g − T = m2·a → T = 9·(9.81 − 2.384) = 66.84 N
Answer
T = 66.84 N
5.Projectile motion
Answer: x = 79.96 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 = 32·cos 65° = 13.52 m/s
vy = v0·sin θ = 32·sin 65° = 29 m/s
Time in the air
Starting and landing at the same height: t = 2vy/g.
t = 2·29/9.81 = 5.913 s
Horizontal distance
x = vx·t = 13.52·5.913 = 79.96 m
Answer
x = 79.96 m
6.Projectile motion
Answer: t = 5.257 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 50° = 12.86 m/s
vy = v0·sin θ = 20·sin 50° = 15.32 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 55 + 15.32·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (15.32 + √(234.7 + 1079))/9.81 = 5.257 s
Answer
t = 5.257 s
7.Circular motion
Answer: vmax = 18.4 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) = √(0.3·9.81·115) = 18.4 m/s
Answer
vmax = 18.4 m/s (66.23 km/h)
8.Circular motion
Answer: Ttop = 16.96 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 at the top
Kinetic energy is traded for potential energy over a height 2r.
vtop² = 6.5² − 4·9.81·0.7 = 14.78 m²/s²
Forces at the top
Tension and gravity both point down, towards the centre.
T + m·g = m·vtop²/r
T = 1.5·14.78/0.7 − 1.5·9.81 = 16.96 N
Answer
Ttop = 16.96 N
9.Work & energy
Answer: Wf = 48.22 J
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).
Friction force
N = mg·cos θ = 24.53·0.8192 = 20.09 N
F = μN = 0.4·20.09 = 8.036 N
Work against friction
Wf = F·d = 8.036·6 = 48.22 J
Answer
Wf = 48.22 J
10.Work & energy
Answer: v = 5.051 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).
Forces along the slope
m·g = 2·9.81 = 19.62 N
mg·sin θ = 6.71 N
μ·mg·cos θ = 5.531 N
Work–energy
½mv² = (25 − 12.24)·2 = 25.52 J
v = √(2·25.52/2) = 5.051 m/s
Answer
v = 5.051 m/s
11.Momentum & collisions
Answer: ΔK = 119000 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) = 1400·17/3400 = 7 m/s
Kinetic energy before and after
Kbefore = ½·1400·17² = 202300 J
Kafter = ½·3400·7² = 83300 J
Energy lost
ΔK = 202300 − 83300 = 119000 J
Answer
ΔK = 119000 J
The lost energy goes into deformation, heat and sound.
12.Momentum & collisions
Answer: v = 480.3 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) = 0.5·(1 − cos 25°) = 0.04685 m
Speed after impact
V = √(2·9.81·0.04685) = 0.9587 m/s
Collision
v = (m + M)·V/m = 4.008·0.9587/0.008 = 480.3 m/s
Answer
v = 480.3 m/s
13.Springs & pendulums
Answer: v = 0.7635 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 = √(170/3.5) = 6.969 rad/s
Speed at x
v = ω·√(A² − x²) = 6.969·√(0.13² − 0.07²) = 0.7635 m/s
Answer
v = 0.7635 m/s
14.Springs & pendulums
Answer: A = 7.062 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.
Speed after impact
V = m·v/(m + M) = 0.005·400/2.005 = 0.9975 m/s
Angular frequency
ω = √(k/(m + M)) = √(400/2.005) = 14.12 rad/s
Amplitude
A = V/ω = 0.9975/14.12 = 0.07062 m = 7.062 cm
Answer
A = 7.062 cm
15.Waves, sound & optics
Answer: hi = 3.333 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 = 1/30 − 1/12
di = −20 cm
Magnification and height
m = −(−20)/12 = 1.667
hi = 1.667·2 = 3.333 cm
Answer
hi = 3.333 cm
16.Waves, sound & optics
Answer: di2 = 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.