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 3 m/s² until it stops. Find the braking distance s.
v₀ = 36 km/ha = −3 m/s²v = 0
s = m
2.KinematicsHard
A tram starts from rest and accelerates at 2.5 m/s² for 12 s, then runs at constant speed for 9 s, and finally brakes at 5 m/s² until it stops. Find the total travel time t.
a1 = 2.5 m/s²t1 = 12 st2 = 9 sa3 = −5 m/s²
t = s
3.Newton’s lawsMedium
Masses of 5 kg and 4.5 kg hang on either side of a light, frictionless pulley, connected by a light string. The system is released from rest. Use g = 9.81 m/s². Find the acceleration a of the masses.
m1 = 5 kgm2 = 4.5 kgg = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
A 5.5 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 2 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 = 5.5 kgm2 = 2 kgμ = 0.2g = 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 maximum height H the ball reaches.
v0 = 20 m/sθ = 65°g = 9.81 m/s²
H = m
6.Projectile motionHard
A ball is launched at 31 m/s and 25° above the horizontal from the top of a 10 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 = 31 m/sθ = 25°h = 10 mg = 9.81 m/s²
H = m
7.Circular motionMedium
A car takes a flat curve of radius 20 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 = 20 mμs = 0.5g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 0.6 kg ball on a light string 0.5 m long is swung in a vertical circle. At the lowest point its speed is 5.5 m/s. Ignore air resistance and use g = 9.81 m/s². Find the tension Ttop in the string at the highest point.
m = 0.6 kgr = 0.5 mvbottom = 5.5 m/sg = 9.81 m/s²
Ttop = N
9.Work & energyMedium
A 2 kg block slides 15 m down a slope at 40° to the horizontal, with a coefficient of kinetic friction of 0.1. Find the work Wf done against friction.
m = 2 kgθ = 40°μ = 0.1d = 15 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
A 20 kg block starts from rest at the bottom of a 25° slope and is pulled up the slope by a constant force of 165 N parallel to the slope. The coefficient of kinetic friction is 0.1. Find the speed v of the block after it has moved 2 m up the slope.
A 12 g bullet is fired horizontally into a 3.5 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 6 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 12 gM = 3.5 kgh = 6 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
At a crossing a 2000 kg car travelling east at 8 m/s collides with a 1300 kg car travelling north at 18 m/s. The cars lock together. Find the speed v′ of the wreck just after the collision.
m1 = 2000 kgv1 = 8 m/s (E)m2 = 1300 kgv2 = 18 m/s (N)
v′ = m/s
13.Springs & pendulumsMedium
How long must a simple pendulum be to have a period of 1.3 s? Use g = 9.81 m/s². Find L.
T = 1.3 sg = 9.81 m/s²
L = m
14.Springs & pendulumsHard
An 8 g bullet moving at 200 m/s hits and stays in a 3 kg block resting on a frictionless surface. The block is attached to a horizontal spring with k = 1000 N/m. Find the amplitude A of the resulting oscillation in cm.
m = 8 gv = 200 m/sM = 3 kgk = 1000 N/m
A = cm
15.Waves, sound & opticsMedium
An object 4 cm tall stands 4 cm in front of a thin converging lens with a focal length of 25 cm. Find the image height hi (negative for an inverted image).
ho = 4 cmdo = 4 cmf = 25 cm
hi = cm
16.Waves, sound & opticsHard
An object is 30 cm in front of a concave mirror with a radius of curvature of 20 cm. Find the image distance di, positive for an image in front of the mirror (real) and negative for one behind it (virtual).
do = 30 cmR = 20 cm
di = cm
3Solutions
1.Kinematics
Answer: s = 16.67 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/(2·3) = 16.67 m
Answer
s = 16.67 m
2.Kinematics
Answer: t = 27 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 = 2.5·12 = 30 m/s
Braking time
t3 = v/a3 = 30/5 = 6 s
Total time
t = 12 + 9 + 6 = 27 s
Answer
t = 27 s
3.Newton’s laws
Answer: a = 0.5163 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.
Acceleration
The net driving force is the difference of the weights; it moves the total mass.
a = (5 − 4.5)·9.81/(5 + 4.5) = 0.5163 m/s²
Answer
a = 0.5163 m/s²
4.Newton’s laws
Answer: T = 17.27 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 = 20·cos 65° = 8.452 m/s
vy = v0·sin θ = 20·sin 65° = 18.13 m/s
Highest point
At the top the vertical velocity is zero: vy² = 2gH.
H = vy²/(2g) = 18.13²/(2·9.81) = 16.75 m
Answer
H = 16.75 m
6.Projectile motion
Answer: H = 18.75 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 = 31·sin 25° = 13.1 m/s
Rise above the cliff
Δh = vy²/(2g) = 13.1²/(2·9.81) = 8.748 m
Height above the ground
H = 10 + 8.748 = 18.75 m
Answer
H = 18.75 m
7.Circular motion
Answer: vmax = 9.905 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) = √(0.5·9.81·20) = 9.905 m/s
Answer
vmax = 9.905 m/s (35.66 km/h)
8.Circular motion
Answer: Ttop = 6.87 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 = 5.5² − 4·9.81·0.5 = 10.63 m²/s²
Forces at the top
Tension and gravity both point down, towards the centre.
T + m·g = m·vtop²/r
T = 0.6·10.63/0.5 − 0.6·9.81 = 6.87 N
Answer
Ttop = 6.87 N
9.Work & energy
Answer: Wf = 22.54 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 = 19.62·0.766 = 15.03 N
F = μN = 0.1·15.03 = 1.503 N
Work against friction
Wf = F·d = 1.503·15 = 22.54 J
Answer
Wf = 22.54 J
10.Work & energy
Answer: v = 3.586 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 = 20·9.81 = 196.2 N
mg·sin θ = 82.92 N
μ·mg·cos θ = 17.78 N
Work–energy
½mv² = (165 − 100.7)·2 = 128.6 J
v = √(2·128.6/20) = 3.586 m/s
Answer
v = 3.586 m/s
11.Momentum & collisions
Answer: v = 317.5 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) = √(2·9.81·0.06) = 1.085 m/s
Collision
m·v = (m + M)·V → v = (0.012 + 3.5)·1.085/0.012 = 317.5 m/s
Answer
v = 317.5 m/s
12.Momentum & collisions
Answer: v′ = 8.59 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.
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.
Solve for the length
L = g·(T/2π)² = 9.81·(1.3/2π)² = 0.4199 m
Answer
L = 0.4199 m
14.Springs & pendulums
Answer: A = 2.917 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) = 0.008·200/3.008 = 0.5319 m/s
Angular frequency
ω = √(k/(m + M)) = √(1000/3.008) = 18.23 rad/s
Amplitude
A = V/ω = 0.5319/18.23 = 0.02917 m = 2.917 cm
Answer
A = 2.917 cm
15.Waves, sound & optics
Answer: hi = 4.762 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 = 1/25 − 1/4
di = −4.762 cm
Magnification and height
m = −(−4.762)/4 = 1.19
hi = 1.19·4 = 4.762 cm
Answer
hi = 4.762 cm
16.Waves, sound & optics
Answer: di = 15 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.