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 4.5 m/s² until it stops. How long does braking take? Find t.
v₀ = 50 km/ha = −4.5 m/s²v = 0
t = s
2.KinematicsHard
A tram starts from rest and accelerates at 2.5 m/s² for 4 s, then runs at constant speed for 19 s, and finally brakes at 2.5 m/s² until it stops. Find the total travel time t.
a1 = 2.5 m/s²t1 = 4 st2 = 19 sa3 = −2.5 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 tension T in the string.
m1 = 5 kgm2 = 4.5 kgg = 9.81 m/s²
T = N
4.Newton’s lawsHard
A block is given a push so that it starts sliding up a ramp at 35° with a speed of 9 m/s. The coefficient of kinetic friction is 0.1. Use g = 9.81 m/s². How far d along the ramp does it slide before it stops?
v₀ = 9 m/sθ = 35°μ = 0.1g = 9.81 m/s²
d = m
5.Projectile motionMedium
A ball is kicked from level ground at 36 m/s and 30° above the horizontal towards a wall 69 m away. Ignore air resistance and use g = 9.81 m/s². At what height y does the ball pass the wall?
v₀ = 36 m/sθ = 30°x = 69 mg = 9.81 m/s²
y = m
6.Projectile motionHard
A ball is launched at 17 m/s and 35° above the horizontal from the top of a 30 m cliff. Ignore air resistance and use g = 9.81 m/s². At what angle α below the horizontal does it hit the ground?
v₀ = 17 m/sθ = 35°h = 30 mg = 9.81 m/s²
α = °
7.Circular motionMedium
A car takes a flat curve of radius 100 m. The coefficient of static friction between tyres and road is 0.8. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 100 mμs = 0.8g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 0.5 kg cart is released from rest at a height of 12.95 m on a frictionless track and runs through a vertical loop of radius 3.5 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 = 0.5 kgh = 12.95 mR = 3.5 mg = 9.81 m/s²
Ntop = N
9.Work & energyMedium
A 1200 kg car drives up an 8° slope at a constant 90 km/h. Air resistance and rolling resistance together amount to 700 N. Find the power P the wheels must deliver, in kW.
A 4 kg block is released from rest at height 3.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 1.5 m long (μ = 0.2) and then runs onto a spring (k = 800 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 4 kgh = 3.5 mL = 1.5 mμ = 0.2k = 800 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 19 kg projectile moving horizontally at 13 m/s explodes into two fragments that move along the same line. The 16 kg fragment continues forwards at 26 m/s. Find the velocity v′2 of the 3 kg fragment (positive in the original direction).
M = 19 kgu = 13 m/sm1 = 16 kgv′1 = 26 m/sm2 = 3 kg
v′2 = m/s
12.Momentum & collisionsHard
A 0.45 kg ball hits a wall at 16 m/s and bounces straight back at 9 m/s. The contact lasts 5 ms. Find the magnitude of the impulse J the wall gives the ball.
m = 0.45 kgvin = 16 m/svout = 9 m/sΔt = 5 ms
J = N·s
13.Springs & pendulumsMedium
An astronaut on another planet finds that a simple pendulum 1.4 m long has a period of 3.86 s for small swings. Find the gravitational acceleration g on that planet.
L = 1.4 mT = 3.86 s
g = m/s²
14.Springs & pendulumsHard
A 2.6 kg mass on a spring with k = 370 N/m is pulled 7 cm from equilibrium and released. Find the total energy E of the oscillation.
m = 2.6 kgk = 370 N/mA = 7 cm
E = J
15.Waves, sound & opticsMedium
An object stands 15 cm in front of a thin converging lens with a focal length of 8 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 = 15 cmf = 8 cm
di = cm
16.Waves, sound & opticsHard
An object is 12 cm in front of a concave mirror with a radius of curvature of 30 cm. Find the magnification m (negative for an inverted image).
do = 12 cmR = 30 cm
m =
3Solutions
1.Kinematics
Answer: t = 3.086 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.
Units
v₀ = 50/3.6 = 13.89 m/s
Braking time
0 = v₀ − a·t → t = v₀/a = 13.89/4.5 = 3.086 s
Answer
t = 3.086 s
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 = 2.5·4 = 10 m/s
Braking time
t3 = v/a3 = 10/2.5 = 4 s
Total time
t = 4 + 19 + 4 = 27 s
Answer
t = 27 s
3.Newton’s laws
Answer: T = 46.47 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.
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
Tension from the lighter mass
T − m2·g = m2·a → T = 4.5·(9.81 + 0.5163) = 46.47 N
Answer
T = 46.47 N
4.Newton’s laws
Answer: d = 6.298 m
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
Both forces point down the slope; the mass cancels.
a = 9.81·(sin 35° + 0.1·cos 35°) = 6.43 m/s²
Distance
d = v₀²/(2a) = 9²/(2·6.43) = 6.298 m
Answer
d = 6.298 m
5.Projectile motion
Answer: y = 15.81 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 = 36·cos 30° = 31.18 m/s
t = x/vx = 69/31.18 = 2.213 s
Height at that time
y = 18·2.213 − ½·9.81·2.213² = 15.81 m
Answer
y = 15.81 m
The ball is already on its way down at the wall.
6.Projectile motion
Answer: α = 61.96 °
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
vx = 17·cos 35° = 13.93 m/s
vy0 = 17·sin 35° = 9.751 m/s
Vertical speed at impact
Energy or v² = v₀² + 2as in the vertical direction.
vy = √(9.751² + 2·9.81·30) = 26.15 m/s
Impact angle
tan α = vy/vx = 26.15/13.93 → α = 61.96°
Answer
α = 61.96°
7.Circular motion
Answer: vmax = 28.01 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) = √(0.8·9.81·100) = 28.01 m/s
Answer
vmax = 28.01 m/s (100.9 km/h)
8.Circular motion
Answer: Ntop = 11.77 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·(12.95 − 2·3.5) = 116.7 m²/s²
Forces
Towards the centre is positive.
N = m·v²/R − m·g = 0.5·116.7/3.5 − 4.905 = 11.77 N
Answer
Ntop = 11.77 N
9.Work & energy
Answer: P = 58.46 kW
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). Power is force times speed: P = F·v.
Driving force
m·g = 1200·9.81 = 11772 N
mg·sin θ = 11772·sin 8° = 1638 N
F = 1638 + 700 = 2338 N
Power
v = 90/3.6 = 25 m/s
P = F·v = 2338·25 = 58459 W = 58.46 kW
Answer
P = 58.46 kW
10.Work & energy
Answer: x = 56.03 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·(3.5 − 0.2·1.5) = 125.6 J
Maximum compression
½kx² = 125.6 → x = √(2·125.6/800) = 0.5603 m = 56.03 cm
Answer
x = 56.03 cm
11.Momentum & collisions
Answer: v′2 = −56.33 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 = 19·13 = 247 kg·m/s
Momentum after
247 = 16·26 + 3·v′2
v′2 = (247 − 416)/3 = −56.33 m/s
Answer
v′2 = −56.33 m/s
The minus sign means the second fragment flies backwards.
12.Momentum & collisions
Answer: J = 11.25 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 = 0.45·(9 + 16) = 11.25 N·s
Answer
J = 11.25 N·s
13.Springs & pendulums
Answer: g = 3.709 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.
Solve for g
g = 4π²·L/T² = 4π²·1.4/3.86² = 3.709 m/s²
Answer
g = 3.709 m/s²
14.Springs & pendulums
Answer: E = 0.9065 J
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 = √(370/2.6) = 11.93 rad/s
Energy
E = ½·k·A² = ½·370·0.07² = 0.9065 J
Answer
E = 0.9065 J
15.Waves, sound & optics
Answer: di = 17.14 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 = 1/8 − 1/15
di = 17.14 cm
Ray diagram
real, on the far side of the lens
Answer
di = 17.14 cm
16.Waves, sound & optics
Answer: m = 5
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.