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 80 km/h brakes with a constant deceleration of 8 m/s² until it stops. How long does braking take? Find t.
v₀ = 80 km/ha = −8 m/s²v = 0
t = s
2.KinematicsHard
A model rocket is launched straight up from rest. Its engine gives it a constant upward acceleration of 34 m/s² for 3 s, after which the engine stops and the rocket moves freely. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax the rocket reaches.
a = 34 m/s²t1 = 3 sg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
A 50 kg box is pushed up a ramp at 25° to the horizontal at constant speed, by a force parallel to the ramp. The coefficient of kinetic friction is 0.1. Use g = 9.81 m/s². Find the pushing force F.
m = 50 kgθ = 25°μ = 0.1g = 9.81 m/s²
F = N
4.Newton’s lawsHard
A 10 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 4.5 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 = 10 kgm2 = 4.5 kgμ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
A ball is launched from level ground at 30 m/s and must land 70 m away. Ignore air resistance and use g = 9.81 m/s². Find the smaller of the two possible launch angles θ.
v₀ = 30 m/sR = 70 mg = 9.81 m/s²
θ = °
6.Projectile motionHard
A ball is launched at 40° above horizontal from the top of a 5 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 29 m/sθ = 40°h = 5 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A car takes a flat curve of radius 115 m. The coefficient of static friction between tyres and road is 0.4. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 115 mμs = 0.4g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A curve of radius 170 m is banked at 8°, and the coefficient of static friction between tyres and road is 0.4. Use g = 9.81 m/s². Find the highest speed vmax at which a car can take the curve without sliding up and out.
r = 170 mθ = 8°μs = 0.4g = 9.81 m/s²
vmax = m/s
9.Work & energyMedium
A block slides from rest down a slope at 45° to the horizontal. The coefficient of kinetic friction is 0.4. Find its speed v after it has slid 18 m along the slope.
θ = 45°μ = 0.4d = 18 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
A 5 kg block is released from rest at height 3.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 4 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 = 5 kgh = 3.5 mL = 4 mμ = 0.2k = 800 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 1 kg glider moving at 9 m/s makes a perfectly elastic head-on collision with a 3 kg glider at rest on a frictionless track. Find the velocity v′2 of the second glider after the collision.
m1 = 1 kgm2 = 3 kgv1 = 9 m/sv2 = 0
v′2 = m/s
12.Momentum & collisionsHard
A 0.4 kg ball hits a wall at 24 m/s and bounces straight back at 4 m/s. The contact lasts 10 ms. Find the average force Favg of the wall on the ball.
m = 0.4 kgvin = 24 m/svout = 4 m/sΔt = 10 ms
Favg = N
13.Springs & pendulumsMedium
A 0.3 kg mass on a spring with k = 380 N/m is pulled 10 cm in the positive direction and released from rest at t = 0. Find its velocity v at t = 0.85 s (with sign).
m = 0.3 kgk = 380 N/mA = 10 cmt = 0.85 s
v = m/s
14.Springs & pendulumsHard
A 0.7 kg mass is attached to the lower end of an unstretched vertical spring with k = 300 N/m and released from rest. Use g = 9.81 m/s². Find the greatest speed vmax of the mass.
m = 0.7 kgk = 300 N/mg = 9.81 m/s²
vmax = m/s
15.Waves, sound & opticsMedium
Light travels in flint glass (n = 1.62) towards a flat boundary with glycerol (n = 1.47). Find the critical angle θc beyond which it is totally reflected.
n1 = 1.62n2 = 1.47
θc = °
16.Waves, sound & opticsHard
An object is 5 cm in front of a convex mirror with a radius of curvature of 16 cm. Find the image distance di, positive for an image in front of the mirror (real) and negative for one behind it (virtual).
do = 5 cmR = 16 cm
di = cm
3Solutions
1.Kinematics
Answer: t = 2.778 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 = 80/3.6 = 22.22 m/s
Braking time
0 = v₀ − a·t → t = v₀/a = 22.22/8 = 2.778 s
Answer
t = 2.778 s
2.Kinematics
Answer: hmax = 683.3 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.
Powered phase
v₁ = 34·3 = 102 m/s
h₁ = ½·34·3² = 153 m
Free flight up
At the top the velocity is zero.
Δh = v₁²/(2g) = 102²/(2·9.81) = 530.3 m
hmax = 153 + 530.3 = 683.3 m
Answer
hmax = 683.3 m
3.Newton’s laws
Answer: F = 251.7 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 slope
Constant speed: the forces balance.
F = m·g·sin θ + μ·m·g·cos θ
Pushing force
F = 207.3 + 44.45 = 251.7 N
Answer
F = 251.7 N
4.Newton’s laws
Answer: T = 36.53 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.
Range formula
R = v₀²·sin 2θ/g → sin 2θ = 9.81·70/30² = 0.763
Angle
2θ = 49.73° → θ = 24.86°
Answer
θ = 24.86°
The other solution is 90° − θ = 65.14°.
6.Projectile motion
Answer: x = 90.02 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 = 29·cos 40° = 22.22 m/s
vy = v0·sin θ = 29·sin 40° = 18.64 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 5 + 18.64·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (18.64 + √(347.5 + 98.1))/9.81 = 4.052 s
Horizontal distance
x = vx·t = 22.22·4.052 = 90.02 m
Answer
x = 90.02 m
7.Circular motion
Answer: vmax = 21.24 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) = √(0.4·9.81·115) = 21.24 m/s
Answer
vmax = 21.24 m/s (76.47 km/h)
8.Circular motion
Answer: vmax = 30.91 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.
Force balances
Friction points down the slope.
N·(sin θ + μ·cos θ) = m·v²/r
N·(cos θ − μ·sin θ) = m·g
Speed
v = √(170·9.81·0.5353/0.9346) = 30.91 m/s
Answer
vmax = 30.91 m/s
(111.3 km/h)
9.Work & energy
Answer: v = 12.24 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 = √(2·9.81·18·(0.7071 − 0.4·0.7071)) = 12.24 m/s
Answer
v = 12.24 m/s
10.Work & energy
Answer: x = 57.54 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 = 49.05·(3.5 − 0.2·4) = 132.4 J
Maximum compression
½kx² = 132.4 → x = √(2·132.4/800) = 0.5754 m = 57.54 cm
Answer
x = 57.54 cm
11.Momentum & collisions
Answer: v′2 = 4.5 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.
Elastic collision with a target at rest
Solving momentum and energy conservation together gives these two results.
v′1 = (1 − 3)/(1 + 3)·9 = −4.5 m/s
v′2 = 2·1/(1 + 3)·9 = 4.5 m/s
Answer
v′2 = 4.5 m/s
12.Momentum & collisions
Answer: Favg = 1120 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 = 0.4·(4 + 24) = 11.2 N·s
Average force
Favg = J/Δt = 11.2/0.01 = 1120 N
Answer
Favg = 1120 N
13.Springs & pendulums
Answer: v = 3.269 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 = √(380/0.3) = 35.59 rad/s
ω·t = 35.59·0.85 = 30.25 rad
Velocity
v = −A·ω·sin(ω·t) = −0.1·35.59·sin(30.25) = 3.269 m/s
Answer
v = 3.269 m/s
14.Springs & pendulums
Answer: vmax = 0.4739 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.
Equilibrium and amplitude
x₀ = m·g/k = 0.7·9.81/300 = 0.02289 m = A
Greatest speed
The speed is largest at equilibrium.
ω = √(300/0.7) = 20.7 rad/s
vmax = A·ω = 0.02289·20.7 = 0.4739 m/s
Answer
vmax = 0.4739 m/s
15.Waves, sound & optics
Answer: θc = 65.15 °
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.
Refraction at 90°
sin θc = n₂/n₁ = 1.47/1.62 = 0.9074
Critical angle
θc = arcsin(0.9074) = 65.15°
Answer
θc = 65.15°
16.Waves, sound & optics
Answer: di = −3.077 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.