Physics 1: Mechanics: cram pack

Formula sheets, the key method steps for every topic and 16 practice problems, with full solutions at the back.

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1Formulas and methods

Kinematics

Velocity
v = v0 + a·t
Displacement
s = v0·t + ½·a·t²
Without time
v² = v0² + 2·a·s
Units
1 m/s = 3.6 km/h

Method

  1. List what you know: v₀, v, a, t and s, with units.
  2. Pick the equation that contains your three knowns and the unknown.
  3. Choose one positive direction and give opposing quantities a minus sign.
  4. Solve and check that the answer is physically sensible.

Newton’s laws

Second law
ΣF = m·a
Weight
W = m·g
Kinetic friction
Ff = μ·N
On a slope
along: m·g·sin θ, normal: m·g·cos θ

Method

  1. Draw a free-body diagram with every force on the body.
  2. Choose a positive direction along the motion.
  3. Split forces into components along and perpendicular to the motion (on a slope: m·g·sin θ along, m·g·cos θ into the slope).
  4. Apply ΣF = m·a along the motion; friction is μ·N against the motion.

Projectile motion

Components
vx = v0cos θ, vy = v0sin θ
Position
x = vxt, y = h + vyt − ½gt²
Maximum height
H = h + vy²/(2g)
Range (level ground)
R = v0²·sin 2θ/g

Method

  1. Split the launch velocity into components: vx = v₀·cos θ and vy = v₀·sin θ.
  2. The horizontal motion has constant speed; the vertical motion has acceleration −g.
  3. Find the time of flight from the vertical motion.
  4. Use that time in the horizontal motion to find the range.

Circular motion

Centripetal acceleration
a = v²/r = ω²·r
Centripetal force
F = m·v²/r
Angular speed
ω = v/r = 2π/T
Flat curve, friction
vmax = √(μ·g·r)

Work & energy

Kinetic energy
Ek = ½mv²
Potential energy
Ep = mgh
Energy balance
Ek1 + Ep1 + W = Ek2 + Ep2 + losses
Work of friction
W = μ·N·d
Power
P = W/t

Momentum & collisions

Momentum
p = m·v
Conservation
Σm·v (before) = Σm·v (after)
Perfectly inelastic
v′ = (m1v1 + m2v2)/(m1 + m2)
Impulse
J = F·Δt = m·Δv

Method

  1. Choose a positive direction and give each velocity its sign.
  2. Write the total momentum before: Σm·v.
  3. For a perfectly inelastic collision the bodies move together after; for an elastic one kinetic energy is also conserved.
  4. Set momentum before equal to momentum after and solve.

Springs & pendulums

Mass on a spring
T = 2π·√(m/k)
Pendulum
T = 2π·√(L/g)
Angular frequency
ω = √(k/m) = 2π/T
Maximum speed and acceleration
vmax = A·ω, amax = A·ω²
Energy
E = ½·k·A²

Waves, sound & optics

Wave speed
v = fλ, T = 1/f
String
v = √(T/μ), μ = m/L
Sound level
β = 10·log10(I/I0), I0 = 10⁻¹² W/m²
Point source
I = P/(4πr²)
Standing waves
string or open pipe fn = n·v/(2L); closed pipe fn = n·v/(4L), n odd
Doppler effect
f′ = f·(v ± vo)/(v ∓ vs)
Refractive index
n = c/v, λ = λ0/n
Snell’s law
n1sin θ1 = n2sin θ2
Critical angle
sin θc = n2/n1
Optical fibre
NA = sin θa = √(n1² − n2²)
Lenses and mirrors
1/f = 1/do + 1/di, m = −di/do, mirror f = R/2
Double slit
d·sin θ = mλ, ym = mλL/d
Grating
d = 1/N, d·sin θ = mλ, mmax ≤ d/λ
Single slit
a·sin θ = mλ (dark), w = 2λL/a

Method

  1. Identify the kind of problem: wave speed, sound level, standing wave, Doppler, refraction, lens or mirror, or interference.
  2. Write the relation first: v = fλ, β = 10·log(I/I₀), fₙ = n·v/(2L) or n·v/(4L), f′ = f(v ± vo)/(v ∓ vs), n₁sin θ₁ = n₂sin θ₂, 1/f = 1/d₀ + 1/dᵢ, d·sin θ = mλ.
  3. Convert units: nm, mm and cm to m, and check whether angles are measured from the normal.
  4. 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
t (s)v (m/s)2.857 s20

A car driving at 72 km/h brakes with a constant deceleration of 7 m/s² until it stops. Find the braking distance s.

v₀ = 72 km/ha = −7 m/s²v = 0
s = m
2.KinematicsHard
t (s)v (m/s)12 s

A tram starts from rest and accelerates at 2 m/s² for 12 s, then runs at constant speed for 24 s, and finally brakes at 5 m/s² until it stops. Find the total distance s between the two stops.

a1 = 2 m/s²t1 = 12 st2 = 24 sa3 = −5 m/s²
s = m
3.Newton’s lawsMedium
15 kg35°F = 60 Nμ = 0.3

A 15 kg sled is pulled across level snow by a rope at 35° above the horizontal with a force of 60 N. The coefficient of kinetic friction is 0.3. Use g = 9.81 m/s². Find the acceleration a of the sled.

m = 15 kgF = 60 Nθ = 35°μ = 0.3g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
m1​ = 4.5 kg9.5 kgθ = 30°

A 4.5 kg block on a ramp at 30° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 9.5 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.25. Use g = 9.81 m/s². Find the acceleration a of the system.

m1 = 4.5 kgm2 = 9.5 kgθ = 30°μ = 0.25g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
v0​ = ?40°R = 95 m

A ball is launched from level ground at 40° above the horizontal and lands 95 m away. Ignore air resistance and use g = 9.81 m/s². Find the launch speed v0.

θ = 40°R = 95 mg = 9.81 m/s²
v0 = m/s
6.Projectile motionHard
h = 60 mv0​ = 14 m/s20°range

A ball is launched at 20° above horizontal from the top of a 60 m cliff. Ignore air resistance. Find the time of flight t until the ball hits the ground.

v0 = 14 m/sθ = 20°h = 60 mg = 9.81 m/s²
t = s
7.Circular motionMedium
r = 85 mvmax​ = ?

A car takes a flat curve of radius 85 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 = 85 mμs = 0.4g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
θ = 18°to centrevmax​ = ?

A curve of radius 210 m is banked at 18°, and the coefficient of static friction between tyres and road is 0.2. 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 = 210 mθ = 18°μs = 0.2g = 9.81 m/s²
vmax = m/s
9.Work & energyMedium
30°d = 19 mμ = 0.1

A block slides from rest down a slope at 30° to the horizontal. The coefficient of kinetic friction is 0.1. Find its speed v after it has slid 19 m along the slope.

θ = 30°μ = 0.1d = 19 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
h = 3.5 mL = 2.5 mμ = 0.2k = 5000 N/m

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 2.5 m long (μ = 0.2) and then runs onto a spring (k = 5000 N/m) on a smooth surface. Find the speed v of the block when it reaches the spring.

m = 4 kgh = 3.5 mL = 2.5 mμ = 0.2k = 5000 N/mg = 9.81 m/s²
v = m/s
11.Momentum & collisionsMedium
2.5 kgv = ?20 gh = 19 cm

A 20 g bullet is fired horizontally into a 2.5 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 19 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.

m = 20 gM = 2.5 kgh = 19 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
1200 kg17 m/s1000 kg10 m/sθ = ?

At a crossing a 1200 kg car travelling east at 17 m/s collides with a 1000 kg car travelling north at 10 m/s. The cars lock together. Find the direction θ of the wreck just after the collision, measured north of east.

m1 = 1200 kgv1 = 17 m/s (E)m2 = 1000 kgv2 = 10 m/s (N)
θ = °
13.Springs & pendulumsMedium
t (s)x (cm)0.1 s

A 3.5 kg mass on a spring with k = 360 N/m is pulled 10 cm in the positive direction and released from rest at t = 0. Find its displacement x from equilibrium at t = 0.1 s (with sign).

m = 3.5 kgk = 360 N/mA = 10 cmt = 0.1 s
x = cm
14.Springs & pendulumsHard
mk = 240 N/mA = 5 cmx = 1 cm

A mass on a spring with k = 240 N/m oscillates on a frictionless surface with an amplitude of 5 cm. Find the energy U stored in the spring when the mass is 1 cm from equilibrium.

k = 240 N/mA = 5 cmx = 1 cm
U = J
15.Waves, sound & opticsMedium
FFdo​ = 8 cmf = 10 cm

An object stands 8 cm in front of a thin converging lens with a focal length of 10 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 = 8 cmf = 10 cm
di = cm
16.Waves, sound & opticsHard
λ = 486 nmθ

A diffraction grating has 1200 lines per millimetre. Light of wavelength 486 nm falls on it at right angles. What is the highest order mmax of bright maximum that can be seen?

N = 1200 mm⁻¹λ = 486 nm
mmax =

3Solutions

1.Kinematics

Answer: s = 28.57 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 = 72/3.6 = 20 m/s

Braking distance

0 = v₀² − 2·a·s → s = v₀²/(2a) = 20²/(2·7) = 28.57 m

Answer

s = 28.57 m
2.Kinematics

Answer: s = 777.6 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.

Top speed

v = a1·t1 = 2·12 = 24 m/s

Distance of each phase

s1 = ½·2·12² = 144 m
s2 = 24·24 = 576 m
s3 = 24²/(2·5) = 57.6 m

Total distance

s = 144 + 576 + 57.6 = 777.6 m

Answer

s = 777.6 m
3.Newton’s laws

Answer: a = 1.022 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.

Normal force

N = m·g − F·sin θ = 147.2 − 60·sin 35° = 112.7 N

Horizontal forces

m·a = F·cos θ − μ·N = 49.15 − 33.82
a = 15.33/15 = 1.022 m/s²

Answer

a = 1.022 m/s²
4.Newton’s laws

Answer: a = 4.397 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.

Forces along the string

m₂·g = 93.2 N
m₁·g·sin θ = 22.07 N
μ·m₁·g·cos θ = 9.558 N

Acceleration of the system

a = (93.2 − 22.07 − 9.558)/14 = 4.397 m/s²

Answer

a = 4.397 m/s²
5.Projectile motion

Answer: v0 = 30.76 m/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.

Solve the range formula for v₀

v₀ = √(g·R/sin 2θ) = √(9.81·95/sin 80°) = 30.76 m/s

Answer

v0 = 30.76 m/s
6.Projectile motion

Answer: t = 4.019 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 = 14·cos 20° = 13.16 m/s
vy = v0·sin θ = 14·sin 20° = 4.788 m/s

Time in the air

Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.

0 = 60 + 4.788·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (4.788 + √(22.93 + 1177))/9.81 = 4.019 s

Answer

t = 4.019 s
7.Circular motion

Answer: vmax = 18.26 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) = √(0.4·9.81·85) = 18.26 m/s

Answer

vmax = 18.26 m/s (65.75 km/h)
8.Circular motion

Answer: vmax = 34.01 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.

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 = √(210·9.81·0.4992/0.8893) = 34.01 m/s

Answer

vmax = 34.01 m/s
(122.4 km/h)
9.Work & energy

Answer: v = 12.41 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 = √(2·9.81·19·(0.5 − 0.1·0.866)) = 12.41 m/s

Answer

v = 12.41 m/s
10.Work & energy

Answer: v = 7.672 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 left after the rough strip

The ramp is smooth, so only the strip removes energy.

mgh − μmgL = 39.24·(3.5 − 0.2·2.5) = 117.7 J

Speed at the spring

½mv² = mg(h − μL) → v = √(2g(h − μL)) = √(2·9.81·3) = 7.672 m/s

Answer

v = 7.672 m/s
11.Momentum & collisions

Answer: v = 243.3 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.

Swing after impact

V = √(2·g·h) = √(2·9.81·0.19) = 1.931 m/s

Collision

m·v = (m + M)·V → v = (0.02 + 2.5)·1.931/0.02 = 243.3 m/s

Answer

v = 243.3 m/s
12.Momentum & collisions

Answer: θ = 26.11 °

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 components

px = 1200·17 = 20400 kg·m/s
py = 1000·10 = 10000 kg·m/s

Direction

tan θ = py/px = 0.4902 → θ = 26.11°

Answer

θ = 26.11°
13.Springs & pendulums

Answer: x = 5.283 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.

Angular frequency

ω = √(360/3.5) = 10.14 rad/s
ω·t = 10.14·0.1 = 1.014 rad

Displacement

x = A·cos(ω·t) = 10·cos(1.014) = 5.283 cm

Answer

x = 5.283 cm
14.Springs & pendulums

Answer: U = 0.012 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.

Total energy

E = ½·k·A² = ½·240·0.05² = 0.3 J

Spring energy at x

U = ½·k·x² = ½·240·0.01² = 0.012 J

Answer

U = 0.012 J
15.Waves, sound & optics

Answer: di = −40 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 = 1/10 − 1/8
di = −40 cm
FF8 cmf = 10 cm

Ray diagram

virtual, on the same side as the object

Answer

di = −40 cm
16.Waves, sound & optics

Answer: mmax = 1

Interference and diffraction: light from two slits a distance d apart reaches a point on the screen with a path difference d·sin θ. Bright fringes appear where d·sin θ = mλ (m = 0, ±1, ±2, …). For a screen at distance L ≫ d, the m-th bright fringe is at y = mλL/d and the fringe spacing is Δy = λL/d. A grating with N lines per millimetre has d = 1/N and sharp maxima at d·sin θ = mλ; since sin θ ≤ 1, the highest order is the largest whole number m ≤ d/λ. A single slit of width a has dark fringes at a·sin θ = mλ, m = ±1, ±2, …

Line spacing

d = 1/1200 mm = 0.8333 µm

Highest order

m ≤ d/λ = 0.8333 µm/0.486 µm = 1.715 → mmax = 1

Answer

mmax = 1

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