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
policea = 4 m/s²car35 m/s

A car speeding at a constant 35 m/s passes a police car standing still. At that moment the police car starts and accelerates uniformly at 4 m/s². How far from its starting point does the police car catch up? Find d.

vcar = 35 m/sapolice = 4 m/s²v0,police = 0
d = m
2.KinematicsHard
v₀ = 20 m/s20 m

A ball is thrown straight up at 20 m/s from the edge of a roof 20 m above the ground. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax above the ground.

v₀ = 20 m/sh₀ = 20 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
θ = 30°a = ?m = 6 kg

A 6 kg block slides down a frictionless slope at 30° to the horizontal. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.

m = 6 kgθ = 30°μ = 0g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
m1​ = 8.5 kg8 kgθ = 40°

An 8.5 kg block on a ramp at 40° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 8 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.1. Use g = 9.81 m/s². Find the tension T in the string.

m1 = 8.5 kgm2 = 8 kgθ = 40°μ = 0.1g = 9.81 m/s²
T = N
5.Projectile motionMedium
v0​ = 38 m/sθ = ?R = 105 m

A ball is launched from level ground at 38 m/s and must land 105 m away. Ignore air resistance and use g = 9.81 m/s². Find the smaller of the two possible launch angles θ.

v₀ = 38 m/sR = 105 mg = 9.81 m/s²
θ = °
6.Projectile motionHard
h = 45 mv0​ = 23 m/s55°x = ?

A ball is launched at 55° above horizontal from the top of a 45 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.

v0 = 23 m/sθ = 55°h = 45 mg = 9.81 m/s²
x = m
7.Circular motionMedium
r = 95 mvmax​ = ?

A car takes a flat curve of radius 95 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 = 95 mμs = 0.8g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
r = 0.8 mv = 7 m/s

A 1.7 kg ball on a string 0.8 m long is swung in a vertical circle. At the lowest point it moves at 7 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.

m = 1.7 kgr = 0.8 mv = 7 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
45°d = 10 mμ = 0.4

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 10 m along the slope.

θ = 45°μ = 0.4d = 10 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
h = 9 mR = 3 mN = ?

A 0.5 kg cart is released from rest at a height of 9 m on a smooth track that leads into a vertical circular loop of radius 3 m. Find the normal force N the track exerts on the cart at the top of the loop.

m = 0.5 kgh = 9 mR = 3 mg = 9.81 m/s²
N = N
11.Momentum & collisionsMedium
12 kg74 m/s1 kgv′2​ = ?

A 13 kg projectile moving horizontally at 29 m/s explodes into two fragments that move along the same line. The 12 kg fragment continues forwards at 74 m/s. Find the velocity v′2 of the 1 kg fragment (positive in the original direction).

M = 13 kgu = 29 m/sm1 = 12 kgv′1 = 74 m/sm2 = 1 kg
v′2 = m/s
12.Momentum & collisionsHard
1400 kg11 m/s800 kg20 m/sθ = ?

At a crossing a 1400 kg car travelling east at 11 m/s collides with an 800 kg car travelling north at 20 m/s. The cars lock together. Find the direction θ of the wreck just after the collision, measured north of east.

m1 = 1400 kgv1 = 11 m/s (E)m2 = 800 kgv2 = 20 m/s (N)
θ = °
13.Springs & pendulumsMedium
L = 0.8 mT = 1.89 s

An astronaut on another planet finds that a simple pendulum 0.8 m long has a period of 1.89 s for small swings. Find the gravitational acceleration g on that planet.

L = 0.8 mT = 1.89 s
g = m/s²
14.Springs & pendulumsHard
3.5 kgk = 1200 N/m400 m/s

A 12 g bullet moving at 400 m/s hits and stays in a 3.5 kg block resting on a frictionless surface. The block is attached to a horizontal spring with k = 1200 N/m. Find the amplitude A of the resulting oscillation in cm.

m = 12 gv = 400 m/sM = 3.5 kgk = 1200 N/m
A = cm
15.Waves, sound & opticsMedium
FFdo​ = 10 cmf = 15 cm

An object stands 10 cm in front of a thin converging lens with a focal length of 15 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 = 10 cmf = 15 cm
di = cm
16.Waves, sound & opticsHard
λ = 500 nmθ1​ = ?

Light of wavelength 500 nm falls at right angles on a grating with 1200 lines per millimetre. Find the angle θ1 of the order-1 maximum.

N = 1200 mm⁻¹λ = 500 nmm = 1
θ1 = °

3Solutions

1.Kinematics

Answer: d = 612.5 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.

Equal distances

v·t = ½·a·t² → t = 2v/a = 2·35/4 = 17.5 s

Distance

d = v·t = 35·17.5 = 612.5 m

Answer

d = 612.5 m

At that moment the police car moves at 2v = 70 m/s.

2.Kinematics

Answer: hmax = 40.39 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.

Rise above the roof

0 = v₀² − 2·g·Δh → Δh = 20²/(2·9.81) = 20.39 m

Height above the ground

hmax = 20 + 20.39 = 40.39 m

Answer

hmax = 40.39 m
3.Newton’s laws

Answer: a = 4.905 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 slope

Down the slope is positive.

m·a = m·g·sin θ − μ·m·g·cos θ

Acceleration

a = 9.81·(sin 30° − 0·cos 30°) = 9.81·(0.5 − 0) = 4.905 m/s²

Answer

a = 4.905 m/s²
4.Newton’s laws

Answer: T = 69.51 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 = 78.48 N
m₁·g·sin θ = 53.6 N
μ·m₁·g·cos θ = 6.388 N

Acceleration of the system

a = (78.48 − 53.6 − 6.388)/16.5 = 1.121 m/s²

Tension from the hanging mass

m2·g − T = m2·a → T = 8·(9.81 − 1.121) = 69.51 N

Answer

T = 69.51 N
5.Projectile motion

Answer: θ = 22.75 °

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 = 9.81·105/38² = 0.7133

Angle

2θ = 45.51° → θ = 22.75°

Answer

θ = 22.75°

The other solution is 90° − θ = 67.25°.

6.Projectile motion

Answer: x = 72.65 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 = 23·cos 55° = 13.19 m/s
vy = v0·sin θ = 23·sin 55° = 18.84 m/s

Time in the air

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

0 = 45 + 18.84·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (18.84 + √(355 + 882.9))/9.81 = 5.507 s

Horizontal distance

x = vx·t = 13.19·5.507 = 72.65 m

Answer

x = 72.65 m
7.Circular motion

Answer: vmax = 27.3 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) = √(0.8·9.81·95) = 27.3 m/s

Answer

vmax = 27.3 m/s (98.3 km/h)
8.Circular motion

Answer: T = 120.8 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.

Forces at the lowest point

Towards the centre (up) is positive.

T − m·g = m·v²/r
T = 1.7·9.81 + 1.7·7²/0.8 = 16.68 + 104.1 = 120.8 N

Answer

T = 120.8 N
9.Work & energy

Answer: v = 9.124 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 = √(2·9.81·10·(0.7071 − 0.4·0.7071)) = 9.124 m/s

Answer

v = 9.124 m/s
10.Work & energy

Answer: N = 4.905 N

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). On a circular path the net force towards the centre must equal mv²/r.

Speed at the top

v² = 2g(h − 2R) = 2·9.81·(9 − 6) = 58.86 m²/s²

Forces at the top

m·g = 0.5·9.81 = 4.905 N
N = mv²/R − mg = 0.5·58.86/3 − 4.905 = 4.905 N

Answer

N = 4.905 N

The cart just makes it round when h = 2.5R; then N = 0 at the top.

11.Momentum & collisions

Answer: v′2 = −511 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 = 13·29 = 377 kg·m/s

Momentum after

377 = 12·74 + 1·v′2
v′2 = (377 − 888)/1 = −511 m/s

Answer

v′2 = −511 m/s

The minus sign means the second fragment flies backwards.

12.Momentum & collisions

Answer: θ = 46.09 °

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 = 1400·11 = 15400 kg·m/s
py = 800·20 = 16000 kg·m/s

Direction

tan θ = py/px = 1.039 → θ = 46.09°

Answer

θ = 46.09°
13.Springs & pendulums

Answer: g = 8.842 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 = 4π²·0.8/1.89² = 8.842 m/s²

Answer

g = 8.842 m/s²
14.Springs & pendulums

Answer: A = 7.394 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) = 0.012·400/3.512 = 1.367 m/s

Angular frequency

ω = √(k/(m + M)) = √(1200/3.512) = 18.48 rad/s

Amplitude

A = V/ω = 1.367/18.48 = 0.07394 m = 7.394 cm

Answer

A = 7.394 cm
15.Waves, sound & optics

Answer: di = −30 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 = 1/15 − 1/10
di = −30 cm
FF10 cmf = 15 cm

Ray diagram

virtual, on the same side as the object

Answer

di = −30 cm
16.Waves, sound & optics

Answer: θ1 = 36.87 °

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 = 8.333 × 10⁻⁷ m

Grating equation

sin θ = mλ/d = 1·500 × 10⁻⁹/(8.333 × 10⁻⁷) = 0.6
θ = 36.87°

Answer

θ1 = 36.87°

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