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
a = ?s = 2200 m

A plane needs a speed of 260 km/h to take off and the usable runway is 2200 m long. Starting from rest, what constant acceleration a does it need?

v₀ = 0v = 260 km/hs = 2200 m
a = m/s²
2.KinematicsHard
v₀ = 8 m/s15 m

A ball is thrown straight up at 8 m/s from the edge of a roof 15 m above the ground. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax above the ground.

v₀ = 8 m/sh₀ = 15 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
θ = 45°a = ?m = 19 kg

A 19 kg block slides down a slope at 45° to the horizontal. The coefficient of kinetic friction is 0.2. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.

m = 19 kgθ = 45°μ = 0.2g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
m1​ = 7 kg1 kgno friction

A 7 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 1 kg mass. The table is frictionless. Use g = 9.81 m/s². Find the acceleration a of the system.

m1 = 7 kgm2 = 1 kgμ = 0g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
v0​ = 22 m/s70°range

A ball is launched at 70° above horizontal from level ground. Ignore air resistance. Find the time of flight t until the ball hits the ground.

v0 = 22 m/sθ = 70°g = 9.81 m/s²
t = s
6.Projectile motionHard
h = 45 mv0​ = 13 m/s45°α = ?

A ball is launched at 13 m/s and 45° above the horizontal from the top of a 45 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 = 13 m/sθ = 45°h = 45 mg = 9.81 m/s²
α = °
7.Circular motionMedium
r = 65 mvmax​ = ?

A car takes a flat curve of radius 65 m. The coefficient of static friction between tyres and road is 0.7. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.

r = 65 mμs = 0.7g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
r = 0.9 mv = 5.5 m/s

A 1.2 kg ball on a string 0.9 m long is swung in a vertical circle. At the lowest point it moves at 5.5 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.

m = 1.2 kgr = 0.9 mv = 5.5 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
25°d = 20 mμ = 0.3

A block slides from rest down a slope at 25° to the horizontal. The coefficient of kinetic friction is 0.3. Find its speed v after it has slid 20 m along the slope.

θ = 25°μ = 0.3d = 20 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
h = 4.5 mL = 1 mμ = 0.1k = 2500 N/m

A 0.5 kg block is released from rest at height 4.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 1 m long (μ = 0.1) and then runs onto a spring (k = 2500 N/m) on a smooth surface. Find the maximum compression x of the spring.

m = 0.5 kgh = 4.5 mL = 1 mμ = 0.1k = 2500 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
4 kg10 m/s3 kg

A 4 kg glider moving at 10 m/s makes a perfectly elastic head-on collision with a 3 kg glider at rest on a frictionless track. Find the velocity v′1 of the first glider after the collision (positive in its original direction).

m1 = 4 kgm2 = 3 kgv1 = 10 m/sv2 = 0
v′1 = m/s
12.Momentum & collisionsHard
3 kgv = ?12 g40°L = 1.4 m

A 12 g bullet is fired horizontally into a 3 kg block hanging from light strings 1.4 m long, and stays in it. The strings swing out to a largest angle of 40° from the vertical. Use g = 9.81 m/s². Find the speed v of the bullet before impact.

m = 12 gM = 3 kgL = 1.4 mθ = 40°g = 9.81 m/s²
v = m/s
13.Springs & pendulumsMedium
t (s)x (cm)1 s

A 2.8 kg mass on a spring with k = 390 N/m is pulled 11 cm in the positive direction and released from rest at t = 0. Find its displacement x from equilibrium at t = 1 s (with sign).

m = 2.8 kgk = 390 N/mA = 11 cmt = 1 s
x = cm
14.Springs & pendulumsHard
0.6 kgk = 170 N/mA = 12 cm

A 0.6 kg mass on a spring with k = 170 N/m is pulled 12 cm from equilibrium and released. Find its maximum acceleration amax.

m = 0.6 kgk = 170 N/mA = 12 cm
amax = m/s²
15.Waves, sound & opticsMedium
θc = ?90°flint glass, n1​ = 1.62water, n2​ = 1.33

Light travels in flint glass (n = 1.62) towards a flat boundary with water (n = 1.33). Find the critical angle θc beyond which it is totally reflected.

n1 = 1.62n2 = 1.33
θc = °
16.Waves, sound & opticsHard
f1​ = 8 cmf2​ = 5 cm15 cm15 cm

Two thin lenses share an axis, 15 cm apart: lens 1 with f₁ = 8 cm and lens 2 with f₂ = 5 cm. An object stands 15 cm in front of lens 1. Find the position di2 of the final image measured from lens 2 (positive beyond lens 2, real-is-positive convention).

f1 = 8 cmf2 = 5 cms = 15 cmdo = 15 cm
di2 = cm

3Solutions

1.Kinematics

Answer: a = 1.185 m/s²

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

v = 260/3.6 = 72.22 m/s

Acceleration

a = v²/(2s) = 72.22²/(2·2200) = 1.185 m/s²

Answer

a = 1.185 m/s²
2.Kinematics

Answer: hmax = 18.26 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 = 8²/(2·9.81) = 3.262 m

Height above the ground

hmax = 15 + 3.262 = 18.26 m

Answer

hmax = 18.26 m
3.Newton’s laws

Answer: a = 5.549 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 45° − 0.2·cos 45°) = 9.81·(0.7071 − 0.1414) = 5.549 m/s²

Answer

a = 5.549 m/s²
4.Newton’s laws

Answer: a = 1.226 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 of the system

a = (m2·g − μ·m1·g)/(m1 + m2) = (9.81 − 0)/8 = 1.226 m/s²

Answer

a = 1.226 m/s²
5.Projectile motion

Answer: t = 4.215 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 = 22·cos 70° = 7.524 m/s
vy = v0·sin θ = 22·sin 70° = 20.67 m/s

Time in the air

Starting and landing at the same height: t = 2vy/g.

t = 2·20.67/9.81 = 4.215 s

Answer

t = 4.215 s
6.Projectile motion

Answer: α = 73.54 °

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 = 13·cos 45° = 9.192 m/s
vy0 = 13·sin 45° = 9.192 m/s

Vertical speed at impact

Energy or v² = v₀² + 2as in the vertical direction.

vy = √(9.192² + 2·9.81·45) = 31.1 m/s

Impact angle

tan α = vy/vx = 31.1/9.192 → α = 73.54°

Answer

α = 73.54°
7.Circular motion

Answer: vmax = 21.13 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) = √(0.7·9.81·65) = 21.13 m/s

Answer

vmax = 21.13 m/s (76.06 km/h)
8.Circular motion

Answer: T = 52.11 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 = 1.2·9.81 + 1.2·5.5²/0.9 = 11.77 + 40.33 = 52.11 N

Answer

T = 52.11 N
9.Work & energy

Answer: v = 7.691 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 = √(2·9.81·20·(0.4226 − 0.3·0.9063)) = 7.691 m/s

Answer

v = 7.691 m/s
10.Work & energy

Answer: x = 13.14 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 = 4.905·(4.5 − 0.1·1) = 21.58 J

Maximum compression

½kx² = 21.58 → x = √(2·21.58/2500) = 0.1314 m = 13.14 cm

Answer

x = 13.14 cm
11.Momentum & collisions

Answer: v′1 = 1.429 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 = (4 − 3)/(4 + 3)·10 = 1.429 m/s
v′2 = 2·4/(4 + 3)·10 = 11.43 m/s

Answer

v′1 = 1.429 m/s
12.Momentum & collisions

Answer: v = 636.3 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.

Rise of the block

h = L·(1 − cos θ) = 1.4·(1 − cos 40°) = 0.3275 m

Speed after impact

V = √(2·9.81·0.3275) = 2.535 m/s

Collision

v = (m + M)·V/m = 3.012·2.535/0.012 = 636.3 m/s

Answer

v = 636.3 m/s
13.Springs & pendulums

Answer: x = 7.94 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 = √(390/2.8) = 11.8 rad/s
ω·t = 11.8·1 = 11.8 rad

Displacement

x = A·cos(ω·t) = 11·cos(11.8) = 7.94 cm

Answer

x = 7.94 cm
14.Springs & pendulums

Answer: amax = 34 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.

Angular frequency

ω = √(170/0.6) = 16.83 rad/s

Maximum acceleration

amax = A·ω² = 0.12·16.83² = 34 m/s²

Answer

amax = 34 m/s²
15.Waves, sound & optics

Answer: θc = 55.18 °

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 = 1.33/1.62 = 0.821

Critical angle

θc = arcsin(0.821) = 55.18°

Answer

θc = 55.18°
16.Waves, sound & optics

Answer: di2 = 1.5 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.

Image of lens 1

1/di1 = 1/8 − 1/15 → di1 = 17.14 cm

Object for lens 2

do2 = 15 − 17.14 = −2.143 cm

Final image

1/di2 = 1/5 − 1/(−2.143) → di2 = 1.5 cm

Answer

di2 = 1.5 cm

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