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 = 2.25 m/s²s = 2000 m

A plane starts from rest and accelerates uniformly at 2.25 m/s² along a runway. Find its speed v after 2000 m.

v₀ = 0a = 2.25 m/s²s = 2000 m
v = m/s
2.KinematicsHard
t (s)v (m/s)5 s

A tram starts from rest and accelerates at 2.5 m/s² for 5 s, then runs at constant speed for 21 s, and finally brakes at 2.5 m/s² until it stops. Find the total distance s between the two stops.

a1 = 2.5 m/s²t1 = 5 st2 = 21 sa3 = −2.5 m/s²
s = m
3.Newton’s lawsMedium
20 kg35°F = 120 Nμ = 0.4

A 20 kg sled is pulled across level snow by a rope at 35° above the horizontal with a force of 120 N. The coefficient of kinetic friction is 0.4. Use g = 9.81 m/s². Find the acceleration a of the sled.

m = 20 kgF = 120 Nθ = 35°μ = 0.4g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
m1​ = 8 kg5 kgμ = 0.2

An 8 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 5 kg mass. The coefficient of kinetic friction between block and table is 0.2. Use g = 9.81 m/s². Find the acceleration a of the system.

m1 = 8 kgm2 = 5 kgμ = 0.2g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
v0​ = ?65°R = 10 m

A ball is launched from level ground at 65° above the horizontal and lands 10 m away. Ignore air resistance and use g = 9.81 m/s². Find the launch speed v0.

θ = 65°R = 10 mg = 9.81 m/s²
v0 = m/s
6.Projectile motionHard
h = 40 mv0​ = 27 m/s20°x = ?

A ball is launched at 20° above horizontal from the top of a 40 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.

v0 = 27 m/sθ = 20°h = 40 mg = 9.81 m/s²
x = m
7.Circular motionMedium
r = 90 mvmax​ = ?

A car takes a flat curve of radius 90 m. The coefficient of static friction between tyres and road is 0.5. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.

r = 90 mμs = 0.5g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
R = 8.5 mh = ?

A small cart is released from rest on a frictionless track and runs into a vertical loop of radius 8.5 m. From what minimum height hmin above the bottom of the loop must it start to stay on the track at the top of the loop?

R = 8.5 mv₀ = 0
hmin = m
9.Work & energyMedium
90°L = 3 m1 kg

A 1 kg ball hangs on a light string of length 3 m. It is pulled aside until the string makes 90° with the vertical and released from rest. Find the tension T in the string when the ball passes the lowest point.

m = 1 kgL = 3 mθ = 90°g = 9.81 m/s²
T = N
10.Work & energyHard
h = 5 mL = 1.5 mμ = 0.3k = 4000 N/m

A 10 kg block is released from rest at height 5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 1.5 m long (μ = 0.3) and then runs onto a spring (k = 4000 N/m) on a smooth surface. Find the speed v of the block when it reaches the spring.

m = 10 kgh = 5 mL = 1.5 mμ = 0.3k = 4000 N/mg = 9.81 m/s²
v = m/s
11.Momentum & collisionsMedium
1300 kg20 m/s900 kg

A 1300 kg car moving at 20 m/s runs into a stationary 900 kg car and the two lock together. How much kinetic energy ΔK is lost in the collision?

m1 = 1300 kgv1 = 20 m/sm2 = 900 kgv2 = 0
ΔK = J
12.Momentum & collisionsHard
2 kgv = ?5 g25°L = 1.5 m

A 5 g bullet is fired horizontally into a 2 kg block hanging from light strings 1.5 m long, and stays in it. The strings swing out to a largest angle of 25° from the vertical. Use g = 9.81 m/s². Find the speed v of the bullet before impact.

m = 5 gM = 2 kgL = 1.5 mθ = 25°g = 9.81 m/s²
v = m/s
13.Springs & pendulumsMedium
0.9 kgk = 160 N/mA = 18 cmx = 2 cm

A 0.9 kg mass on a spring with k = 160 N/m oscillates on a frictionless surface with an amplitude of 18 cm. Find its speed v when it is 2 cm from equilibrium.

m = 0.9 kgk = 160 N/mA = 18 cmx = 2 cm
v = m/s
14.Springs & pendulumsHard
3.5 kgk = 600 N/m100 m/s

A 10 g bullet moving at 100 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 = 600 N/m. Find the amplitude A of the resulting oscillation in cm.

m = 10 gv = 100 m/sM = 3.5 kgk = 600 N/m
A = cm
15.Waves, sound & opticsMedium
90 dB?2 m20 m

A small source sends sound equally in all directions. The level is 90 dB at 2 m from it. Find the level β2 at 20 m, ignoring absorption and reflections.

β1 = 90 dBr1 = 2 mr2 = 20 m
β2 = dB
16.Waves, sound & opticsHard
λ = 600 nma = 0.12 mm

Light of wavelength 600 nm passes through a single slit 0.12 mm wide. Find the angle θ3 of dark fringe number 3 from the straight-through direction.

λ = 600 nma = 0.12 mmm = 3
θ3 = °

3Solutions

1.Kinematics

Answer: v = 94.87 m/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.

Speed after the distance

v = √(2·a·s) = √(2·2.25·2000) = 94.87 m/s

Answer

v = 94.87 m/s
(341.5 km/h)
2.Kinematics

Answer: s = 325 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 = 2.5·5 = 12.5 m/s

Distance of each phase

s1 = ½·2.5·5² = 31.25 m
s2 = 12.5·21 = 262.5 m
s3 = 12.5²/(2·2.5) = 31.25 m

Total distance

s = 31.25 + 262.5 + 31.25 = 325 m

Answer

s = 325 m
3.Newton’s laws

Answer: a = 2.367 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 = 196.2 − 120·sin 35° = 127.4 N

Horizontal forces

m·a = F·cos θ − μ·N = 98.3 − 50.95
a = 47.35/20 = 2.367 m/s²

Answer

a = 2.367 m/s²
4.Newton’s laws

Answer: a = 2.566 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) = (49.05 − 15.7)/13 = 2.566 m/s²

Answer

a = 2.566 m/s²
5.Projectile motion

Answer: v0 = 11.32 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) = √(9.81·10/sin 130°) = 11.32 m/s

Answer

v0 = 11.32 m/s
6.Projectile motion

Answer: x = 100.2 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 = 27·cos 20° = 25.37 m/s
vy = v0·sin θ = 27·sin 20° = 9.235 m/s

Time in the air

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

0 = 40 + 9.235·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (9.235 + √(85.28 + 784.8))/9.81 = 3.948 s

Horizontal distance

x = vx·t = 25.37·3.948 = 100.2 m

Answer

x = 100.2 m
7.Circular motion

Answer: vmax = 21.01 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) = √(0.5·9.81·90) = 21.01 m/s

Answer

vmax = 21.01 m/s (75.64 km/h)
8.Circular motion

Answer: hmin = 21.25 m

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.

Condition at the top

m·g = m·v²/R → v² = g·R

Energy conservation

g·h = g·2R + ½·g·R → h = 2.5·R = 2.5·8.5 = 21.25 m

Answer

hmin = 21.25 m
9.Work & energy

Answer: T = 29.43 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 bottom

v² = 2gL(1 − cos θ) = 2·9.81·3·(1 − cos 90°) = 58.86 m²/s²

Forces at the bottom

m·g = 1·9.81 = 9.81 N
T = mg + mv²/L = 9.81 + 1·58.86/3 = 29.43 N

Answer

T = 29.43 N

Check: T = mg(3 − 2cos θ) = 29.43 N.

10.Work & energy

Answer: v = 9.448 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 = 98.1·(5 − 0.3·1.5) = 446.4 J

Speed at the spring

½mv² = mg(h − μL) → v = √(2g(h − μL)) = √(2·9.81·4.55) = 9.448 m/s

Answer

v = 9.448 m/s
11.Momentum & collisions

Answer: ΔK = 106364 J

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.

Common velocity

v′ = m1·v1/(m1 + m2) = 1300·20/2200 = 11.82 m/s

Kinetic energy before and after

Kbefore = ½·1300·20² = 260000 J
Kafter = ½·2200·11.82² = 153636 J

Energy lost

ΔK = 260000 − 153636 = 106364 J

Answer

ΔK = 106364 J

The lost energy goes into deformation, heat and sound.

12.Momentum & collisions

Answer: v = 665.9 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) = 1.5·(1 − cos 25°) = 0.1405 m

Speed after impact

V = √(2·9.81·0.1405) = 1.661 m/s

Collision

v = (m + M)·V/m = 2.005·1.661/0.005 = 665.9 m/s

Answer

v = 665.9 m/s
13.Springs & pendulums

Answer: v = 2.385 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 = √(160/0.9) = 13.33 rad/s

Speed at x

v = ω·√(A² − x²) = 13.33·√(0.18² − 0.02²) = 2.385 m/s

Answer

v = 2.385 m/s
14.Springs & pendulums

Answer: A = 2.179 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) = 0.01·100/3.51 = 0.2849 m/s

Angular frequency

ω = √(k/(m + M)) = √(600/3.51) = 13.07 rad/s

Amplitude

A = V/ω = 0.2849/13.07 = 0.02179 m = 2.179 cm

Answer

A = 2.179 cm
15.Waves, sound & optics

Answer: β2 = 70 dB

Waves: a periodic wave moves one wavelength in one period, so v = fλ with T = 1/f. On a stretched string v = √(T/μ), with T the tension and μ = m/L the mass per metre. Sound in air travels at about 343 m/s at 20 °C, light in vacuum at c = 3.00 × 10⁸ m/s. Loudness is measured on a logarithmic scale: β = 10·log₁₀(I/I₀) dB with I₀ = 10⁻¹² W/m², so 10 times the intensity adds 10 dB. A point source spreads its power P over a sphere: I = P/(4πr²).

Intensity ratio

I₂/I₁ = (2/20)² = 0.01

New level

β₂ = 90 + 10·log₁₀(0.01) = 70 dB

Answer

β2 = 70 dB

Every doubling of the distance lowers the level by about 6 dB.

16.Waves, sound & optics

Answer: θ3 = 0.8595 °

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, …

Dark fringe condition

sin θ = mλ/a = 0.015
θ = 0.8595°

Answer

θ3 = 0.8595°

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