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)5.556 s25

A car driving at 90 km/h brakes with a constant deceleration of 4.5 m/s² until it stops. Find the braking distance s.

v₀ = 90 km/ha = −4.5 m/s²v = 0
s = m
2.KinematicsHard
t (s)v (m/s)7 s

A tram starts from rest and accelerates at 2.5 m/s² for 7 s, then runs at constant speed for 19 s, and finally brakes at 2.5 m/s² until it stops. Find the total travel time t.

a1 = 2.5 m/s²t1 = 7 st2 = 19 sa3 = −2.5 m/s²
t = s
3.Newton’s lawsMedium
boxv₀ = 11.5 m/sμ = ?

A box slides across a level floor with an initial speed of 11.5 m/s and comes to rest after 130 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.

v₀ = 11.5 m/sd = 130 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
m1​ = 4.5 kg4 kgθ = 20°

A 4.5 kg block on a ramp at 20° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 4 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.2. Use g = 9.81 m/s². Find the tension T in the string.

m1 = 4.5 kgm2 = 4 kgθ = 20°μ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
v0​ = 16 m/s60°range

A ball is launched at 60° above horizontal from level ground. Ignore air resistance. Find the time of flight t until the ball hits the ground.

v0 = 16 m/sθ = 60°g = 9.81 m/s²
t = s
6.Projectile motionHard
h = 60 mv0​ = 16 m/s45°range

A ball is launched at 45° 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 = 16 m/sθ = 45°h = 60 mg = 9.81 m/s²
t = s
7.Circular motionMedium
θ = 25°to centrev = ?

A curve of radius 200 m is banked at 25°. At what speed v can a car take it with no friction needed at all? Use g = 9.81 m/s².

r = 200 mθ = 25°g = 9.81 m/s²
v = m/s
8.Circular motionHard
r = 1.2 mv = 3.5 m/s

A 1.9 kg ball on a string 1.2 m long is swung in a vertical circle. At the lowest point it moves at 3.5 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.

m = 1.9 kgr = 1.2 mv = 3.5 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
110 km/h

A car travelling at 110 km/h on a level road brakes hard and skids to a stop with its wheels locked. The coefficient of kinetic friction between the tyres and the road is 0.5. Find the stopping distance d.

v = 110 km/hμ = 0.5g = 9.81 m/s²
d = m
10.Work & energyHard
h = 2 mL = 2.5 mμ = 0.1k = 5000 N/m

A 0.5 kg block is released from rest at height 2 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 2.5 m long (μ = 0.1) and then runs onto a spring (k = 5000 N/m) on a smooth surface. Find the maximum compression x of the spring.

m = 0.5 kgh = 2 mL = 2.5 mμ = 0.1k = 5000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
500 kg10 m/s900 kg

A 500 kg car moving at 10 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 = 500 kgv1 = 10 m/sm2 = 900 kgv2 = 0
ΔK = J
12.Momentum & collisionsHard
4 kgv = ?8 g35°L = 0.7 m

An 8 g bullet is fired horizontally into a 4 kg block hanging from light strings 0.7 m long, and stays in it. The strings swing out to a largest angle of 35° from the vertical. Use g = 9.81 m/s². Find the speed v of the bullet before impact.

m = 8 gM = 4 kgL = 0.7 mθ = 35°g = 9.81 m/s²
v = m/s
13.Springs & pendulumsMedium
L = ?

How long must a simple pendulum be to have a period of 1.1 s? Use g = 9.81 m/s². Find L.

T = 1.1 sg = 9.81 m/s²
L = m
14.Springs & pendulumsHard
2.9 kgk = 160 N/m

A 2.9 kg mass is attached to the lower end of an unstretched vertical spring with k = 160 N/m and released from rest. Use g = 9.81 m/s². Find the greatest extension xmax of the spring in cm.

m = 2.9 kgk = 160 N/mg = 9.81 m/s²
xmax = cm
15.Waves, sound & opticsMedium
L = ?n = 3

A pipe open at both ends must have its harmonic number 3 at 515 Hz. The speed of sound is 343 m/s. Find the length L of the pipe.

v = 343 m/sn = 3f3 = 515 Hz
L = m
16.Waves, sound & opticsHard
λ = 650 nmd = 0.25 mmL = 2.5 m

Light of wavelength 650 nm falls on two slits 0.25 mm apart. The screen is 2.5 m away. Find the spacing Δy between neighbouring bright fringes.

λ = 650 nmd = 0.25 mmL = 2.5 m
Δy = mm

3Solutions

1.Kinematics

Answer: s = 69.44 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 = 90/3.6 = 25 m/s

Braking distance

0 = v₀² − 2·a·s → s = v₀²/(2a) = 25²/(2·4.5) = 69.44 m

Answer

s = 69.44 m
2.Kinematics

Answer: t = 33 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.

Top speed

v = a1·t1 = 2.5·7 = 17.5 m/s

Braking time

t3 = v/a3 = 17.5/2.5 = 7 s

Total time

t = 7 + 19 + 7 = 33 s

Answer

t = 33 s
3.Newton’s laws

Answer: μ = 0.05185

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.

Deceleration

a = v₀²/(2d) = 11.5²/(2·130) = 0.5087 m/s²

Friction coefficient

μ·m·g = m·a → μ = a/g = 0.5087/9.81 = 0.05185

Answer

μ = 0.05185
4.Newton’s laws

Answer: T = 31.78 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 = 39.24 N
m₁·g·sin θ = 15.1 N
μ·m₁·g·cos θ = 8.297 N

Acceleration of the system

a = (39.24 − 15.1 − 8.297)/8.5 = 1.864 m/s²

Tension from the hanging mass

m2·g − T = m2·a → T = 4·(9.81 − 1.864) = 31.78 N

Answer

T = 31.78 N
5.Projectile motion

Answer: t = 2.825 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 = 16·cos 60° = 8 m/s
vy = v0·sin θ = 16·sin 60° = 13.86 m/s

Time in the air

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

t = 2·13.86/9.81 = 2.825 s

Answer

t = 2.825 s
6.Projectile motion

Answer: t = 4.836 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 = 16·cos 45° = 11.31 m/s
vy = v0·sin θ = 16·sin 45° = 11.31 m/s

Time in the air

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

0 = 60 + 11.31·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (11.31 + √(128 + 1177))/9.81 = 4.836 s

Answer

t = 4.836 s
7.Circular motion

Answer: v = 30.25 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 balance

N·cos θ = m·g, N·sin θ = m·v²/r → tan θ = v²/(r·g)

Speed

v = √(r·g·tan θ) = √(200·9.81·tan 25°) = 30.25 m/s

Answer

v = 30.25 m/s
(108.9 km/h)
8.Circular motion

Answer: T = 38.03 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 = 1.9·9.81 + 1.9·3.5²/1.2 = 18.64 + 19.4 = 38.03 N

Answer

T = 38.03 N
9.Work & energy

Answer: d = 95.17 m

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).

Speed in base units

v = 110/3.6 = 30.56 m/s

Energy balance

½mv² = μmg·d → d = v²/(2μg) = 30.56²/(2·0.5·9.81) = 95.17 m

Answer

d = 95.17 m

Doubling the speed makes the stopping distance four times longer.

10.Work & energy

Answer: x = 5.86 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·(2 − 0.1·2.5) = 8.584 J

Maximum compression

½kx² = 8.584 → x = √(2·8.584/5000) = 0.0586 m = 5.86 cm

Answer

x = 5.86 cm
11.Momentum & collisions

Answer: ΔK = 16071 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) = 500·10/1400 = 3.571 m/s

Kinetic energy before and after

Kbefore = ½·500·10² = 25000 J
Kafter = ½·1400·3.571² = 8929 J

Energy lost

ΔK = 25000 − 8929 = 16071 J

Answer

ΔK = 16071 J

The lost energy goes into deformation, heat and sound.

12.Momentum & collisions

Answer: v = 789.6 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) = 0.7·(1 − cos 35°) = 0.1266 m

Speed after impact

V = √(2·9.81·0.1266) = 1.576 m/s

Collision

v = (m + M)·V/m = 4.008·1.576/0.008 = 789.6 m/s

Answer

v = 789.6 m/s
13.Springs & pendulums

Answer: L = 0.3007 m

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 the length

L = g·(T/2π)² = 9.81·(1.1/2π)² = 0.3007 m

Answer

L = 0.3007 m
14.Springs & pendulums

Answer: xmax = 35.56 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.

Equilibrium and amplitude

x₀ = m·g/k = 2.9·9.81/160 = 0.1778 m = A

Lowest point

xmax = 2A = 0.3556 m = 35.56 cm

Answer

xmax = 35.56 cm
15.Waves, sound & optics

Answer: L = 0.999 m

Standing waves and the Doppler effect: a string fixed at both ends, or a pipe open at both ends, fits whole half-wavelengths: L = nλ/2, fn = n·v/(2L), n = 1, 2, 3, … A pipe closed at one end has a node at the closed end and an antinode at the open end: L = nλ/4 with odd n only, fn = n·v/(4L). When source or observer move along the line between them, f′ = f·(v ± vo)/(v ∓ vs): use the upper signs for motion towards each other. Two waves of nearly equal frequency beat at |f₁ − f₂|.

Wavelength

λ = v/f = 343/515 = 0.666 m

Length

L = nλ/2 = 3·0.666/2 = 0.999 m

Answer

L = 0.999 m
16.Waves, sound & optics

Answer: Δy = 6.5 mm

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, …

Fringe spacing

Δy = λL/d = 650 × 10⁻⁹·2.5/(0.25 × 10⁻³) = 6.5 mm

Answer

Δy = 6.5 mm

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