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)3.333 s10

A car driving at 36 km/h brakes with a constant deceleration of 3 m/s² until it stops. Find the braking distance s.

v₀ = 36 km/ha = −3 m/s²v = 0
s = m
2.KinematicsHard
t (s)v (m/s)12 s

A tram starts from rest and accelerates at 2.5 m/s² for 12 s, then runs at constant speed for 9 s, and finally brakes at 5 m/s² until it stops. Find the total travel time t.

a1 = 2.5 m/s²t1 = 12 st2 = 9 sa3 = −5 m/s²
t = s
3.Newton’s lawsMedium
5 kg4.5 kg

Masses of 5 kg and 4.5 kg hang on either side of a light, frictionless pulley, connected by a light string. The system is released from rest. Use g = 9.81 m/s². Find the acceleration a of the masses.

m1 = 5 kgm2 = 4.5 kgg = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
m1​ = 5.5 kg2 kgμ = 0.2

A 5.5 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 2 kg mass. The coefficient of kinetic friction between block and table is 0.2. Use g = 9.81 m/s². Find the tension T in the string.

m1 = 5.5 kgm2 = 2 kgμ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
v0​ = 20 m/s65°rangeH = ?

A ball is launched at 65° above horizontal from level ground. Ignore air resistance. Find the maximum height H the ball reaches.

v0 = 20 m/sθ = 65°g = 9.81 m/s²
H = m
6.Projectile motionHard
h = 10 mv0​ = 31 m/s25°H = ?

A ball is launched at 31 m/s and 25° above the horizontal from the top of a 10 m cliff. Ignore air resistance and use g = 9.81 m/s². Find the greatest height H above the ground at the foot of the cliff.

v₀ = 31 m/sθ = 25°h = 10 mg = 9.81 m/s²
H = m
7.Circular motionMedium
r = 20 mvmax​ = ?

A car takes a flat curve of radius 20 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 = 20 mμs = 0.5g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
r = 0.5 m5.5 m/svtop​ = ?

A 0.6 kg ball on a light string 0.5 m long is swung in a vertical circle. At the lowest point its speed is 5.5 m/s. Ignore air resistance and use g = 9.81 m/s². Find the tension Ttop in the string at the highest point.

m = 0.6 kgr = 0.5 mvbottom = 5.5 m/sg = 9.81 m/s²
Ttop = N
9.Work & energyMedium
40°d = 15 mμ = 0.1

A 2 kg block slides 15 m down a slope at 40° to the horizontal, with a coefficient of kinetic friction of 0.1. Find the work Wf done against friction.

m = 2 kgθ = 40°μ = 0.1d = 15 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
F = 165 N25°μ = 0.120 kg

A 20 kg block starts from rest at the bottom of a 25° slope and is pulled up the slope by a constant force of 165 N parallel to the slope. The coefficient of kinetic friction is 0.1. Find the speed v of the block after it has moved 2 m up the slope.

m = 20 kgF = 165 Nθ = 25°μ = 0.1d = 2 mg = 9.81 m/s²
v = m/s
11.Momentum & collisionsMedium
3.5 kgv = ?12 gh = 6 cm

A 12 g bullet is fired horizontally into a 3.5 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 6 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.

m = 12 gM = 3.5 kgh = 6 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
2000 kg8 m/s1300 kg18 m/sv′ = ?

At a crossing a 2000 kg car travelling east at 8 m/s collides with a 1300 kg car travelling north at 18 m/s. The cars lock together. Find the speed v′ of the wreck just after the collision.

m1 = 2000 kgv1 = 8 m/s (E)m2 = 1300 kgv2 = 18 m/s (N)
v′ = m/s
13.Springs & pendulumsMedium
L = ?

How long must a simple pendulum be to have a period of 1.3 s? Use g = 9.81 m/s². Find L.

T = 1.3 sg = 9.81 m/s²
L = m
14.Springs & pendulumsHard
3 kgk = 1000 N/m200 m/s

An 8 g bullet moving at 200 m/s hits and stays in a 3 kg block resting on a frictionless surface. The block is attached to a horizontal spring with k = 1000 N/m. Find the amplitude A of the resulting oscillation in cm.

m = 8 gv = 200 m/sM = 3 kgk = 1000 N/m
A = cm
15.Waves, sound & opticsMedium
FF4 cmdo​ = 4 cmf = 25 cm

An object 4 cm tall stands 4 cm in front of a thin converging lens with a focal length of 25 cm. Find the image height hi (negative for an inverted image).

ho = 4 cmdo = 4 cmf = 25 cm
hi = cm
16.Waves, sound & opticsHard
Fdo​ = 30 cmR = 20 cm

An object is 30 cm in front of a concave mirror with a radius of curvature of 20 cm. Find the image distance di, positive for an image in front of the mirror (real) and negative for one behind it (virtual).

do = 30 cmR = 20 cm
di = cm

3Solutions

1.Kinematics

Answer: s = 16.67 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 = 36/3.6 = 10 m/s

Braking distance

0 = v₀² − 2·a·s → s = v₀²/(2a) = 10²/(2·3) = 16.67 m

Answer

s = 16.67 m
2.Kinematics

Answer: t = 27 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 = 2.5·12 = 30 m/s

Braking time

t3 = v/a3 = 30/5 = 6 s

Total time

t = 12 + 9 + 6 = 27 s

Answer

t = 27 s
3.Newton’s laws

Answer: a = 0.5163 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

The net driving force is the difference of the weights; it moves the total mass.

a = (5 − 4.5)·9.81/(5 + 4.5) = 0.5163 m/s²

Answer

a = 0.5163 m/s²
4.Newton’s laws

Answer: T = 17.27 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.

Acceleration of the system

a = (m2·g − μ·m1·g)/(m1 + m2) = (19.62 − 10.79)/7.5 = 1.177 m/s²

Tension from the block on the table

T − μ·m1·g = m1·a
T = 5.5·1.177 + 10.79 = 17.27 N

Answer

T = 17.27 N
5.Projectile motion

Answer: H = 16.75 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 = 20·cos 65° = 8.452 m/s
vy = v0·sin θ = 20·sin 65° = 18.13 m/s

Highest point

At the top the vertical velocity is zero: vy² = 2gH.

H = vy²/(2g) = 18.13²/(2·9.81) = 16.75 m

Answer

H = 16.75 m
6.Projectile motion

Answer: H = 18.75 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.

Vertical launch speed

vy = 31·sin 25° = 13.1 m/s

Rise above the cliff

Δh = vy²/(2g) = 13.1²/(2·9.81) = 8.748 m

Height above the ground

H = 10 + 8.748 = 18.75 m

Answer

H = 18.75 m
7.Circular motion

Answer: vmax = 9.905 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) = √(0.5·9.81·20) = 9.905 m/s

Answer

vmax = 9.905 m/s (35.66 km/h)
8.Circular motion

Answer: Ttop = 6.87 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.

Speed at the top

Kinetic energy is traded for potential energy over a height 2r.

vtop² = 5.5² − 4·9.81·0.5 = 10.63 m²/s²

Forces at the top

Tension and gravity both point down, towards the centre.

T + m·g = m·vtop²/r
T = 0.6·10.63/0.5 − 0.6·9.81 = 6.87 N

Answer

Ttop = 6.87 N
9.Work & energy

Answer: Wf = 22.54 J

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).

Friction force

N = mg·cos θ = 19.62·0.766 = 15.03 N
F = μN = 0.1·15.03 = 1.503 N

Work against friction

Wf = F·d = 1.503·15 = 22.54 J

Answer

Wf = 22.54 J
10.Work & energy

Answer: v = 3.586 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).

Forces along the slope

m·g = 20·9.81 = 196.2 N
mg·sin θ = 82.92 N
μ·mg·cos θ = 17.78 N

Work–energy

½mv² = (165 − 100.7)·2 = 128.6 J
v = √(2·128.6/20) = 3.586 m/s

Answer

v = 3.586 m/s
11.Momentum & collisions

Answer: v = 317.5 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) = √(2·9.81·0.06) = 1.085 m/s

Collision

m·v = (m + M)·V → v = (0.012 + 3.5)·1.085/0.012 = 317.5 m/s

Answer

v = 317.5 m/s
12.Momentum & collisions

Answer: v′ = 8.59 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 components

px = 2000·8 = 16000 kg·m/s
py = 1300·18 = 23400 kg·m/s

Speed of the wreck

v′ = √(px² + py²)/(m1 + m2) = 28347/3300 = 8.59 m/s

Answer

v′ = 8.59 m/s
13.Springs & pendulums

Answer: L = 0.4199 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·(1.3/2π)² = 0.4199 m

Answer

L = 0.4199 m
14.Springs & pendulums

Answer: A = 2.917 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) = 0.008·200/3.008 = 0.5319 m/s

Angular frequency

ω = √(k/(m + M)) = √(1000/3.008) = 18.23 rad/s

Amplitude

A = V/ω = 0.5319/18.23 = 0.02917 m = 2.917 cm

Answer

A = 2.917 cm
15.Waves, sound & optics

Answer: hi = 4.762 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 = 1/25 − 1/4
di = −4.762 cm
FF4 cmf = 25 cm

Magnification and height

m = −(−4.762)/4 = 1.19
hi = 1.19·4 = 4.762 cm

Answer

hi = 4.762 cm
16.Waves, sound & optics

Answer: di = 15 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.

Focal length

f = R/2 = 10 cm

Mirror equation

1/di = 1/f − 1/do = 1/10 − 1/30
di = 15 cm
F30 cmf = 10 cm

Ray diagram

real image in front of the mirror

Answer

di = 15 cm

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