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)t = ?13.89

A car driving at 50 km/h brakes with a constant deceleration of 4.5 m/s² until it stops. How long does braking take? Find t.

v₀ = 50 km/ha = −4.5 m/s²v = 0
t = s
2.KinematicsHard
t (s)v (m/s)4 s

A tram starts from rest and accelerates at 2.5 m/s² for 4 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 = 4 st2 = 19 sa3 = −2.5 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 tension T in the string.

m1 = 5 kgm2 = 4.5 kgg = 9.81 m/s²
T = N
4.Newton’s lawsHard
θ = 35°blockv₀ = 9 m/s

A block is given a push so that it starts sliding up a ramp at 35° with a speed of 9 m/s. The coefficient of kinetic friction is 0.1. Use g = 9.81 m/s². How far d along the ramp does it slide before it stops?

v₀ = 9 m/sθ = 35°μ = 0.1g = 9.81 m/s²
d = m
5.Projectile motionMedium
69 my = ?v0​ = 36 m/s30°

A ball is kicked from level ground at 36 m/s and 30° above the horizontal towards a wall 69 m away. Ignore air resistance and use g = 9.81 m/s². At what height y does the ball pass the wall?

v₀ = 36 m/sθ = 30°x = 69 mg = 9.81 m/s²
y = m
6.Projectile motionHard
h = 30 mv0​ = 17 m/s35°α = ?

A ball is launched at 17 m/s and 35° above the horizontal from the top of a 30 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 = 17 m/sθ = 35°h = 30 mg = 9.81 m/s²
α = °
7.Circular motionMedium
r = 100 mvmax​ = ?

A car takes a flat curve of radius 100 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 = 100 mμs = 0.8g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
R = 3.5 mh = 12.95 m

A 0.5 kg cart is released from rest at a height of 12.95 m on a frictionless track and runs through a vertical loop of radius 3.5 m. Use g = 9.81 m/s². Find the normal force Ntop of the track on the cart at the top of the loop.

m = 0.5 kgh = 12.95 mR = 3.5 mg = 9.81 m/s²
Ntop = N
9.Work & energyMedium
90 km/h8°1200 kg

A 1200 kg car drives up an 8° slope at a constant 90 km/h. Air resistance and rolling resistance together amount to 700 N. Find the power P the wheels must deliver, in kW.

m = 1200 kgθ = 8°Fr = 700 Nv = 90 km/hg = 9.81 m/s²
P = kW
10.Work & energyHard
h = 3.5 mL = 1.5 mμ = 0.2k = 800 N/m

A 4 kg block is released from rest at height 3.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 1.5 m long (μ = 0.2) and then runs onto a spring (k = 800 N/m) on a smooth surface. Find the maximum compression x of the spring.

m = 4 kgh = 3.5 mL = 1.5 mμ = 0.2k = 800 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
16 kg26 m/s3 kgv′2​ = ?

A 19 kg projectile moving horizontally at 13 m/s explodes into two fragments that move along the same line. The 16 kg fragment continues forwards at 26 m/s. Find the velocity v′2 of the 3 kg fragment (positive in the original direction).

M = 19 kgu = 13 m/sm1 = 16 kgv′1 = 26 m/sm2 = 3 kg
v′2 = m/s
12.Momentum & collisionsHard
16 m/s9 m/s

A 0.45 kg ball hits a wall at 16 m/s and bounces straight back at 9 m/s. The contact lasts 5 ms. Find the magnitude of the impulse J the wall gives the ball.

m = 0.45 kgvin = 16 m/svout = 9 m/sΔt = 5 ms
J = N·s
13.Springs & pendulumsMedium
L = 1.4 mT = 3.86 s

An astronaut on another planet finds that a simple pendulum 1.4 m long has a period of 3.86 s for small swings. Find the gravitational acceleration g on that planet.

L = 1.4 mT = 3.86 s
g = m/s²
14.Springs & pendulumsHard
2.6 kgk = 370 N/mA = 7 cm

A 2.6 kg mass on a spring with k = 370 N/m is pulled 7 cm from equilibrium and released. Find the total energy E of the oscillation.

m = 2.6 kgk = 370 N/mA = 7 cm
E = J
15.Waves, sound & opticsMedium
FFdo​ = 15 cmf = 8 cm

An object stands 15 cm in front of a thin converging lens with a focal length of 8 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 = 15 cmf = 8 cm
di = cm
16.Waves, sound & opticsHard
Fdo​ = 12 cmR = 30 cm

An object is 12 cm in front of a concave mirror with a radius of curvature of 30 cm. Find the magnification m (negative for an inverted image).

do = 12 cmR = 30 cm
m =

3Solutions

1.Kinematics

Answer: t = 3.086 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.

Units

v₀ = 50/3.6 = 13.89 m/s

Braking time

0 = v₀ − a·t → t = v₀/a = 13.89/4.5 = 3.086 s

Answer

t = 3.086 s
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 = 2.5·4 = 10 m/s

Braking time

t3 = v/a3 = 10/2.5 = 4 s

Total time

t = 4 + 19 + 4 = 27 s

Answer

t = 27 s
3.Newton’s laws

Answer: T = 46.47 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

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

Tension from the lighter mass

T − m2·g = m2·a → T = 4.5·(9.81 + 0.5163) = 46.47 N

Answer

T = 46.47 N
4.Newton’s laws

Answer: d = 6.298 m

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

Both forces point down the slope; the mass cancels.

a = 9.81·(sin 35° + 0.1·cos 35°) = 6.43 m/s²

Distance

d = v₀²/(2a) = 9²/(2·6.43) = 6.298 m

Answer

d = 6.298 m
5.Projectile motion

Answer: y = 15.81 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.

Time to the wall

vx = 36·cos 30° = 31.18 m/s
t = x/vx = 69/31.18 = 2.213 s

Height at that time

y = 18·2.213 − ½·9.81·2.213² = 15.81 m

Answer

y = 15.81 m

The ball is already on its way down at the wall.

6.Projectile motion

Answer: α = 61.96 °

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 = 17·cos 35° = 13.93 m/s
vy0 = 17·sin 35° = 9.751 m/s

Vertical speed at impact

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

vy = √(9.751² + 2·9.81·30) = 26.15 m/s

Impact angle

tan α = vy/vx = 26.15/13.93 → α = 61.96°

Answer

α = 61.96°
7.Circular motion

Answer: vmax = 28.01 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) = √(0.8·9.81·100) = 28.01 m/s

Answer

vmax = 28.01 m/s (100.9 km/h)
8.Circular motion

Answer: Ntop = 11.77 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 from energy conservation

v² = 2·9.81·(12.95 − 2·3.5) = 116.7 m²/s²

Forces

Towards the centre is positive.

N = m·v²/R − m·g = 0.5·116.7/3.5 − 4.905 = 11.77 N

Answer

Ntop = 11.77 N
9.Work & energy

Answer: P = 58.46 kW

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). Power is force times speed: P = F·v.

Driving force

m·g = 1200·9.81 = 11772 N
mg·sin θ = 11772·sin 8° = 1638 N
F = 1638 + 700 = 2338 N

Power

v = 90/3.6 = 25 m/s
P = F·v = 2338·25 = 58459 W = 58.46 kW

Answer

P = 58.46 kW
10.Work & energy

Answer: x = 56.03 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 = 39.24·(3.5 − 0.2·1.5) = 125.6 J

Maximum compression

½kx² = 125.6 → x = √(2·125.6/800) = 0.5603 m = 56.03 cm

Answer

x = 56.03 cm
11.Momentum & collisions

Answer: v′2 = −56.33 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 = 19·13 = 247 kg·m/s

Momentum after

247 = 16·26 + 3·v′2
v′2 = (247 − 416)/3 = −56.33 m/s

Answer

v′2 = −56.33 m/s

The minus sign means the second fragment flies backwards.

12.Momentum & collisions

Answer: J = 11.25 N·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.

Change in momentum

J = m·Δv = 0.45·(9 + 16) = 11.25 N·s

Answer

J = 11.25 N·s
13.Springs & pendulums

Answer: g = 3.709 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 = 4π²·1.4/3.86² = 3.709 m/s²

Answer

g = 3.709 m/s²
14.Springs & pendulums

Answer: E = 0.9065 J

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 = √(370/2.6) = 11.93 rad/s

Energy

E = ½·k·A² = ½·370·0.07² = 0.9065 J

Answer

E = 0.9065 J
15.Waves, sound & optics

Answer: di = 17.14 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 = 1/8 − 1/15
di = 17.14 cm
FF15 cmf = 8 cm

Ray diagram

real, on the far side of the lens

Answer

di = 17.14 cm
16.Waves, sound & optics

Answer: m = 5

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 = 15 cm

Mirror equation

1/di = 1/f − 1/do = 1/15 − 1/12
di = −60 cm
F12 cmf = 15 cm

Magnification

m = −(−60)/12 = 5

Answer

m = 5

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