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)2.222 s10

A car driving at 36 km/h brakes with a constant deceleration of 4.5 m/s² until it stops. Find the braking distance s.

v₀ = 36 km/ha = −4.5 m/s²v = 0
s = m
2.KinematicsHard
v₀ = 9 m/s30 m

A ball is thrown straight up at 9 m/s from the edge of a roof 30 m above the ground. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax above the ground.

v₀ = 9 m/sh₀ = 30 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
θ = 35°a = ?m = 9 kg

A 9 kg block slides down a frictionless slope at 35° to the horizontal. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.

m = 9 kgθ = 35°μ = 0g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
m1​ = 4.5 kg3 kgμ = 0.1

A 4.5 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 3 kg mass. The coefficient of kinetic friction between block and table is 0.1. Use g = 9.81 m/s². Find the acceleration a of the system.

m1 = 4.5 kgm2 = 3 kgμ = 0.1g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
9 my = ?v0​ = 20 m/s25°

A ball is kicked from level ground at 20 m/s and 25° above the horizontal towards a wall 9 m away. Ignore air resistance and use g = 9.81 m/s². At what height y does the ball pass the wall?

v₀ = 20 m/sθ = 25°x = 9 mg = 9.81 m/s²
y = m
6.Projectile motionHard
h = 15 mv0​ = 20 m/s35°range

A ball is launched at 35° above horizontal from the top of a 15 m cliff. Ignore air resistance. Find the time of flight t until the ball hits the ground.

v0 = 20 m/sθ = 35°h = 15 mg = 9.81 m/s²
t = s
7.Circular motionMedium
r = 50 mvmax​ = ?

A car takes a flat curve of radius 50 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 = 50 mμs = 0.5g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
R = 6.5 mh = 20.15 m

A 3.5 kg cart is released from rest at a height of 20.15 m on a frictionless track and runs through a vertical loop of radius 6.5 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 = 3.5 kgh = 20.15 mR = 6.5 mg = 9.81 m/s²
Ntop = N
9.Work & energyMedium
40 km/h

A car travelling at 40 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 0.6. Find the stopping distance d.

v = 40 km/hμ = 0.6g = 9.81 m/s²
d = m
10.Work & energyHard
h = 3 mL = 4 mμ = 0.1k = 1000 N/m

A 2.5 kg block is released from rest at height 3 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 4 m long (μ = 0.1) and then runs onto a spring (k = 1000 N/m) on a smooth surface. Find the maximum compression x of the spring.

m = 2.5 kgh = 3 mL = 4 mμ = 0.1k = 1000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
6 kg86 m/s6 kgv′2​ = ?

A 12 kg projectile moving horizontally at 40 m/s explodes into two fragments that move along the same line. The 6 kg fragment continues forwards at 86 m/s. Find the velocity v′2 of the 6 kg fragment (positive in the original direction).

M = 12 kgu = 40 m/sm1 = 6 kgv′1 = 86 m/sm2 = 6 kg
v′2 = m/s
12.Momentum & collisionsHard
21 m/s9 m/s

A 0.1 kg ball hits a wall at 21 m/s and bounces straight back at 9 m/s. The contact lasts 10 ms. Find the average force Favg of the wall on the ball.

m = 0.1 kgvin = 21 m/svout = 9 m/sΔt = 10 ms
Favg = N
13.Springs & pendulumsMedium
1.2 kgk = 260 N/mA = 7 cmx = 1 cm

A 1.2 kg mass on a spring with k = 260 N/m oscillates on a frictionless surface with an amplitude of 7 cm. Find its speed v when it is 1 cm from equilibrium.

m = 1.2 kgk = 260 N/mA = 7 cmx = 1 cm
v = m/s
14.Springs & pendulumsHard
2.4 kgk = 40 N/mA = 11 cm

A 2.4 kg mass on a spring with k = 40 N/m is pulled 11 cm from equilibrium and released. Find its maximum speed vmax.

m = 2.4 kgk = 40 N/mA = 11 cm
vmax = m/s
15.Waves, sound & opticsMedium
FFdo​ = 6 cmf = −15 cm

An object stands 6 cm in front of a thin diverging lens with a focal length of 15 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 = 6 cmf = −15 cm
di = cm
16.Waves, sound & opticsHard
Fdo​ = 4 cmR = 60 cm

An object is 4 cm in front of a concave mirror with a radius of curvature of 60 cm. Find the image distance di, positive for an image in front of the mirror (real) and negative for one behind it (virtual).

do = 4 cmR = 60 cm
di = cm

3Solutions

1.Kinematics

Answer: s = 11.11 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/(2·4.5) = 11.11 m

Answer

s = 11.11 m
2.Kinematics

Answer: hmax = 34.13 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 = 9²/(2·9.81) = 4.128 m

Height above the ground

hmax = 30 + 4.128 = 34.13 m

Answer

hmax = 34.13 m
3.Newton’s laws

Answer: a = 5.627 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 35° − 0·cos 35°) = 9.81·(0.5736 − 0) = 5.627 m/s²

Answer

a = 5.627 m/s²
4.Newton’s laws

Answer: a = 3.335 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) = (29.43 − 4.415)/7.5 = 3.335 m/s²

Answer

a = 3.335 m/s²
5.Projectile motion

Answer: y = 2.988 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 = 20·cos 25° = 18.13 m/s
t = x/vx = 9/18.13 = 0.4965 s

Height at that time

y = 8.452·0.4965 − ½·9.81·0.4965² = 2.988 m

Answer

y = 2.988 m

The ball is still rising at the wall.

6.Projectile motion

Answer: t = 3.273 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 = 20·cos 35° = 16.38 m/s
vy = v0·sin θ = 20·sin 35° = 11.47 m/s

Time in the air

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

0 = 15 + 11.47·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (11.47 + √(131.6 + 294.3))/9.81 = 3.273 s

Answer

t = 3.273 s
7.Circular motion

Answer: vmax = 15.66 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) = √(0.5·9.81·50) = 15.66 m/s

Answer

vmax = 15.66 m/s (56.38 km/h)
8.Circular motion

Answer: Ntop = 41.2 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·(20.15 − 2·6.5) = 140.3 m²/s²

Forces

Towards the centre is positive.

N = m·v²/R − m·g = 3.5·140.3/6.5 − 34.34 = 41.2 N

Answer

Ntop = 41.2 N
9.Work & energy

Answer: d = 10.49 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 = 40/3.6 = 11.11 m/s

Energy balance

½mv² = μmg·d → d = v²/(2μg) = 11.11²/(2·0.6·9.81) = 10.49 m

Answer

d = 10.49 m

Doubling the speed makes the stopping distance four times longer.

10.Work & energy

Answer: x = 35.71 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 = 24.53·(3 − 0.1·4) = 63.77 J

Maximum compression

½kx² = 63.77 → x = √(2·63.77/1000) = 0.3571 m = 35.71 cm

Answer

x = 35.71 cm
11.Momentum & collisions

Answer: v′2 = −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.

Momentum before

p = M·u = 12·40 = 480 kg·m/s

Momentum after

480 = 6·86 + 6·v′2
v′2 = (480 − 516)/6 = −6 m/s

Answer

v′2 = −6 m/s

The minus sign means the second fragment flies backwards.

12.Momentum & collisions

Answer: Favg = 300 N

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 = 0.1·(9 + 21) = 3 N·s

Average force

Favg = J/Δt = 3/0.01 = 300 N

Answer

Favg = 300 N
13.Springs & pendulums

Answer: v = 1.02 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 = √(260/1.2) = 14.72 rad/s

Speed at x

v = ω·√(A² − x²) = 14.72·√(0.07² − 0.01²) = 1.02 m/s

Answer

v = 1.02 m/s
14.Springs & pendulums

Answer: vmax = 0.4491 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 = √(40/2.4) = 4.082 rad/s

Maximum speed

vmax = A·ω = 0.11·4.082 = 0.4491 m/s

Answer

vmax = 0.4491 m/s
15.Waves, sound & optics

Answer: di = −4.286 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/(−15) − 1/6
di = −4.286 cm
FF6 cmf = −15 cm

Ray diagram

virtual, on the same side as the object

Answer

di = −4.286 cm
16.Waves, sound & optics

Answer: di = −4.615 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 = 30 cm

Mirror equation

1/di = 1/f − 1/do = 1/30 − 1/4
di = −4.615 cm
F4 cmf = 30 cm

Ray diagram

virtual image behind the mirror

Answer

di = −4.615 cm

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