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 = ?s = 2400 m

A plane needs a speed of 240 km/h to take off and the usable runway is 2400 m long. Starting from rest, what constant acceleration a does it need?

v₀ = 0v = 240 km/hs = 2400 m
a = m/s²
2.KinematicsHard
t (s)v (m/s)12 s

A tram starts from rest and accelerates at 3 m/s² for 12 s, then runs at constant speed for 5 s, and finally brakes at 3 m/s² until it stops. Find the total distance s between the two stops.

a1 = 3 m/s²t1 = 12 st2 = 5 sa3 = −3 m/s²
s = m
3.Newton’s lawsMedium
boxv₀ = 6 m/sμ = ?

A box slides across a level floor with an initial speed of 6 m/s and comes to rest after 7.3 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.

v₀ = 6 m/sd = 7.3 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
m1​ = 5.5 kg5.5 kgθ = 20°

A 5.5 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 5.5 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.1. Use g = 9.81 m/s². Find the acceleration a of the system.

m1 = 5.5 kgm2 = 5.5 kgθ = 20°μ = 0.1g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
v0​ = 16 m/s25°range

A ball is launched at 25° above horizontal from level ground. Ignore air resistance. Find the time of flight t until the ball hits the ground.

v0 = 16 m/sθ = 25°g = 9.81 m/s²
t = s
6.Projectile motionHard
h = 45 mv0​ = 23 m/s35°H = ?

A ball is launched at 23 m/s and 35° above the horizontal from the top of a 45 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 = 23 m/sθ = 35°h = 45 mg = 9.81 m/s²
H = m
7.Circular motionMedium
r = 25 m6 m/s

An 800 kg car drives over the top of a hump in the road at 6 m/s. The top of the hump is a circular arc of radius 25 m. Use g = 9.81 m/s². Find the normal force N of the road on the car at the top.

m = 800 kgv = 6 m/sr = 25 mg = 9.81 m/s²
N = N
8.Circular motionHard
r = 1.4 mv = 6 m/s

A 0.3 kg ball on a string 1.4 m long is swung in a vertical circle. At the lowest point it moves at 6 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.

m = 0.3 kgr = 1.4 mv = 6 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
45°d = 5 mμ = 0.3

A 10 kg block slides 5 m down a slope at 45° to the horizontal, with a coefficient of kinetic friction of 0.3. Find the work Wf done against friction.

m = 10 kgθ = 45°μ = 0.3d = 5 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
h = 3 mL = 4.5 mμ = 0.25k = 1000 N/m

A 10 kg block is released from rest at height 3 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 4.5 m long (μ = 0.25) and then runs onto a spring (k = 1000 N/m) on a smooth surface. Find the maximum compression x of the spring.

m = 10 kgh = 3 mL = 4.5 mμ = 0.25k = 1000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
3.5 kg5 m/s1 kg

A 3.5 kg glider moving at 5 m/s makes a perfectly elastic head-on collision with a 1 kg glider at rest on a frictionless track. Find the velocity v′1 of the first glider after the collision (positive in its original direction).

m1 = 3.5 kgm2 = 1 kgv1 = 5 m/sv2 = 0
v′1 = m/s
12.Momentum & collisionsHard
30 m/s16 m/s

A 0.1 kg ball hits a wall at 30 m/s and bounces straight back at 16 m/s. The contact lasts 20 ms. Find the magnitude of the impulse J the wall gives the ball.

m = 0.1 kgvin = 30 m/svout = 16 m/sΔt = 20 ms
J = N·s
13.Springs & pendulumsMedium
L = 0.6 mT = 2.52 s

An astronaut on another planet finds that a simple pendulum 0.6 m long has a period of 2.52 s for small swings. Find the gravitational acceleration g on that planet.

L = 0.6 mT = 2.52 s
g = m/s²
14.Springs & pendulumsHard
t (s)x (cm)t = ?5.6

A body oscillates in simple harmonic motion with period 2.8 s and amplitude 8 cm. What is the shortest time t it needs to move from equilibrium to a displacement of 5.6 cm?

T = 2.8 sA = 8 cmx = 5.6 cm
t = s
15.Waves, sound & opticsMedium
FFdo​ = 10 cmf = 20 cm

An object stands 10 cm in front of a thin converging lens with a focal length of 20 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 = 10 cmf = 20 cm
di = cm
16.Waves, sound & opticsHard
λ = 532 nmθ1​ = ?

Light of wavelength 532 nm falls at right angles on a grating with 300 lines per millimetre. Find the angle θ1 of the order-1 maximum.

N = 300 mm⁻¹λ = 532 nmm = 1
θ1 = °

3Solutions

1.Kinematics

Answer: a = 0.9259 m/s²

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

v = 240/3.6 = 66.67 m/s

Acceleration

a = v²/(2s) = 66.67²/(2·2400) = 0.9259 m/s²

Answer

a = 0.9259 m/s²
2.Kinematics

Answer: s = 612 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 = 3·12 = 36 m/s

Distance of each phase

s1 = ½·3·12² = 216 m
s2 = 36·5 = 180 m
s3 = 36²/(2·3) = 216 m

Total distance

s = 216 + 180 + 216 = 612 m

Answer

s = 612 m
3.Newton’s laws

Answer: μ = 0.2514

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) = 6²/(2·7.3) = 2.466 m/s²

Friction coefficient

μ·m·g = m·a → μ = a/g = 2.466/9.81 = 0.2514

Answer

μ = 0.2514
4.Newton’s laws

Answer: a = 2.766 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 string

m₂·g = 53.96 N
m₁·g·sin θ = 18.45 N
μ·m₁·g·cos θ = 5.07 N

Acceleration of the system

a = (53.96 − 18.45 − 5.07)/11 = 2.766 m/s²

Answer

a = 2.766 m/s²
5.Projectile motion

Answer: t = 1.379 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 25° = 14.5 m/s
vy = v0·sin θ = 16·sin 25° = 6.762 m/s

Time in the air

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

t = 2·6.762/9.81 = 1.379 s

Answer

t = 1.379 s
6.Projectile motion

Answer: H = 53.87 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 = 23·sin 35° = 13.19 m/s

Rise above the cliff

Δh = vy²/(2g) = 13.19²/(2·9.81) = 8.87 m

Height above the ground

H = 45 + 8.87 = 53.87 m

Answer

H = 53.87 m
7.Circular motion

Answer: N = 6696 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 top

Towards the centre (down) is positive.

m·g − N = m·v²/r
N = 800·9.81 − 800·6²/25 = 7848 − 1152 = 6696 N

Answer

N = 6696 N

The car feels lighter: the road pushes up less than its weight.

8.Circular motion

Answer: T = 10.66 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 = 0.3·9.81 + 0.3·6²/1.4 = 2.943 + 7.714 = 10.66 N

Answer

T = 10.66 N
9.Work & energy

Answer: Wf = 104.1 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 = 98.1·0.7071 = 69.37 N
F = μN = 0.3·69.37 = 20.81 N

Work against friction

Wf = F·d = 20.81·5 = 104.1 J

Answer

Wf = 104.1 J
10.Work & energy

Answer: x = 60.65 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 = 98.1·(3 − 0.25·4.5) = 183.9 J

Maximum compression

½kx² = 183.9 → x = √(2·183.9/1000) = 0.6065 m = 60.65 cm

Answer

x = 60.65 cm
11.Momentum & collisions

Answer: v′1 = 2.778 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.

Elastic collision with a target at rest

Solving momentum and energy conservation together gives these two results.

v′1 = (3.5 − 1)/(3.5 + 1)·5 = 2.778 m/s
v′2 = 2·3.5/(3.5 + 1)·5 = 7.778 m/s

Answer

v′1 = 2.778 m/s
12.Momentum & collisions

Answer: J = 4.6 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 = 0.1·(16 + 30) = 4.6 N·s

Answer

J = 4.6 N·s
13.Springs & pendulums

Answer: g = 3.73 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 = 4π²·0.6/2.52² = 3.73 m/s²

Answer

g = 3.73 m/s²
14.Springs & pendulums

Answer: t = 0.3455 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 = 2π/T = 2π/2.8 = 2.244 rad/s

Solve for the time

0.7 = sin(ω·t) → ω·t = arcsin(0.7) = 0.7754 rad
t = 0.7754/2.244 = 0.3455 s

Answer

t = 0.3455 s
15.Waves, sound & optics

Answer: di = −20 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 = 1/20 − 1/10
di = −20 cm
FF10 cmf = 20 cm

Ray diagram

virtual, on the same side as the object

Answer

di = −20 cm
16.Waves, sound & optics

Answer: θ1 = 9.184 °

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, …

Line spacing

d = 1/300 mm = 3.333 × 10⁻⁶ m

Grating equation

sin θ = mλ/d = 1·532 × 10⁻⁹/(3.333 × 10⁻⁶) = 0.1596
θ = 9.184°

Answer

θ1 = 9.184°

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