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)0.5 s10

A driver moving at 36 km/h sees an obstacle. Her reaction time is 0.5 s, after which the car brakes with a constant deceleration of 5.5 m/s². Find the total stopping distance d from the moment she sees the obstacle.

v₀ = 36 km/htr = 0.5 sa = −5.5 m/s²
d = m
2.KinematicsHard
t (s)v (m/s)9 s

A tram starts from rest and accelerates at 2 m/s² for 9 s, then runs at constant speed for 26 s, and finally brakes at 3.5 m/s² until it stops. Find the total distance s between the two stops.

a1 = 2 m/s²t1 = 9 st2 = 26 sa3 = −3.5 m/s²
s = m
3.Newton’s lawsMedium
boxv₀ = 2.5 m/sμ = ?

A box slides across a level floor with an initial speed of 2.5 m/s and comes to rest after 0.8 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.

v₀ = 2.5 m/sd = 0.8 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
m1​ = 6 kg3.5 kgμ = 0.2

A 6 kg block on a horizontal table is connected by a light string over a frictionless pulley to a hanging 3.5 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 = 6 kgm2 = 3.5 kgμ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
v0​ = 40 m/s45°x = ?

A ball is launched at 45° above horizontal from level ground. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.

v0 = 40 m/sθ = 45°g = 9.81 m/s²
x = m
6.Projectile motionHard
h = 20 mv0​ = 24 m/s55°H = ?

A ball is launched at 24 m/s and 55° above the horizontal from the top of a 20 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 = 24 m/sθ = 55°h = 20 mg = 9.81 m/s²
H = m
7.Circular motionMedium
r = 40 mvmax​ = ?

A car takes a flat curve of radius 40 m. The coefficient of static friction between tyres and road is 0.4. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.

r = 40 mμs = 0.4g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
θ = 12°to centrevmax​ = ?

A curve of radius 190 m is banked at 12°, and the coefficient of static friction between tyres and road is 0.3. Use g = 9.81 m/s². Find the highest speed vmax at which a car can take the curve without sliding up and out.

r = 190 mθ = 12°μs = 0.3g = 9.81 m/s²
vmax = m/s
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 0.7. Find the stopping distance d.

v = 110 km/hμ = 0.7g = 9.81 m/s²
d = m
10.Work & energyHard
h = 4.5 mL = 5 mμ = 0.15k = 5000 N/m

A 2.5 kg block is released from rest at height 4.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 5 m long (μ = 0.15) and then runs onto a spring (k = 5000 N/m) on a smooth surface. Find the maximum compression x of the spring.

m = 2.5 kgh = 4.5 mL = 5 mμ = 0.15k = 5000 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
5 kgv = ?8 gh = 22 cm

An 8 g bullet is fired horizontally into a 5 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 22 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.

m = 8 gM = 5 kgh = 22 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
18 m/s10 m/s

A 0.5 kg ball hits a wall at 18 m/s and bounces straight back at 10 m/s. The contact lasts 40 ms. Find the magnitude of the impulse J the wall gives the ball.

m = 0.5 kgvin = 18 m/svout = 10 m/sΔt = 40 ms
J = N·s
13.Springs & pendulumsMedium
L = 0.6 m

A simple pendulum has a length of 0.6 m. Use g = 9.81 m/s². Find its period T for small swings.

L = 0.6 mg = 9.81 m/s²
T = s
14.Springs & pendulumsHard
1.9 kgk = 220 N/m

A 1.9 kg mass is attached to the lower end of an unstretched vertical spring with k = 220 N/m and released from rest. Use g = 9.81 m/s². Find the greatest speed vmax of the mass.

m = 1.9 kgk = 220 N/mg = 9.81 m/s²
vmax = m/s
15.Waves, sound & opticsMedium
θc = ?90°diamond, n1​ = 2.42air, n2​ = 1.00

Light travels in diamond (n = 2.42) towards a flat boundary with air (n = 1.00). Find the critical angle θc beyond which it is totally reflected.

n1 = 2.42n2 = 1.00
θc = °
16.Waves, sound & opticsHard
f1​ = 10 cmf2​ = 20 cm25 cm10 cm

Two thin lenses share an axis, 10 cm apart: lens 1 with f₁ = 10 cm and lens 2 with f₂ = 20 cm. An object stands 25 cm in front of lens 1. Find the position di2 of the final image measured from lens 2 (positive beyond lens 2, real-is-positive convention).

f1 = 10 cmf2 = 20 cms = 10 cmdo = 25 cm
di2 = cm

3Solutions

1.Kinematics

Answer: d = 14.09 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

Reaction distance

d1 = v₀·tr = 10·0.5 = 5 m

Braking distance

d2 = v₀²/(2a) = 10²/(2·5.5) = 9.091 m

Total

d = 5 + 9.091 = 14.09 m

Answer

d = 14.09 m
2.Kinematics

Answer: s = 595.3 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 = 2·9 = 18 m/s

Distance of each phase

s1 = ½·2·9² = 81 m
s2 = 18·26 = 468 m
s3 = 18²/(2·3.5) = 46.29 m

Total distance

s = 81 + 468 + 46.29 = 595.3 m

Answer

s = 595.3 m
3.Newton’s laws

Answer: μ = 0.3982

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) = 2.5²/(2·0.8) = 3.906 m/s²

Friction coefficient

μ·m·g = m·a → μ = a/g = 3.906/9.81 = 0.3982

Answer

μ = 0.3982
4.Newton’s laws

Answer: T = 26.02 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) = (34.34 − 11.77)/9.5 = 2.375 m/s²

Tension from the block on the table

T − μ·m1·g = m1·a
T = 6·2.375 + 11.77 = 26.02 N

Answer

T = 26.02 N
5.Projectile motion

Answer: x = 163.1 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 = 40·cos 45° = 28.28 m/s
vy = v0·sin θ = 40·sin 45° = 28.28 m/s

Time in the air

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

t = 2·28.28/9.81 = 5.766 s

Horizontal distance

x = vx·t = 28.28·5.766 = 163.1 m

Answer

x = 163.1 m
6.Projectile motion

Answer: H = 39.7 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 = 24·sin 55° = 19.66 m/s

Rise above the cliff

Δh = vy²/(2g) = 19.66²/(2·9.81) = 19.7 m

Height above the ground

H = 20 + 19.7 = 39.7 m

Answer

H = 39.7 m
7.Circular motion

Answer: vmax = 12.53 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) = √(0.4·9.81·40) = 12.53 m/s

Answer

vmax = 12.53 m/s (45.1 km/h)
8.Circular motion

Answer: vmax = 31.94 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 balances

Friction points down the slope.

N·(sin θ + μ·cos θ) = m·v²/r
N·(cos θ − μ·sin θ) = m·g

Speed

v = √(190·9.81·0.5014/0.9158) = 31.94 m/s

Answer

vmax = 31.94 m/s
(115 km/h)
9.Work & energy

Answer: d = 67.98 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/(2·0.7·9.81) = 67.98 m

Answer

d = 67.98 m

Doubling the speed makes the stopping distance four times longer.

10.Work & energy

Answer: x = 19.18 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·(4.5 − 0.15·5) = 91.97 J

Maximum compression

½kx² = 91.97 → x = √(2·91.97/5000) = 0.1918 m = 19.18 cm

Answer

x = 19.18 cm
11.Momentum & collisions

Answer: v = 1301 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) = √(2·9.81·0.22) = 2.078 m/s

Collision

m·v = (m + M)·V → v = (0.008 + 5)·2.078/0.008 = 1301 m/s

Answer

v = 1301 m/s
12.Momentum & collisions

Answer: J = 14 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 = 0.5·(10 + 18) = 14 N·s

Answer

J = 14 N·s
13.Springs & pendulums

Answer: T = 1.554 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.

Pendulum period

T = 2π·√(0.6/9.81) = 1.554 s

Answer

T = 1.554 s
14.Springs & pendulums

Answer: vmax = 0.9117 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.

Equilibrium and amplitude

x₀ = m·g/k = 1.9·9.81/220 = 0.08472 m = A

Greatest speed

The speed is largest at equilibrium.

ω = √(220/1.9) = 10.76 rad/s
vmax = A·ω = 0.08472·10.76 = 0.9117 m/s

Answer

vmax = 0.9117 m/s
15.Waves, sound & optics

Answer: θc = 24.41 °

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.

Refraction at 90°

sin θc = n₂/n₁ = 1.00/2.42 = 0.4132

Critical angle

θc = arcsin(0.4132) = 24.41°

Answer

θc = 24.41°
16.Waves, sound & optics

Answer: di2 = 5 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.

Image of lens 1

1/di1 = 1/10 − 1/25 → di1 = 16.67 cm

Object for lens 2

do2 = 10 − 16.67 = −6.667 cm

Final image

1/di2 = 1/20 − 1/(−6.667) → di2 = 5 cm

Answer

di2 = 5 cm

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