Physics 1: Mechanics: cram pack

Formula sheets, the key method steps for every topic and 16 practice problems, with full solutions at the back.

New problemsBack to the course

In the print window, choose “Save as PDF” to keep a copy on your device.

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
policea = 5 m/s²car17 m/s

A car speeding at a constant 17 m/s passes a police car standing still. At that moment the police car starts and accelerates uniformly at 5 m/s². How long does it take the police car to catch up? Find t.

vcar = 17 m/sapolice = 5 m/s²v0,police = 0
t = s
2.KinematicsHard
t (s)v (m/s)5 s

A tram starts from rest and accelerates at 1.5 m/s² for 5 s, then runs at constant speed for 20 s, and finally brakes at 2 m/s² until it stops. Find the total travel time t.

a1 = 1.5 m/s²t1 = 5 st2 = 20 sa3 = −2 m/s²
t = s
3.Newton’s lawsMedium
40 kg35°F = 130 Nμ = 0.2

A 40 kg sled is pulled across level snow by a rope at 35° above the horizontal with a force of 130 N. The coefficient of kinetic friction is 0.2. Use g = 9.81 m/s². Find the acceleration a of the sled.

m = 40 kgF = 130 Nθ = 35°μ = 0.2g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
m1​ = 5 kg10 kgθ = 20°

A 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 10 kg mass. The hanging mass moves down. The coefficient of kinetic friction between block and ramp is 0.2. Use g = 9.81 m/s². Find the tension T in the string.

m1 = 5 kgm2 = 10 kgθ = 20°μ = 0.2g = 9.81 m/s²
T = N
5.Projectile motionMedium
23 my = ?v0​ = 24 m/s50°

A ball is kicked from level ground at 24 m/s and 50° above the horizontal towards a wall 23 m away. Ignore air resistance and use g = 9.81 m/s². At what height y does the ball pass the wall?

v₀ = 24 m/sθ = 50°x = 23 mg = 9.81 m/s²
y = m
6.Projectile motionHard
h = 60 mv0​ = 12 m/s15°x = ?

A ball is launched at 15° above horizontal from the top of a 60 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.

v0 = 12 m/sθ = 15°h = 60 mg = 9.81 m/s²
x = m
7.Circular motionMedium
θ = 12°to centrev = ?

A curve of radius 350 m is banked at 12°. At what speed v can a car take it with no friction needed at all? Use g = 9.81 m/s².

r = 350 mθ = 12°g = 9.81 m/s²
v = m/s
8.Circular motionHard
r = 1.5 mv = 9 m/s

A 1.9 kg ball on a string 1.5 m long is swung in a vertical circle. At the lowest point it moves at 9 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.

m = 1.9 kgr = 1.5 mv = 9 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
25°d = 7 mμ = 0.1

A block slides from rest down a slope at 25° to the horizontal. The coefficient of kinetic friction is 0.1. Find its speed v after it has slid 7 m along the slope.

θ = 25°μ = 0.1d = 7 mg = 9.81 m/s²
v = m/s
10.Work & energyHard
h = 4 mL = 2.5 mμ = 0.25k = 1500 N/m

A 4 kg block is released from rest at height 4 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 2.5 m long (μ = 0.25) and then runs onto a spring (k = 1500 N/m) on a smooth surface. Find the maximum compression x of the spring.

m = 4 kgh = 4 mL = 2.5 mμ = 0.25k = 1500 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
2 kgv = ?12 gh = 18 cm

A 12 g bullet is fired horizontally into a 2 kg wooden block hanging on light strings, and stays inside it. The block swings up until it is 18 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.

m = 12 gM = 2 kgh = 18 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
16 m/s20 m/s

A 0.45 kg ball hits a wall at 16 m/s and bounces straight back at 20 m/s. The contact lasts 10 ms. Find the average force Favg of the wall on the ball.

m = 0.45 kgvin = 16 m/svout = 20 m/sΔt = 10 ms
Favg = N
13.Springs & pendulumsMedium
L = 2.9 m

A simple pendulum has a length of 2.9 m. Use g = 9.81 m/s². Find its period T for small swings.

L = 2.9 mg = 9.81 m/s²
T = s
14.Springs & pendulumsHard
3.1 kgk = 120 N/mA = 18 cm

A 3.1 kg mass on a spring with k = 120 N/m is pulled 18 cm from equilibrium and released. Find its maximum acceleration amax.

m = 3.1 kgk = 120 N/mA = 18 cm
amax = m/s²
15.Waves, sound & opticsMedium
FFdo​ = 4 cmf = 20 cm

An object stands 4 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 = 4 cmf = 20 cm
di = cm
16.Waves, sound & opticsHard
f1​ = 8 cmf2​ = 5 cm20 cm10 cm

Two thin lenses share an axis, 10 cm apart: lens 1 with f₁ = 8 cm and lens 2 with f₂ = 5 cm. An object stands 20 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 = 8 cmf2 = 5 cms = 10 cmdo = 20 cm
di2 = cm

3Solutions

1.Kinematics

Answer: t = 6.8 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.

Equal distances

v·t = ½·a·t² → t = 2v/a = 2·17/5 = 6.8 s

Answer

t = 6.8 s

At that moment the police car moves at 2v = 34 m/s.

2.Kinematics

Answer: t = 28.75 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 = 1.5·5 = 7.5 m/s

Braking time

t3 = v/a3 = 7.5/2 = 3.75 s

Total time

t = 5 + 20 + 3.75 = 28.75 s

Answer

t = 28.75 s
3.Newton’s laws

Answer: a = 1.073 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.

Normal force

N = m·g − F·sin θ = 392.4 − 130·sin 35° = 317.8 N

Horizontal forces

m·a = F·cos θ − μ·N = 106.5 − 63.57
a = 42.92/40 = 1.073 m/s²

Answer

a = 1.073 m/s²
4.Newton’s laws

Answer: T = 50.03 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.

Forces along the string

m₂·g = 98.1 N
m₁·g·sin θ = 16.78 N
μ·m₁·g·cos θ = 9.218 N

Acceleration of the system

a = (98.1 − 16.78 − 9.218)/15 = 4.807 m/s²

Tension from the hanging mass

m2·g − T = m2·a → T = 10·(9.81 − 4.807) = 50.03 N

Answer

T = 50.03 N
5.Projectile motion

Answer: y = 16.51 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 = 24·cos 50° = 15.43 m/s
t = x/vx = 23/15.43 = 1.491 s

Height at that time

y = 18.39·1.491 − ½·9.81·1.491² = 16.51 m

Answer

y = 16.51 m

The ball is still rising at the wall.

6.Projectile motion

Answer: x = 44.38 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 = 12·cos 15° = 11.59 m/s
vy = v0·sin θ = 12·sin 15° = 3.106 m/s

Time in the air

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

0 = 60 + 3.106·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (3.106 + √(9.646 + 1177))/9.81 = 3.828 s

Horizontal distance

x = vx·t = 11.59·3.828 = 44.38 m

Answer

x = 44.38 m
7.Circular motion

Answer: v = 27.02 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 balance

N·cos θ = m·g, N·sin θ = m·v²/r → tan θ = v²/(r·g)

Speed

v = √(r·g·tan θ) = √(350·9.81·tan 12°) = 27.02 m/s

Answer

v = 27.02 m/s
(97.25 km/h)
8.Circular motion

Answer: T = 121.2 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.

Forces at the lowest point

Towards the centre (up) is positive.

T − m·g = m·v²/r
T = 1.9·9.81 + 1.9·9²/1.5 = 18.64 + 102.6 = 121.2 N

Answer

T = 121.2 N
9.Work & energy

Answer: v = 6.752 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).

Energy balance along the slope

The block drops a height d·sin θ while friction removes μmg·cos θ·d. The mass cancels.

½v² = g·d·(sin θ − μ·cos θ)
v = √(2·9.81·7·(0.4226 − 0.1·0.9063)) = 6.752 m/s

Answer

v = 6.752 m/s
10.Work & energy

Answer: x = 42.02 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·(4 − 0.25·2.5) = 132.4 J

Maximum compression

½kx² = 132.4 → x = √(2·132.4/1500) = 0.4202 m = 42.02 cm

Answer

x = 42.02 cm
11.Momentum & collisions

Answer: v = 315.1 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) = √(2·9.81·0.18) = 1.879 m/s

Collision

m·v = (m + M)·V → v = (0.012 + 2)·1.879/0.012 = 315.1 m/s

Answer

v = 315.1 m/s
12.Momentum & collisions

Answer: Favg = 1620 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 = 0.45·(20 + 16) = 16.2 N·s

Average force

Favg = J/Δt = 16.2/0.01 = 1620 N

Answer

Favg = 1620 N
13.Springs & pendulums

Answer: T = 3.416 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·√(2.9/9.81) = 3.416 s

Answer

T = 3.416 s
14.Springs & pendulums

Answer: amax = 6.968 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.

Angular frequency

ω = √(120/3.1) = 6.222 rad/s

Maximum acceleration

amax = A·ω² = 0.18·6.222² = 6.968 m/s²

Answer

amax = 6.968 m/s²
15.Waves, sound & optics

Answer: di = −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.

Thin-lens equation

1/di = 1/f − 1/do = 1/20 − 1/4
di = −5 cm
FF4 cmf = 20 cm

Ray diagram

virtual, on the same side as the object

Answer

di = −5 cm
16.Waves, sound & optics

Answer: di2 = 2 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 = 1/8 − 1/20 → di1 = 13.33 cm

Object for lens 2

do2 = 10 − 13.33 = −3.333 cm

Final image

1/di2 = 1/5 − 1/(−3.333) → di2 = 2 cm

Answer

di2 = 2 cm

ExamProblems · examproblems.com/courses/physics-1-mechanics