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
List what you know: v₀, v, a, t and s, with units.
Pick the equation that contains your three knowns and the unknown.
Choose one positive direction and give opposing quantities a minus sign.
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
Draw a free-body diagram with every force on the body.
Choose a positive direction along the motion.
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).
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
Split the launch velocity into components: vx = v₀·cos θ and vy = v₀·sin θ.
The horizontal motion has constant speed; the vertical motion has acceleration −g.
Find the time of flight from the vertical motion.
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
Choose a positive direction and give each velocity its sign.
Write the total momentum before: Σm·v.
For a perfectly inelastic collision the bodies move together after; for an elastic one kinetic energy is also conserved.
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
2Practice problems
1.KinematicsMedium
A driver moving at 80 km/h sees an obstacle. Her reaction time is 1.5 s, after which the car brakes with a constant deceleration of 8 m/s². Find the total stopping distance d from the moment she sees the obstacle.
v₀ = 80 km/htr = 1.5 sa = −8 m/s²
d = m
2.KinematicsHard
A stone is dropped from rest into a well. The splash is heard 2 s after the stone is released. The speed of sound is 343 m/s; ignore air resistance and use g = 9.81 m/s². Find the depth d of the well.
ttotal = 2 svsound = 343 m/sg = 9.81 m/s²
d = m
3.Newton’s lawsMedium
A 12 kg block slides down a slope at 25° to the horizontal. The coefficient of kinetic friction is 0.2. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.
m = 12 kgθ = 25°μ = 0.2g = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
Two blocks on a level floor are joined by a light horizontal string. A horizontal force of 70 N pulls the front 8 kg block; the 2 kg block is towed behind. The coefficient of kinetic friction is 0.2 for both blocks. Use g = 9.81 m/s². Find the acceleration a of the blocks.
m1 = 2 kgm2 = 8 kgF = 70 Nμ = 0.2g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
A ball is launched from level ground at 32 m/s and must land 45 m away. Ignore air resistance and use g = 9.81 m/s². Find the smaller of the two possible launch angles θ.
v₀ = 32 m/sR = 45 mg = 9.81 m/s²
θ = °
6.Projectile motionHard
A ball is thrown from ground level at 34 m/s and 65° above the horizontal. It lands on the flat roof of a building 25 m high, on its way down. Ignore air resistance and use g = 9.81 m/s². Find the horizontal distance x from the launch point to where it lands.
v₀ = 34 m/sθ = 65°hroof = 25 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A small ball on a string of length 1.8 m moves in a horizontal circle so that the string makes a constant angle of 30° with the vertical (a conical pendulum). Use g = 9.81 m/s². Find the speed v of the ball.
L = 1.8 mθ = 30°g = 9.81 m/s²
v = m/s
8.Circular motionHard
A ball on a string 0.9 m long is swung in a vertical circle. Use g = 9.81 m/s². What is the smallest speed vmin at the top for which the string just stays tight?
r = 0.9 mg = 9.81 m/s²
vmin = m/s
9.Work & energyMedium
A 1800 kg car drives up a 4° slope at a constant 90 km/h. Air resistance and rolling resistance together amount to 400 N. Find the power P the wheels must deliver, in kW.
An 8 kg block rests on a rough horizontal table (μ = 0.4). A light string runs from it over a smooth pulley at the edge of the table to a hanging 4 kg block. The system is released from rest. Find the speed v of the blocks after the hanging block has dropped 0.5 m.
m1 = 8 kgm2 = 4 kgμ = 0.4d = 0.5 mg = 9.81 m/s²
v = m/s
11.Momentum & collisionsMedium
A 20 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 24 cm higher than at the start. Use g = 9.81 m/s². Find the speed v of the bullet before impact.
m = 20 gM = 2 kgh = 24 cmg = 9.81 m/s²
v = m/s
12.Momentum & collisionsHard
A 0.15 kg ball hits a wall at 19 m/s and bounces straight back at 16 m/s. The contact lasts 50 ms. Find the magnitude of the impulse J the wall gives the ball.
m = 0.15 kgvin = 19 m/svout = 16 m/sΔt = 50 ms
J = N·s
13.Springs & pendulumsMedium
How long must a simple pendulum be to have a period of 2 s? Use g = 9.81 m/s². Find L.
T = 2 sg = 9.81 m/s²
L = m
14.Springs & pendulumsHard
A 1.2 kg mass on a spring with k = 480 N/m is pulled 11 cm from equilibrium and released. Find its maximum speed vmax.
m = 1.2 kgk = 480 N/mA = 11 cm
vmax = m/s
3Solutions
1.Kinematics
Answer: d = 64.2 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 = 80/3.6 = 22.22 m/s
Reaction distance
d1 = v₀·tr = 22.22·1.5 = 33.33 m
Braking distance
d2 = v₀²/(2a) = 22.22²/(2·8) = 30.86 m
Total
d = 33.33 + 30.86 = 64.2 m
Answer
d = 64.2 m
2.Kinematics
Answer: d = 18.57 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.
Two parts of the time
½·9.81·t₁² = 343·(2 − t₁)
4.905·t₁² + 343·t₁ − 686 = 0
Fall time
Only the positive root is physical.
t₁ = (−343 + √(343² + 4·4.905·686))/(2·4.905) = 1.946 s
Depth
d = ½·9.81·1.946² = 18.57 m
Answer
d = 18.57 m
3.Newton’s laws
Answer: a = 2.368 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.
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.
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.
Range formula
R = v₀²·sin 2θ/g → sin 2θ = 9.81·45/32² = 0.4311
Angle
2θ = 25.54° → θ = 12.77°
Answer
θ = 12.77°
The other solution is 90° − θ = 77.23°.
6.Projectile motion
Answer: x = 76.52 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
vx = 14.37 m/s, vy = 30.81 m/s
Time to land on the roof
Larger root: the ball is descending.
25 = 30.81·t − 4.905·t²
t = (30.81 + √(30.81² − 2·9.81·25))/9.81 = 5.325 s
Horizontal distance
x = vx·t = 14.37·5.325 = 76.52 m
Answer
x = 76.52 m
7.Circular motion
Answer: v = 2.258 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.
Radius of the circle
r = L·sin θ = 1.8·sin 30° = 0.9 m
Speed
Tension and gravity together give the inward force: tan θ = v²/(r·g).
v = √(r·g·tan θ) = √(0.9·9.81·tan 30°) = 2.258 m/s
Answer
v = 2.258 m/s
8.Circular motion
Answer: vmin = 2.971 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.
Tension zero at the top
m·g = m·v²/r → v = √(g·r) = √(9.81·0.9) = 2.971 m/s
Answer
vmin = 2.971 m/s
9.Work & energy
Answer: P = 40.79 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 = 1800·9.81 = 17658 N
mg·sin θ = 17658·sin 4° = 1232 N
F = 1232 + 400 = 1632 N
Power
v = 90/3.6 = 25 m/s
P = F·v = 1632·25 = 40794 W = 40.79 kW
Answer
P = 40.79 kW
10.Work & energy
Answer: v = 0.8087 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 released and lost
m₂g·d = 39.24·0.5 = 19.62 J
μ·m₁g·d = 0.4·78.48·0.5 = 15.7 J
Kinetic energy of both blocks
½(m₁ + m₂)v² = 3.924 → v = √(2·3.924/12) = 0.8087 m/s
Answer
v = 0.8087 m/s
11.Momentum & collisions
Answer: v = 219.2 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) = √(2·9.81·0.24) = 2.17 m/s
Collision
m·v = (m + M)·V → v = (0.02 + 2)·2.17/0.02 = 219.2 m/s
Answer
v = 219.2 m/s
12.Momentum & collisions
Answer: J = 5.25 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 = 0.15·(16 + 19) = 5.25 N·s
Answer
J = 5.25 N·s
13.Springs & pendulums
Answer: L = 0.994 m
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 the length
L = g·(T/2π)² = 9.81·(2/2π)² = 0.994 m
Answer
L = 0.994 m
14.Springs & pendulums
Answer: vmax = 2.2 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.