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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
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 car driving at 54 km/h brakes with a constant deceleration of 8 m/s² until it stops. Find the braking distance s.
v₀ = 54 km/ha = −8 m/s²v = 0
s = m
2.KinematicsHard
A ball is thrown straight up at 19 m/s from the edge of a roof 40 m above the ground. Ignore air resistance and use g = 9.81 m/s². Find the greatest height hmax above the ground.
v₀ = 19 m/sh₀ = 40 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
Masses of 6 kg and 2 kg hang on either side of a light, frictionless pulley, connected by a light string. The system is released from rest. Use g = 9.81 m/s². Find the tension T in the string.
m1 = 6 kgm2 = 2 kgg = 9.81 m/s²
T = N
4.Newton’s lawsHard
A 9.5 kg block on a ramp at 35° is connected by a light string over a frictionless pulley at the top of the ramp to a hanging 8.5 kg mass. The hanging mass moves down. The ramp is frictionless. Use g = 9.81 m/s². Find the acceleration a of the system.
m1 = 9.5 kgm2 = 8.5 kgθ = 35°μ = 0g = 9.81 m/s²
a = m/s²
5.Projectile motionMedium
A ball is kicked from level ground at 26 m/s and 40° above the horizontal towards a wall 48 m away. Ignore air resistance and use g = 9.81 m/s². At what height y does the ball pass the wall?
v₀ = 26 m/sθ = 40°x = 48 mg = 9.81 m/s²
y = m
6.Projectile motionHard
A ball is launched at 25 m/s and 15° above the horizontal from the top of a 15 m cliff. Ignore air resistance and use g = 9.81 m/s². Find the speed v with which it hits the ground.
v₀ = 25 m/sθ = 15°h = 15 mg = 9.81 m/s²
v = m/s
7.Circular motionMedium
In a fairground rotor, riders stand against the wall of a vertical cylinder of radius 2.5 m. When it spins fast enough the floor drops away and friction holds them up. The coefficient of static friction is 0.5. Use g = 9.81 m/s². Find the smallest angular speed ω in rad/s.
r = 2.5 mμs = 0.5g = 9.81 m/s²
ω = rad/s
8.Circular motionHard
A curve of radius 260 m is banked at 18°, and the coefficient of static friction between tyres and road is 0.2. 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 = 260 mθ = 18°μs = 0.2g = 9.81 m/s²
vmax = m/s
9.Work & energyMedium
A 10 kg block slides 4 m down a slope at 40° to the horizontal, with a coefficient of kinetic friction of 0.3. Find the work Wf done against friction.
m = 10 kgθ = 40°μ = 0.3d = 4 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
A 3 kg block rests on a rough horizontal table (μ = 0.25). A light string runs from it over a smooth pulley at the edge of the table to a hanging 5 kg block. The system is released from rest. Find the speed v of the blocks after the hanging block has dropped 1.5 m.
m1 = 3 kgm2 = 5 kgμ = 0.25d = 1.5 mg = 9.81 m/s²
v = m/s
11.Momentum & collisionsMedium
An 800 kg car moving at 10 m/s runs into a stationary 2000 kg car and the two lock together. How much kinetic energy ΔK is lost in the collision?
m1 = 800 kgv1 = 10 m/sm2 = 2000 kgv2 = 0
ΔK = J
12.Momentum & collisionsHard
A 0.45 kg ball hits a wall at 13 m/s and bounces straight back at 14 m/s. The contact lasts 25 ms. Find the magnitude of the impulse J the wall gives the ball.
m = 0.45 kgvin = 13 m/svout = 14 m/sΔt = 25 ms
J = N·s
13.Springs & pendulumsMedium
A 1.9 kg mass on a spring with k = 390 N/m is pulled 3 cm in the positive direction and released from rest at t = 0. Find its displacement x from equilibrium at t = 1.25 s (with sign).
m = 1.9 kgk = 390 N/mA = 3 cmt = 1.25 s
x = cm
14.Springs & pendulumsHard
A 3.5 kg mass on a spring with k = 430 N/m is pulled 8 cm from equilibrium and released. Find its maximum acceleration amax.
m = 3.5 kgk = 430 N/mA = 8 cm
amax = m/s²
3Solutions
1.Kinematics
Answer: s = 14.06 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 = 54/3.6 = 15 m/s
Braking distance
0 = v₀² − 2·a·s → s = v₀²/(2a) = 15²/(2·8) = 14.06 m
Answer
s = 14.06 m
2.Kinematics
Answer: hmax = 58.4 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 = 19²/(2·9.81) = 18.4 m
Height above the ground
hmax = 40 + 18.4 = 58.4 m
Answer
hmax = 58.4 m
3.Newton’s laws
Answer: T = 29.43 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
The net driving force is the difference of the weights; it moves the total mass.
a = (6 − 2)·9.81/(6 + 2) = 4.905 m/s²
Tension from the lighter mass
T − m2·g = m2·a → T = 2·(9.81 + 4.905) = 29.43 N
Answer
T = 29.43 N
4.Newton’s laws
Answer: a = 1.663 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 = 83.39 N
m₁·g·sin θ = 53.45 N
μ·m₁·g·cos θ = 0 N
Acceleration of the system
a = (83.39 − 53.45 − 0)/18 = 1.663 m/s²
Answer
a = 1.663 m/s²
5.Projectile motion
Answer: y = 11.79 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 = 26·cos 40° = 19.92 m/s
t = x/vx = 48/19.92 = 2.41 s
Height at that time
y = 16.71·2.41 − ½·9.81·2.41² = 11.79 m
Answer
y = 11.79 m
The ball is already on its way down at the wall.
6.Projectile motion
Answer: v = 30.32 m/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
vx = 25·cos 15° = 24.15 m/s
vy0 = 25·sin 15° = 6.47 m/s
Vertical speed at impact
Energy or v² = v₀² + 2as in the vertical direction.
vy = √(6.47² + 2·9.81·15) = 18.33 m/s
Impact speed
v = √(24.15² + 18.33²) = 30.32 m/s
Answer
v = 30.32 m/s
Check with energy: v² = v₀² + 2gh.
7.Circular motion
Answer: ω = 2.801 rad/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.
Forces on a rider
N = m·ω²·r
μ·N = m·g → μ·ω²·r = g
Angular speed
ω = √(g/(μ·r)) = √(9.81/(0.5·2.5)) = 2.801 rad/s
Answer
ω = 2.801 rad/s
8.Circular motion
Answer: vmax = 37.84 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 = √(260·9.81·0.4992/0.8893) = 37.84 m/s
Answer
vmax = 37.84 m/s
(136.2 km/h)
9.Work & energy
Answer: Wf = 90.18 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 = 98.1·0.766 = 75.15 N
F = μN = 0.3·75.15 = 22.54 N
Work against friction
Wf = F·d = 22.54·4 = 90.18 J
Answer
Wf = 90.18 J
10.Work & energy
Answer: v = 3.954 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 = 49.05·1.5 = 73.58 J
μ·m₁g·d = 0.25·29.43·1.5 = 11.04 J
Kinetic energy of both blocks
½(m₁ + m₂)v² = 62.54 → v = √(2·62.54/8) = 3.954 m/s
Answer
v = 3.954 m/s
11.Momentum & collisions
Answer: ΔK = 28571 J
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.
Common velocity
v′ = m1·v1/(m1 + m2) = 800·10/2800 = 2.857 m/s
Kinetic energy before and after
Kbefore = ½·800·10² = 40000 J
Kafter = ½·2800·2.857² = 11429 J
Energy lost
ΔK = 40000 − 11429 = 28571 J
Answer
ΔK = 28571 J
The lost energy goes into deformation, heat and sound.
12.Momentum & collisions
Answer: J = 12.15 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 = 0.45·(14 + 13) = 12.15 N·s
Answer
J = 12.15 N·s
13.Springs & pendulums
Answer: x = 1.767 cm
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 = √(390/1.9) = 14.33 rad/s
ω·t = 14.33·1.25 = 17.91 rad
Displacement
x = A·cos(ω·t) = 3·cos(17.91) = 1.767 cm
Answer
x = 1.767 cm
14.Springs & pendulums
Answer: amax = 9.829 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.