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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 60 km/h brakes with a constant deceleration of 6.5 m/s² until it stops. Find the braking distance s.
v₀ = 60 km/ha = −6.5 m/s²v = 0
s = m
2.KinematicsHard
A ball is thrown straight up at 6 m/s from the edge of a roof 20 m above the ground. Ignore air resistance and use g = 9.81 m/s². How long after the throw does it hit the ground? Find t.
v₀ = 6 m/sh₀ = 20 mg = 9.81 m/s²
t = s
3.Newton’s lawsMedium
Masses of 8 kg and 1 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 acceleration a of the masses.
m1 = 8 kgm2 = 1 kgg = 9.81 m/s²
a = m/s²
4.Newton’s lawsHard
A block is given a push so that it starts sliding up a ramp at 25° with a speed of 11 m/s. The coefficient of kinetic friction is 0.2. Use g = 9.81 m/s². How far d along the ramp does it slide before it stops?
v₀ = 11 m/sθ = 25°μ = 0.2g = 9.81 m/s²
d = m
5.Projectile motionMedium
A ball is launched from level ground at 70° above the horizontal and lands 100 m away. Ignore air resistance and use g = 9.81 m/s². Find the launch speed v0.
θ = 70°R = 100 mg = 9.81 m/s²
v0 = m/s
6.Projectile motionHard
A ball is launched at 60° above horizontal from the top of a 30 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 30 m/sθ = 60°h = 30 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A car takes a flat curve of radius 85 m. The coefficient of static friction between tyres and road is 0.5. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 85 mμs = 0.5g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 0.8 kg ball on a light string 1 m long is swung in a vertical circle. At the lowest point its speed is 8.5 m/s. Ignore air resistance and use g = 9.81 m/s². Find the tension Ttop in the string at the highest point.
m = 0.8 kgr = 1 mvbottom = 8.5 m/sg = 9.81 m/s²
Ttop = N
9.Work & energyMedium
A 10 kg block slides 13 m down a slope at 40° to the horizontal, with a coefficient of kinetic friction of 0.15. Find the work Wf done against friction.
m = 10 kgθ = 40°μ = 0.15d = 13 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
A 4 kg block rests on a rough horizontal table (μ = 0.15). A light string runs from it over a smooth pulley at the edge of the table to a hanging 3 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 = 4 kgm2 = 3 kgμ = 0.15d = 0.5 mg = 9.81 m/s²
v = m/s
11.Momentum & collisionsMedium
A 13 kg projectile moving horizontally at 28 m/s explodes into two fragments that move along the same line. The 5 kg fragment continues forwards at 49 m/s. Find the velocity v′2 of the 8 kg fragment (positive in the original direction).
M = 13 kgu = 28 m/sm1 = 5 kgv′1 = 49 m/sm2 = 8 kg
v′2 = m/s
12.Momentum & collisionsHard
A 1300 kg car runs into a parked 1000 kg car. The wrecks lock together and skid 23 m before stopping. The coefficient of kinetic friction between tyres and road is 0.5. Use g = 9.81 m/s². Find the speed v1 of the first car just before the collision.
How long must a simple pendulum be to have a period of 2.8 s? Use g = 9.81 m/s². Find L.
T = 2.8 sg = 9.81 m/s²
L = m
14.Springs & pendulumsHard
A 2.8 kg mass is attached to the lower end of an unstretched vertical spring with k = 210 N/m and released from rest. Use g = 9.81 m/s². Find the greatest extension xmax of the spring in cm.
m = 2.8 kgk = 210 N/mg = 9.81 m/s²
xmax = cm
3Solutions
1.Kinematics
Answer: s = 21.37 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 = 60/3.6 = 16.67 m/s
Braking distance
0 = v₀² − 2·a·s → s = v₀²/(2a) = 16.67²/(2·6.5) = 21.37 m
Answer
s = 21.37 m
2.Kinematics
Answer: t = 2.721 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.
Equation of motion
Up is positive and the ground is h₀ below the starting point.
−20 = 6·t − ½·9.81·t²
4.905·t² − 6·t − 20 = 0
Solve the quadratic
Only the positive root is physical.
t = (6 + √(6² + 2·9.81·20))/9.81 = 2.721 s
Answer
t = 2.721 s
3.Newton’s laws
Answer: a = 7.63 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.
Acceleration
The net driving force is the difference of the weights; it moves the total mass.
a = (8 − 1)·9.81/(8 + 1) = 7.63 m/s²
Answer
a = 7.63 m/s²
4.Newton’s laws
Answer: d = 10.21 m
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
Both forces point down the slope; the mass cancels.
a = 9.81·(sin 25° + 0.2·cos 25°) = 5.924 m/s²
Distance
d = v₀²/(2a) = 11²/(2·5.924) = 10.21 m
Answer
d = 10.21 m
5.Projectile motion
Answer: v0 = 39.07 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.
Solve the range formula for v₀
v₀ = √(g·R/sin 2θ) = √(9.81·100/sin 140°) = 39.07 m/s
Answer
v0 = 39.07 m/s
6.Projectile motion
Answer: x = 94.08 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 = 30·cos 60° = 15 m/s
vy = v0·sin θ = 30·sin 60° = 25.98 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 30 + 25.98·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (25.98 + √(675 + 588.6))/9.81 = 6.272 s
Horizontal distance
x = vx·t = 15·6.272 = 94.08 m
Answer
x = 94.08 m
7.Circular motion
Answer: vmax = 20.42 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) = √(0.5·9.81·85) = 20.42 m/s
Answer
vmax = 20.42 m/s (73.51 km/h)
8.Circular motion
Answer: Ttop = 18.56 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.
Speed at the top
Kinetic energy is traded for potential energy over a height 2r.
vtop² = 8.5² − 4·9.81·1 = 33.01 m²/s²
Forces at the top
Tension and gravity both point down, towards the centre.
T + m·g = m·vtop²/r
T = 0.8·33.01/1 − 0.8·9.81 = 18.56 N
Answer
Ttop = 18.56 N
9.Work & energy
Answer: Wf = 146.5 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 = 0.15·75.15 = 11.27 N
Work against friction
Wf = F·d = 11.27·13 = 146.5 J
Answer
Wf = 146.5 J
10.Work & energy
Answer: v = 1.834 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 = 29.43·0.5 = 14.71 J
μ·m₁g·d = 0.15·39.24·0.5 = 2.943 J
Kinetic energy of both blocks
½(m₁ + m₂)v² = 11.77 → v = √(2·11.77/7) = 1.834 m/s
Answer
v = 1.834 m/s
11.Momentum & collisions
Answer: v′2 = 14.88 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.
Momentum before
p = M·u = 13·28 = 364 kg·m/s
Momentum after
364 = 5·49 + 8·v′2
v′2 = (364 − 245)/8 = 14.88 m/s
Answer
v′2 = 14.88 m/s
12.Momentum & collisions
Answer: v1 = 26.58 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.
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·(2.8/2π)² = 1.948 m
Answer
L = 1.948 m
14.Springs & pendulums
Answer: xmax = 26.16 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.