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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 72 km/h brakes with a constant deceleration of 3 m/s² until it stops. Find the braking distance s.
v₀ = 72 km/ha = −3 m/s²v = 0
s = 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 20° to the horizontal. The coefficient of kinetic friction is 0.25. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.
m = 12 kgθ = 20°μ = 0.25g = 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 30° with a speed of 8.5 m/s. The coefficient of kinetic friction is 0.25. Use g = 9.81 m/s². How far d along the ramp does it slide before it stops?
v₀ = 8.5 m/sθ = 30°μ = 0.25g = 9.81 m/s²
d = m
5.Projectile motionMedium
A ball is launched at 25° above horizontal from level ground. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 20 m/sθ = 25°g = 9.81 m/s²
x = m
6.Projectile motionHard
A ball is launched at 20° above horizontal from the top of a 55 m cliff. Ignore air resistance. Find the horizontal distance x travelled before the ball hits the ground.
v0 = 33 m/sθ = 20°h = 55 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A small ball on a string of length 2 m moves in a horizontal circle so that the string makes a constant angle of 25° with the vertical (a conical pendulum). Use g = 9.81 m/s². Find the speed v of the ball.
L = 2 mθ = 25°g = 9.81 m/s²
v = m/s
8.Circular motionHard
A 1.9 kg ball on a string 0.7 m long is swung in a vertical circle. At the lowest point it moves at 7.5 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.
m = 1.9 kgr = 0.7 mv = 7.5 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
A 2 kg ball hangs on a light string of length 0.8 m. It is pulled aside until the string makes 70° with the vertical and released from rest. Find the tension T in the string when the ball passes the lowest point.
m = 2 kgL = 0.8 mθ = 70°g = 9.81 m/s²
T = N
10.Work & energyHard
A 10 kg block is released from rest at height 5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 4.5 m long (μ = 0.3) and then runs onto a spring (k = 800 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 10 kgh = 5 mL = 4.5 mμ = 0.3k = 800 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 2 kg glider moving at 6 m/s makes a perfectly elastic head-on collision with a 0.5 kg glider at rest on a frictionless track. Find the velocity v′2 of the second glider after the collision.
m1 = 2 kgm2 = 0.5 kgv1 = 6 m/sv2 = 0
v′2 = m/s
12.Momentum & collisionsHard
A 0.1 kg ball hits a wall at 13 m/s and bounces straight back at 7 m/s. The contact lasts 20 ms. Find the magnitude of the impulse J the wall gives the ball.
m = 0.1 kgvin = 13 m/svout = 7 m/sΔt = 20 ms
J = N·s
13.Springs & pendulumsMedium
An astronaut on another planet finds that a simple pendulum 1.7 m long has a period of 1.65 s for small swings. Find the gravitational acceleration g on that planet.
L = 1.7 mT = 1.65 s
g = m/s²
14.Springs & pendulumsHard
A mass on a spring with k = 240 N/m oscillates on a frictionless surface with an amplitude of 16 cm. Find the energy U stored in the spring when the mass is 3 cm from equilibrium.
k = 240 N/mA = 16 cmx = 3 cm
U = J
3Solutions
1.Kinematics
Answer: s = 66.67 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 = 72/3.6 = 20 m/s
Braking distance
0 = v₀² − 2·a·s → s = v₀²/(2a) = 20²/(2·3) = 66.67 m
Answer
s = 66.67 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 = 1.051 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.
Deceleration
Both forces point down the slope; the mass cancels.
a = 9.81·(sin 30° + 0.25·cos 30°) = 7.029 m/s²
Distance
d = v₀²/(2a) = 8.5²/(2·7.029) = 5.139 m
Answer
d = 5.139 m
5.Projectile motion
Answer: x = 31.24 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 = 20·cos 25° = 18.13 m/s
vy = v0·sin θ = 20·sin 25° = 8.452 m/s
Time in the air
Starting and landing at the same height: t = 2vy/g.
t = 2·8.452/9.81 = 1.723 s
Horizontal distance
x = vx·t = 18.13·1.723 = 31.24 m
Answer
x = 31.24 m
6.Projectile motion
Answer: x = 145.5 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 = 33·cos 20° = 31.01 m/s
vy = v0·sin θ = 33·sin 20° = 11.29 m/s
Time in the air
Solve y(t) = h + vy·t − ½gt² = 0 for the positive root.
0 = 55 + 11.29·t − 4.905·t²
t = (vy + √(vy² + 2gh))/g = (11.29 + √(127.4 + 1079))/9.81 = 4.691 s
Horizontal distance
x = vx·t = 31.01·4.691 = 145.5 m
Answer
x = 145.5 m
7.Circular motion
Answer: v = 1.966 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 = 2·sin 25° = 0.8452 m
Speed
Tension and gravity together give the inward force: tan θ = v²/(r·g).
v = √(r·g·tan θ) = √(0.8452·9.81·tan 25°) = 1.966 m/s
Answer
v = 1.966 m/s
8.Circular motion
Answer: T = 171.3 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 + 1.9·7.5²/0.7 = 18.64 + 152.7 = 171.3 N
Answer
T = 171.3 N
9.Work & energy
Answer: T = 45.44 N
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). On a circular path the net force towards the centre must equal mv²/r.
Speed at the bottom
v² = 2gL(1 − cos θ) = 2·9.81·0.8·(1 − cos 70°) = 10.33 m²/s²
Forces at the bottom
m·g = 2·9.81 = 19.62 N
T = mg + mv²/L = 19.62 + 2·10.33/0.8 = 45.44 N
Answer
T = 45.44 N
Check: T = mg(3 − 2cos θ) = 45.44 N.
10.Work & energy
Answer: x = 94.61 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 = 98.1·(5 − 0.3·4.5) = 358.1 J
Maximum compression
½kx² = 358.1 → x = √(2·358.1/800) = 0.9461 m = 94.61 cm
Answer
x = 94.61 cm
11.Momentum & collisions
Answer: v′2 = 9.6 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.
Elastic collision with a target at rest
Solving momentum and energy conservation together gives these two results.
v′1 = (2 − 0.5)/(2 + 0.5)·6 = 3.6 m/s
v′2 = 2·2/(2 + 0.5)·6 = 9.6 m/s
Answer
v′2 = 9.6 m/s
12.Momentum & collisions
Answer: J = 2 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 = 0.1·(7 + 13) = 2 N·s
Answer
J = 2 N·s
13.Springs & pendulums
Answer: g = 24.65 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.
Solve for g
g = 4π²·L/T² = 4π²·1.7/1.65² = 24.65 m/s²
Answer
g = 24.65 m/s²
14.Springs & pendulums
Answer: U = 0.108 J
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.