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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 speeding at a constant 26 m/s passes a police car standing still. At that moment the police car starts and accelerates uniformly at 4 m/s². How long does it take the police car to catch up? Find t.
vcar = 26 m/sapolice = 4 m/s²v0,police = 0
t = s
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. Find the greatest height hmax above the ground.
v₀ = 6 m/sh₀ = 20 mg = 9.81 m/s²
hmax = m
3.Newton’s lawsMedium
A box slides across a level floor with an initial speed of 6.5 m/s and comes to rest after 11 m. Use g = 9.81 m/s². Find the coefficient of kinetic friction μ.
v₀ = 6.5 m/sd = 11 mg = 9.81 m/s²
μ =
4.Newton’s lawsHard
A block is given a push so that it starts sliding up a ramp at 35° with a speed of 3 m/s. The coefficient of kinetic friction is 0.3. Use g = 9.81 m/s². How far d along the ramp does it slide before it stops?
v₀ = 3 m/sθ = 35°μ = 0.3g = 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 maximum height H the ball reaches.
v0 = 16 m/sθ = 25°g = 9.81 m/s²
H = m
6.Projectile motionHard
A ball is thrown from ground level at 23 m/s and 45° above the horizontal. It lands on the flat roof of a building 4 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 = 23 m/sθ = 45°hroof = 4 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A small ball on a string of length 1.5 m moves in a horizontal circle so that the string makes a constant angle of 40° with the vertical (a conical pendulum). Use g = 9.81 m/s². Find the time T for one revolution.
L = 1.5 mθ = 40°g = 9.81 m/s²
T = s
8.Circular motionHard
A 4.5 kg cart is released from rest at a height of 29.75 m on a frictionless track and runs through a vertical loop of radius 8.5 m. Use g = 9.81 m/s². Find the normal force Nbottom of the track on the cart at the bottom of the loop.
m = 4.5 kgh = 29.75 mR = 8.5 mg = 9.81 m/s²
Nbottom = N
9.Work & energyMedium
A 2.5 kg block slides 5 m down a slope at 30° to the horizontal, with a coefficient of kinetic friction of 0.4. Find the work Wf done against friction.
m = 2.5 kgθ = 30°μ = 0.4d = 5 mg = 9.81 m/s²
Wf = J
10.Work & energyHard
A spring of stiffness 500 N/m at the bottom of a 40° slope is compressed by 12 cm against a 1 kg block and released. The coefficient of kinetic friction between block and slope is 0.15. How far d up the slope does the block travel from the release point before it stops? (Ignore the short distance over which the spring pushes, i.e. take friction and gravity over the whole distance d.)
A 4.5 kg glider moving at 11 m/s makes a perfectly elastic head-on collision with a 2 kg glider at rest on a frictionless track. Find the velocity v′2 of the second glider after the collision.
m1 = 4.5 kgm2 = 2 kgv1 = 11 m/sv2 = 0
v′2 = m/s
12.Momentum & collisionsHard
At a crossing a 1100 kg car travelling east at 23 m/s collides with a 1800 kg car travelling north at 10 m/s. The cars lock together. Find the speed v′ of the wreck just after the collision.
m1 = 1100 kgv1 = 23 m/s (E)m2 = 1800 kgv2 = 10 m/s (N)
v′ = m/s
13.Springs & pendulumsMedium
An astronaut on another planet finds that a simple pendulum 1.6 m long has a period of 4.13 s for small swings. Find the gravitational acceleration g on that planet.
L = 1.6 mT = 4.13 s
g = m/s²
14.Springs & pendulumsHard
A 0.5 kg mass on a spring with k = 340 N/m is pulled 6 cm from equilibrium and released. Find its maximum speed vmax.
m = 0.5 kgk = 340 N/mA = 6 cm
vmax = m/s
3Solutions
1.Kinematics
Answer: t = 13 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 = 2·26/4 = 13 s
Answer
t = 13 s
At that moment the police car moves at 2v = 52 m/s.
2.Kinematics
Answer: hmax = 21.83 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 = 6²/(2·9.81) = 1.835 m
Height above the ground
hmax = 20 + 1.835 = 21.83 m
Answer
hmax = 21.83 m
3.Newton’s laws
Answer: μ = 0.1958
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
a = v₀²/(2d) = 6.5²/(2·11) = 1.92 m/s²
Friction coefficient
μ·m·g = m·a → μ = a/g = 1.92/9.81 = 0.1958
Answer
μ = 0.1958
4.Newton’s laws
Answer: d = 0.5599 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 35° + 0.3·cos 35°) = 8.038 m/s²
Distance
d = v₀²/(2a) = 3²/(2·8.038) = 0.5599 m
Answer
d = 0.5599 m
5.Projectile motion
Answer: H = 2.33 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 = 16·cos 25° = 14.5 m/s
vy = v0·sin θ = 16·sin 25° = 6.762 m/s
Highest point
At the top the vertical velocity is zero: vy² = 2gH.
H = vy²/(2g) = 6.762²/(2·9.81) = 2.33 m
Answer
H = 2.33 m
6.Projectile motion
Answer: x = 49.57 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 = 16.26 m/s, vy = 16.26 m/s
Time to land on the roof
Larger root: the ball is descending.
4 = 16.26·t − 4.905·t²
t = (16.26 + √(16.26² − 2·9.81·4))/9.81 = 3.048 s
Horizontal distance
x = vx·t = 16.26·3.048 = 49.57 m
Answer
x = 49.57 m
7.Circular motion
Answer: T = 2.15 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 = 1.5·sin 40° = 0.9642 m
Speed
Tension and gravity together give the inward force: tan θ = v²/(r·g).
v = √(r·g·tan θ) = √(0.9642·9.81·tan 40°) = 2.817 m/s
Period
T = 2π·r/v = 2π·0.9642/2.817 = 2.15 s
Answer
T = 2.15 s
8.Circular motion
Answer: Nbottom = 353.2 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.
Speed from energy conservation
v² = 2·9.81·29.75 = 583.7 m²/s²
Forces
Towards the centre is positive.
N = m·v²/R + m·g = 4.5·583.7/8.5 + 44.15 = 353.2 N
Answer
Nbottom = 353.2 N
9.Work & energy
Answer: Wf = 42.48 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 = 24.53·0.866 = 21.24 N
F = μN = 0.4·21.24 = 8.496 N
Work against friction
Wf = F·d = 8.496·5 = 42.48 J
Answer
Wf = 42.48 J
10.Work & energy
Answer: d = 0.4843 m
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).
Spring energy
x = 12 cm = 0.12 m
½kx² = ½·500·0.12² = 3.6 J
Energy balance along the slope
m·g = 1·9.81 = 9.81 N
½kx² = mg·d·(sin θ + μ·cos θ)
d = 3.6/(9.81·(0.6428 + 0.15·0.766)) = 0.4843 m
Answer
d = 0.4843 m
11.Momentum & collisions
Answer: v′2 = 15.23 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 = (4.5 − 2)/(4.5 + 2)·11 = 4.231 m/s
v′2 = 2·4.5/(4.5 + 2)·11 = 15.23 m/s
Answer
v′2 = 15.23 m/s
12.Momentum & collisions
Answer: v′ = 10.71 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 g
g = 4π²·L/T² = 4π²·1.6/4.13² = 3.703 m/s²
Answer
g = 3.703 m/s²
14.Springs & pendulums
Answer: vmax = 1.565 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.