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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 8 m/s² until it stops. How long does braking take? Find t.
v₀ = 60 km/ha = −8 m/s²v = 0
t = s
2.KinematicsHard
Ball A is dropped from rest from a height of 25 m. At the same instant ball B is thrown straight up from the ground directly below at 24 m/s. Ignore air resistance and use g = 9.81 m/s². At what height y above the ground do they meet?
H = 25 mv0B = 24 m/sg = 9.81 m/s²
y = m
3.Newton’s lawsMedium
A 3 kg block slides down a slope at 45° to the horizontal. The coefficient of kinetic friction is 0.15. Use g = 9.81 m/s². Find the acceleration a of the block along the slope.
m = 3 kgθ = 45°μ = 0.15g = 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 20 N pulls the front 1 kg block; the 6 kg block is towed behind. The coefficient of kinetic friction is 0.15 for both blocks. Use g = 9.81 m/s². Find the tension T in the connecting string.
m1 = 6 kgm2 = 1 kgF = 20 Nμ = 0.15g = 9.81 m/s²
T = N
5.Projectile motionMedium
A ball is launched at 35° above horizontal from level ground. Ignore air resistance. Find the time of flight t until the ball hits the ground.
v0 = 22 m/sθ = 35°g = 9.81 m/s²
t = s
6.Projectile motionHard
A ball is thrown from ground level at 27 m/s and 60° above the horizontal. It lands on the flat roof of a building 3 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 = 27 m/sθ = 60°hroof = 3 mg = 9.81 m/s²
x = m
7.Circular motionMedium
A car takes a flat curve of radius 100 m. The coefficient of static friction between tyres and road is 0.7. Use g = 9.81 m/s². Find the highest speed vmax at which the car can take the curve without skidding.
r = 100 mμs = 0.7g = 9.81 m/s²
vmax = m/s
8.Circular motionHard
A 1.5 kg ball on a string 1.2 m long is swung in a vertical circle. At the lowest point it moves at 6.5 m/s. Use g = 9.81 m/s². Find the tension T in the string at the lowest point.
m = 1.5 kgr = 1.2 mv = 6.5 m/sg = 9.81 m/s²
T = N
9.Work & energyMedium
A 1200 kg car drives up a 5° slope at a constant 90 km/h. Air resistance and rolling resistance together amount to 700 N. Find the power P the wheels must deliver, in kW.
A 5 kg block is released from rest at height 1.5 m on a smooth ramp. At the bottom it crosses a rough horizontal strip 1.5 m long (μ = 0.1) and then runs onto a spring (k = 500 N/m) on a smooth surface. Find the maximum compression x of the spring.
m = 5 kgh = 1.5 mL = 1.5 mμ = 0.1k = 500 N/mg = 9.81 m/s²
x = cm
11.Momentum & collisionsMedium
A 10 kg projectile moving horizontally at 27 m/s explodes into two fragments that move along the same line. The 9 kg fragment continues forwards at 85 m/s. Find the velocity v′2 of the 1 kg fragment (positive in the original direction).
M = 10 kgu = 27 m/sm1 = 9 kgv′1 = 85 m/sm2 = 1 kg
v′2 = m/s
12.Momentum & collisionsHard
A 0.5 kg ball hits a wall at 18 m/s and bounces straight back at 9 m/s. The contact lasts 5 ms. Find the average force Favg of the wall on the ball.
m = 0.5 kgvin = 18 m/svout = 9 m/sΔt = 5 ms
Favg = N
13.Springs & pendulumsMedium
How long must a simple pendulum be to have a period of 2.3 s? Use g = 9.81 m/s². Find L.
T = 2.3 sg = 9.81 m/s²
L = m
14.Springs & pendulumsHard
A 0.6 kg mass on a spring with k = 310 N/m is pulled 2 cm from equilibrium and released. Find its maximum speed vmax.
m = 0.6 kgk = 310 N/mA = 2 cm
vmax = m/s
3Solutions
1.Kinematics
Answer: t = 2.083 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.
Units
v₀ = 60/3.6 = 16.67 m/s
Braking time
0 = v₀ − a·t → t = v₀/a = 16.67/8 = 2.083 s
Answer
t = 2.083 s
2.Kinematics
Answer: y = 19.68 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.
Meeting time
Up is positive, the ground is y = 0.
25 − ½g·t² = 24·t − ½g·t² → t = 25/24 = 1.042 s
Meeting height
y = 24·1.042 − ½·9.81·1.042² = 19.68 m
Answer
y = 19.68 m
3.Newton’s laws
Answer: a = 5.896 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.
With the same μ on both blocks, friction drops out: T = F·m₁/(m₁ + m₂).
5.Projectile motion
Answer: t = 2.573 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
Horizontal velocity stays constant; vertical motion has a constant acceleration g = 9.81 m/s² downward.
vx = v0·cos θ = 22·cos 35° = 18.02 m/s
vy = v0·sin θ = 22·sin 35° = 12.62 m/s
Time in the air
Starting and landing at the same height: t = 2vy/g.
t = 2·12.62/9.81 = 2.573 s
Answer
t = 2.573 s
6.Projectile motion
Answer: x = 62.57 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 = 13.5 m/s, vy = 23.38 m/s
Time to land on the roof
Larger root: the ball is descending.
3 = 23.38·t − 4.905·t²
t = (23.38 + √(23.38² − 2·9.81·3))/9.81 = 4.635 s
Horizontal distance
x = vx·t = 13.5·4.635 = 62.57 m
Answer
x = 62.57 m
7.Circular motion
Answer: vmax = 26.2 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) = √(0.7·9.81·100) = 26.2 m/s
Answer
vmax = 26.2 m/s (94.34 km/h)
8.Circular motion
Answer: T = 67.53 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 = 1.5·9.81 + 1.5·6.5²/1.2 = 14.71 + 52.81 = 67.53 N
Answer
T = 67.53 N
9.Work & energy
Answer: P = 43.15 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 = 1200·9.81 = 11772 N
mg·sin θ = 11772·sin 5° = 1026 N
F = 1026 + 700 = 1726 N
Power
v = 90/3.6 = 25 m/s
P = F·v = 1726·25 = 43150 W = 43.15 kW
Answer
P = 43.15 kW
10.Work & energy
Answer: x = 51.47 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 = 49.05·(1.5 − 0.1·1.5) = 66.22 J
Maximum compression
½kx² = 66.22 → x = √(2·66.22/500) = 0.5147 m = 51.47 cm
Answer
x = 51.47 cm
11.Momentum & collisions
Answer: v′2 = −495 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 = 10·27 = 270 kg·m/s
Momentum after
270 = 9·85 + 1·v′2
v′2 = (270 − 765)/1 = −495 m/s
Answer
v′2 = −495 m/s
The minus sign means the second fragment flies backwards.
12.Momentum & collisions
Answer: Favg = 2700 N
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 = 0.5·(9 + 18) = 13.5 N·s
Average force
Favg = J/Δt = 13.5/0.005 = 2700 N
Answer
Favg = 2700 N
13.Springs & pendulums
Answer: L = 1.315 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·(2.3/2π)² = 1.315 m
Answer
L = 1.315 m
14.Springs & pendulums
Answer: vmax = 0.4546 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.