Gunnar loads a hockey puck into a spring launcher on smooth ice. As it fires, the spring exerts a steady 16 N net force on the puck for 0.5 s.
By how much does the puck's momentum change during that 0.5 s?
Start from the momentum form of the second law: F = Δp/Δt.
Multiply both sides by Δt.
FΔt = Δp.
Read the left side: F times Δt is the impulse, J.
So the line says the impulse on an object equals its change in momentum.
This line is called the impulse-momentum theorem.
The launcher's impulse: J = 16 × 0.5 = 8 N·s.
So the puck's momentum changes by 8 kg·m/s: an impulse of 8 N·s changes an object's momentum by 8 kg·m/s.
Impulse arrives in newton-seconds; momentum change arrives in kilogram-meters per second.
The theorem makes the two quantities equal, so the two units must agree. A later lesson checks the units themselves.
A 40 N net force acts on a kick scooter for 0.3 s. Enter the scooter's momentum change in kg·m/s.
An impulse of 18 N·s is delivered to a canoe. What is the canoe's momentum change?
A spring launcher exerts a steady 16 N net force on a hockey puck for 0.5 s. Calculate the puck's momentum change.
Write down the values:
F = 16 N, Δt = 0.5 s, Δp = ?
Write down the equation:
FΔt = Δp
Substitute in the values, and calculate:
Δp = 16 × 0.5 = 8
The puck's momentum changes by 8 kg·m/s (an impulse of 8 N·s delivered it).
A 30 N net force acts on a wheelbarrow for 0.8 s. Calculate its momentum change.
Write down the values:
F = 30 N, Δt = 0.8 s, Δp = ?
Write down the equation:
FΔt = Δp
Substitute in the values, and calculate:
Δp = 30 × 0.8 = 24
The wheelbarrow's momentum changes by 24 kg·m/s.
Esther says: 'An impulse of 6 N·s on a cart changes the cart's momentum by 6 kg·m/s, whatever the cart's mass is.' Is Esther correct?
A 250 N net force acts on a bobsled for 0.4 s. Enter the bobsled's momentum change in kg·m/s.
Which single move turns F = Δp/Δt into FΔt = Δp?
During a serve, Wilma's racket exerts an average 300 N net force on a 0.057 kg tennis ball for 0.005 s. Enter the ball's momentum change in kg·m/s.