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Bengalfan985
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Homework Statement
A boy pulls his brother in a wagon (total mass of 35kg). He exerts a force of 72 N at an angle of 45° above the horizontal.
a) How much power is he using if he walks 10 meters in 6.4 seconds?
b) How fast will his brother be going after 6.4 seconds
c) What is the impulse imparted to the brother and wagon (assume they start at rest)?
Homework Equations
P = W/t
W = ΔK
Δp = mΔv = FΔt
The Attempt at a Solution
a) P = W/t = F_x d/t = Fcosθd/t = 72cos(45°)(10)/6.4 = 79.5 Watts
b) W = ΔK = K_f - K_i = K_f - 0 = 1/2 m (v_f)^2
v_f = √(2Fcosθd/m) = √(2(72)cos(45°)10/35) = 5.4 m/s
c) Δp = mΔv = 35(v_f - v_i) = 35(5.4 - 0) = 189 kg⋅m/s
Hi, I have a question about part c. The answer was correct with 189 kg⋅m/s, but I first tried solving it with Δp = FΔt. Why doesn't Δp = FΔt work?
Δp = FΔt = F_x t = Fcosθt = 72cos(45°)6.4 = 326N⋅s
The units are the same, right? N⋅s = kg⋅m/s