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What did I do wrong with this physics problem?
An object in space is initially stationary relative to the earth. Then, a force begins acting on the object, starting from 0N, and increasing at a uniform rate until the magnitude of the force is 48N after the force has acted for 35s. The body has a mass of 5 kg. What is the speed of the object at 35 s?
This is what I did.
(Average force)(time)=m(change in velocity)
Average force = 48/35
So, (average force)(time)=(48/35)*(35)=45N
48=m(change in velocity)
48=5(Vf-Vo)
9.6=Vf-0
Vf=9.6 m/s
However I know this answer is wrong. It is multiple choice and I cannot figure out where my error was.
Thankyou!
2 Answers
- oldprofLv 77 years ago
The average force Favg = (F1 + F0)/2 when it increases at a "uniform rate." So Favg = (48 + 0)/2 = 24 N; not what you have. So V = U + Aavg T = 0 + (Favg/M)T = 24*35/5 = 168 mps. ANS.
You can use impulse, as you tried to, but you must do it correctly. m dV = Favg dT; so that dV = V = Favg dT/m = 24*35/5 = 168 mps. ANS.
- Steve4PhysicsLv 77 years ago
You average force is wrong.
SInce the force increased uniformly:
Average force = (initial force + final force)/2
= (0+48)/2
= 24N
Impulse = change of momentum
F*t = m*v_final - m*v_initial
24*35 = 5*v_final - m* 0
v_final = 168m/s