# physics 100

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Bob, who has a mass of 60 , can throw a 800 rock with a speed of 26 . The distance through which his hand moves as he accelerates the rock forward from rest until he releases it is 1.0 .

a) What constant force must Bob exert on the rock to throw it with this speed?

b)If Bob is standing on frictionless ice, what is his recoil speed after releasing the rock?

• physics 100 -

You need to provide dimensions with your numbers.

• physics 100 -

m1=60 kg, m2=0.8 kg, v=26m/s, s=1 m
(a). a=v²/2s =26²/2•1=338 m/s²
F=m1•a=60•338=20280 N (inhumanly large force... I believe that velocity has to be 2.6 m/s .... check your given data)
(b)The law of conservation of linear momentum
0= m1•v1-m2•v2
v1= m2•v2/m1= 0.8•26/60=0.347 m/s

• physics 100 -

b. F = m*a = 0.8 * 338 = 270.4 N.

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