(a) A car that weighs 12300 N is initially moving at a speed of 46.0 km/hr when

Question

(a) A car that weighs 12300 N is initially moving at a speed of 46.0 km/hr when the brakes are applied and the car is brought to a stop in 4.5 s. Find the magnitude of the force that stops the car, assuming it is constant. What distance does the car move during this time?

(b) A block of mass 7.2 kg is pulled along a horizontal frictionless floor by a cord that exerts a force of 30.0 N at an angle 21.0 above the horizontal. What is the magnitude of the acceleration of the block? What is the magnitude of the normal force exerted on the mass by the floor?

Explanation / Answer

a) 46 km/hr = 12.78 m/s

v = v0 + at

a = -12.78/4.5=-2.84

so F = ma = 12300*2.84/9.81=3561 N

v^2 = v0^2 + 2 a x

x = v0^2/2a = 12.78^2/(2*2.84)=28.76 m

b) sum forces in the x:

30*cos(21) = 7.2 a

a = 30*cos(21)/7.2=3.89 m/s^2

sum forces in the y

N - mg + 30 *sin(21) = 0

N = 7.2*9.81-30*sin(21)=59.88 N

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