Physics, 27.06.2019 15:30 jamisoncameron000

A20kg boy on a 5 kg skateboard coasts to a rest from a speed of 4.5m/s while traveling a distance of 20m on the sidewalk. draw a free body diagram for this situation and calculate the acceleration of the skateboard, the force of friction on the skate board and the coefficient of friction between the skateboard and the sidewalk.

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Answer from: cbender30p860we

a = -.51 m/s²

coefficient of friction = .05

fk= 12.25 N

Explanation:

mass = 20 + 5kg

weight = 25kg ·9.8 = 245 N

There are many ways to do this I am sure but I did this:

1. found a by using vf²=vi²-2aΔx. accel is - because it is slowing down.

started at 4.5 m/s, final = 0 and Δx=20....a=-.51 m/s²

2. the force acting on the object is the friction force which has to equal the mass(accel).

-fk (going to left on free body diagram) = ma

-mu·n = ma

-mu·245N = 25 (-.51)

mu=.05

fk= .05(245N) = 12.25 N

A20kg boy on a 5 kg skateboard coasts to a rest from a speed of 4.5m/s while traveling a distance of

Answer from: Quest

Kinetic energy = 1/2mv^2. since the mass is constant we can form a ratio of it's kinetic energy at 60 divided by that at 30 to see ke(60)/ke(30) = +(60^2)/30^2). this may be said to 300% increase.

Answer from: Quest

force is based upon mass and acceleration.the statement is not always true.

it can be explained from newton's second law of motion.

as per newton's second law of motion the rate of change of momentum is directly proportional to the applied force and takes place along the direction of force.

mathematically it can be written as -

$f\alpha \frac{dp}{dt}$

⇒ $f= k \frac{dp}{dt}$

here k is the proportionality constant whose value is 1.

$f=\frac{dp}{dt}$

$f= \frac{d[mv]}{dt}$

$f= m\frac{dv}{dt} +v\frac{dm}{dt}$

$f=ma+v\frac{dm}{dt}$ [a is the acceleration=dv/dt]

hence we see that force not only depends on mass and acceleration but also on velocity and rate of change of mass. force f will depend on mass and acceleration only when mass is constant.

hence the statement that force is based upon mass and acceleration is false.

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