A4000 kg truck is parked on a 15 degree slope. how big is the friction force on the truck? the coefficient of static friction between the tires and the road is 0.90.

fr = 10145 N

Explanation:

To solve this exercise let's use Newton's second law

     F = ma

Let's write a reference system, where the x-axis is parallel to the slope and the y-axis is perpendicular

Let's decompose the weight that is not in the direction of the axes

     cos θ = Wy / W

      sin θ = Wx / W

      Wy = W cos θ

      Wx = W sin θ

We write Newton's second law on each axis

Case 1. Truck stopped on the upward slope

X axis

       fr - Wx = 0

       fr = Wx

YAxis

      N- Wy = 0

      N = Wy

The friction force has the equation

      fr = μ N

Let's calculate the force of friction

      fr = W sin θ

      fr = mg sin θ

      fr = 4000 9.8 without 15

      fr = 10145 N

Case2 Truck stopped on the descending slope

X axis

      Wx -fr = 0

      fr = Wx

      fr = W sin θ = m g sin 15

      fr = 10145N

We can see that the value of the friction force is the same regardless of the direction of the slope

true tan speed = ang vel x radius

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.