Average acceleration is equal to , where  is the final velocity and  is the initial velocity.  

If the velocity is constant, the final and initial velocities are equal. Let's say the car is traveling at  constantly.

 .  So  

When velocity is constant, acceleration is always equal to

Average acceleration is equal to  where  is the final velocity and  is the initial velocity.  

In this problem, we know our initial velocity is  (starting from rest). The final velocity is . We must convert these units to .  

Using these values, we can calculate the final velocity to be

Using this number, we can finally calculate the magnitude of acceleration.  

We can solve this problem using the kinematic equation:

Where  is the displacement,  is the initial velocity,  is the acceleration, and  is time.  We are given values for all but the time. Therefore, we may plug these values into the above equation and solve for .

The correct answer is . We can use the kinematics equation:

We are given the values of , , and  (initial velocity, time, and acceleration). Therefore we can solve for  to find the distance traveled by the ball.  

 is negative because the position of the ball is  below its initial point.  

To solve this problem, we can use the kinematics equation:

Where is the initial velocity,  is time, and  is acceleration. We are given all of these variables, so we can plug them in for .  

In this problem,  and  are negative because the rock is initially being thrown downhill and is accelerating downwards.

The first step in doing this problem is figuring out how many forces are acting on the car when the driver slams the brakes. Since there is nothing pushing the car in the direction that it was originally traveling, the only force that is acting on the car is the force of friction. This means that the net force is simply just the force of kinetic friction, net force is also equal to mass multiplied by acceleration, so this equation appears as follows:

where  is the force of kinetic friction. 

The force of friction is also equal to normal force multiplied by the coefficient of kinetic friction for a certain surface. This equation appears as follows:

where  is the force of kinetic friction,  is the coefficient of kinetic friction, and N is normal force (also equal to mass x gravity).

These 2 equations can be connected in order to figure out the coefficient of kinetic friction.

mass can be cancelled from both sides. Acceleration is found as follows:

By plugging in the given values,  is found to be:

You can think of the average velocity of a particle as the difference between the final and initial states. Conventionally, we call this slope. Therefore, we need to plug in our two initial times into the position function, and use the equation for slope between two points to determine the average velocity.

Therefore, our two coordinates are: 

Now, we use the slope equation:

Using the kinematic equation:

Using the kinematic equation:

Using

Initially she is at rest.

Combining equations:

Solving for 

Plugging in values:

Using