Pre AP Physics Chapter 4: Forces and the Laws of Motion Problems

Problem (160)

Solve the problems below on your own paper. Be sure to put each problem on its own page. Clearly label each answer ‘a’, ‘b’, ‘c’ and so on.

1. A physics teacher rolls a very large steel ball bearing across the classroom floor. The VLSBB has an initial velocity of 1.1 m/s. 0.6 seconds later the velocity of the VLSBB is 0.8 m/s.

a) Draw a FBD of the forces acting on the VLSBB.

b) Write the equation for the sum of all horizontal forces.

c) Write the equation for the sum of all vertical forces.

d) What is the acceleration of the VLSBB?

e) What is the coefficient of friction between the VLSBB and the floor?

f) How far did the VLSBB roll in 0.6 seconds?

2. Note the figure below. The block has a mass of 5 kg, it is held in equilibrium with force 'F'. The angle of the incline is 15⁰.

a) Draw a FBD of th eforces acting on the box.

b) Write the equation for the sum of all horizontal forces.

c) Write the equation for the sum of all vertical forces.

d) Find the magnitude of force 'F'

e) Find the magnitude of the normal force.

3. A 100 N box has a force of 40 N applied to it at an angle of 32⁰ below the horizontal. The coefficient of kinetic friction between the box and the floor is 0.13

a) Draw a FBD of the forces acting on the box.

b) Write the equation for the sum of all horizontal forces.

c) Write the equation for the sum of all vertical forces.

d) What is the acceleration of the box?

e) How long will it take the box to move 5m, starting from rest?

Pre AP Physics Chapter 4: Forces and the Laws of Motion Problems

Answer Section

PROBLEM

1.

From the FBD we can write the horizontal and vertical forces equations:

We know that and from the vertical forces equation that

So we can write horizontal forces equation like this:

which gives us or,

So, all we need now is ‘a’.

We can find the acceleation with our old friend

We can find the distance the ball rolled with our other old friend:

wich gives us:

2.

From the FBD we can write the horizontal and vertical force equations; remember that the system is in equlibrium which means that there is no acceleration, there is no net force in either direction. Here are the equations:

To find the applied force we need either or . There is not enough information (yet) to find the ; we do not have the normal force nor the perpendicular force, although we could find that easily enough (). So we will have to find .

From the horizontal forces equation we know that . The FBD tells usthat so now we know that .

The FBD tells us we can use the Cosine function to find the force applied: , so .

To find the normal force we look at the vertical forces equation and we see that . The FBD tells us how to find these two quantities:

and . Putting all this together we get this equation for force normal:

3.

From this FBD we get the horizontal and vertical forces equations:

From the FBD we can see that is equal to and we already know that . When we put these two equations together we can find ma:

If only it were that easy! We have to find the normal force. Fortunately we can use the forces vertical equation to find that! , and from the FBD we can see that . This measn that .

Now, go back to the equation and replace the quantities so that we can find the acceleration:

To find the time we can use:

Of course you know that since that