 Ch 4      Physics Lab Exam – Forces and Newton’s Laws

Read each question carefully, draw a diagram where appropriate and show your work for partial credit.  Remember to put your answers in  significant figures.  Good Luck!

1.         What is the metric unit of force?  (2pts)  The Newton

How is this unit derived?  (That is, where does it come from – the definition) (2pts)

It is the unit of force required to accelerate a 1 kg mass at a rate of 1 m/s2

2.         Can an object with one force acting on it be in equilibrium?  Explain. (4 pts)

No, in order for an object to be in static (not moving) or dynamic (moving with constant velocity) equilibrium  the net force on the object

must be equal to zero.  A single force of any magnitude will not produce a net force of zero newtons.

3.         A clothesline is hung between two poles and then a shirt is hung near the center to dry.  No matter how tightly the line is stretched it will always sag a little at the center.  Why?  Draw a diagram to help explain. (5 pts)

4.         The other day I tied a fairly fine thread to the hook at the top of a 1 kg mass.  If I lift slowly with a constant force, I could lift the mass.  But if I jerked upward on the string quickly, the string would snap.  Why?  What does it have to do with inertia?  Which of Newton’s Laws explains this?   (4 pts)

5.            According to Newton’s 3rd Law each team in a tug-of-war pulls on the other team with equal force?  So what then, determines which team wins?  (4 pts)

6.         A block rests on an inclined plane with enough friction to prevent it from sliding down.  Draw a free body diagram for this block.To start the block moving, is it easier to push it up the plane or down the plane?  Why?  (8 points)

7.         When trying to stop a moving car on ice or wet roads drivers are told to pump the brakes rather than slam on them and hold the pedal down.  Anti-lock brakes do the same thing, applying and releasing the brakes.  Knowing what you do about the coefficients of static and kinetic friction, why do you think this helps the car stop better? (4 points)

8.         Why, when trying to climb up a rope, do you pull DOWN on it?  Which of Newton’s laws is this? (3 points)

9.         What is the weight of a 3.5 kg mass?  What is the normal force working on it if the block is placed on a 25 degree incline?  If the incline somehow gets steeper,  would the normal force change?  If so, how?  What would be the minimum and maximum values of the normal force for this mass? (8 points)

10.       How much force does it take to give a 2.00 x 104-kg locomotive an acceleration of 1.50 m/s2 on a level track with a friction force of 5880 N?  (8 points)

11.       The orthodontal wire brace shown below makes an angle of 80.0 degrees with the

perpendicular to the tooth.  If the tension in the wire is 10.0 N, what force is exerted on the tooth by the brace? (8 points)

12.       For the diagram below, determine the scale reading in the right arm’s spring scale and find the angle theta.  (8 points)

13.       A valuable antique desk is being transported in the back of a truck.  The coefficient of static friction between the desk and the truck’s bed is 0.50.  How hard can the driver hit his brakes (maximum deceleration) if the desk is not to slide? (10 points)

14.       In the diagram below, one hand pushes down on the block at a 45 degree angle with 15 N of force and the other hand pulls up on the block at a 45 degree angle with a force of 15 N.  If the block has a mass of 2kg (and assuming negligible friction) what is the block’s acceleration?  (8 points)

15.       In 1794, George Atwood published a description of a device used to calculate g by “diluting” the effect of gravity.  This “Atwood’s Machine” consists of two masses slung over an essentially massless and frictionless pulley as shown below.  (14 points)

If m2 > m1 prove that the both masses accelerate at a rate of:

a  = [ (m2 – m1)/( m2 + m1) ]g

What is the acceleration of the system if m2 = 1800 N and m1 = 1300 N

Now suppose someone removed m2 and replaced it by pulling downward on the rope with a force of 1800 N.  What would be the system’s acceleration now?  Does the acceleration change?  Why or why not?