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)
How is this unit
derived? (That is, where does it come
from – the definition) (2pts)
2. Can an object with one force acting on
it be in equilibrium? Explain. (4 pts)
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?