Physical Science Lab #4
Forces and Newton's First and Second Laws
Theory
N1: An object at rest or traveling at constant speed in one direction has no net force on it.
(The words in italics are equivalent to zero acceleration or to constant velocity.)
N2: An object of mass m will experience acceleration a if a net force
Fnet
is applied to it as described by the equation:
Fnet = ma
where Fnet and a are vectors that point in the same direction and the units of force
are Newtons (N), mass are kg and accleration are m/s2.
Frictional force: is parallel to the surface between two objects in contact and the direction of the force
acts to slow down the movement between surfaces.
Gravitational force (weight): the force of gravity acts between all objects and increases with the mass
of each object and decreases with the separation between their centers. On the earth's surface, since the earth's mass and the distance
between the earth's center and the object's center is about constant, the weight is given by
weight = mg
where g is the acceleration due to gravity at the earth's surface = 9.8 m/s2, m is the mass in kg and the weight is in N.
Normal force: is perpendicular to the surface of contact bewteen two objects and pushes outward against each object.
Tension force: acts as a pulling force when matter is stretched, as with a spring or rubber band, and acts to pull the material back together.
Procedure
You will be using standard masses and force meters to measure the forces on objects at rest,
moving with constant velocity and accelerating.
- Place a mass of 200 or 500 g on the lab bench. Draw a freebody diagram (FBD) for the mass. (Remember to label the forces using the names from the theory section.) Calculate the weight and normal force and add them to the labels on your FBD.
- Use the force meter to suspend the mass. Again, draw the FBD and calculate the weight
and the tension in the spring inside the force meter.
How does this compare to the forces in part 1? (The force meter's reading is, by definition,
the tension in the spring.) Does the force meter reading agree with your calculated results for parts 1 and 2?
In your own words, explain how the force meter works.
- Drag the mass across the bench at a constant velocity. What is the net force on the mass?
Draw the FBD (it should have four arrows on it). How are the weight and normal force related to those in part 1?
What is the relationship between the tension force from the spring in the force meter and the frictional force?
Use the meter to read off the tension and note it. What is the value of the frictional force? Add all the values for the forces to your FBD.
- Try pulling the mass across the lab bench so that it has a fairly constant acceleration. How does the reading on the force meter change? Explain.