Newton’s 2nd Law

Mr. Kuffer

Concepts of Physics

Complete the Free-Body Diagrams for the following situations

Apply the method described in the paragraph above to construct free-body diagrams for the situations described below. Answers are shown at the bottom of this page.

1. A book is at rest on a table top. Diagram the forces acting on the book.

2. A girl is suspended motionless from a bar which hangs from the ceiling by two ropes. Diagram the forces acting on the girl.

3. An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it falls.

4. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel.

5. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.

6. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.

7. A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. Diagram the vertical forces acting on the backpack.

8. A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver.

9. A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration. Diagram the forces acting upon the sled.

10. A football is moving upwards towards its peak after having been booted by the punter. Neglect air resistance. Diagram the forces acting upon the football as it rises upward towards its peak.

11. A car is coasting to the right and slowing down. Diagram the forces acting upon the car.

Newton’s 2nd Law Lab

Concepts of Physics

Mr. Kuffer

Objective: TSW be able to verify the relationship between mass, force and acceleration in accordance with Newton’s 2nd Law of Motion.

Procedure:

  1. Set up the Vernier Logger Pro Bundle, you know the drill.
  2. Using Logger Pro and the motion detector, determine

the acceleration of the cart. (slope of v/t graph)

Perform 3 trials. Calculate average acceleration.

Record on data table

  1. Calculate “Pulling Force”. Record on data table
  2. Calculate “total mass”. Record on data table.
  3. A-E Graph accel (Y) vs. FP (X)
  4. F-K Graph accel (Y) vs. MTOT (X)

Tips and Reminders:

  1. To attain the total mass: Add column #1 + Column #3 + Column #4
  2. See me after trials A – E
  3. In trials A – E: Mass is constant. Vary the pulling force (a.k.a. add mass to hanger).
  4. In trials F – J: Pulling force is constant. Vary the mass of the cart system.
  5. Convertall distances to meters.
  6. Convertall masses to kilograms.
  7. Using the mass, calculate the weight (or force) multiply by -10 m/s2

Example: Fg = mg

(.35 kg)(-10 m/s2)

= -3.5 N

  1. Do not let the masses crash to the ground!Do not let the carts run off the table! If this happens ONCE your group will receive a zero for the lab! Don’t be “that guy”!

Important Equation:

F = ma

Analysis:

  1. For Trials A – E create a graph of acceleration and pulling force. What is the significance of this relationship?
  2. For Trials F – K create a graph of acceleration and total mass. What is the significance of this relationship?
  3. How does this lab validate Newton’s Second Law? Explain in terms of both graphs.

Data Table:

Trial / Pulling mass (kg) / Pulling Force (N) / Mass of Cart (kg) / Extra mass in cart (kg) / Total mass (kg) / Accel. (m/s2)
Trial #1 / Accel. (m/s2)
Trial #2 / Accel. (m/s2)
Trial #3 / Average Accel. (m/s2)
A / .100 / .5 / .20
B / .150 / .5 / .15
C / .200 / .5 / .10
D / .250 / .5 / .05
E / .300 / .5 / .00
F / .200 / .5 / .00
G / .200 / .5 / .05
H / .200 / .5 / .10
I / .200 / .5 / .15
J / .200 / .5 / .20
K / .200 / .5 / .25

Calculations and Analysis in Lab Notebook!

Graphs on Excel!

  1. Frictional Forces:

A)Ff is proportional to the Fg (weight) and the normal force FN, and depends on the surfaces in contact. Example:

More weightharder to push

On ice  easier to push

On cement  harder to push

B)A model used to represent this is…

Ff =  FN

Where  is the coefficient of friction between the two surfaces

C)A table of all coefficients of friction can be found on page 131 of your text.

Frictional Forces

You push a 25-kg wooden box across a wooden floor at a constant velocity. How much force do you exert on the box? (Hint: b/c the box has a constant velocity Ff has got to equal FA)

If the force you exerted on the box is doubled, what is the resulting acceleration of the box?

Draw a Free-Body diagram for the situation above.

Be careful to identify ALL of the forces involved!!

SHOW ALL WORK ON A SEPARATE SHEET OF PAPER!

Friction Lab

Objective:

Calculate the coefficient of friction between two surfaces.

Theory:

If an applied force pulls horizontally on a mass and moves the mass at a constant speed, then the free-body diagram is as follows:

Applying Newton’s 2nd Law:

FNET = FA – Ff

OR

FNET = FA – ( µFN) b/c Ff = µFN

OR

FNET = FA – ( µmg)

Procedure:

  1. Using the spring scale, measure the weight of the block (Fg)
  2. Using the spring scale, pull the block of wood across the wooden surface at a constant speed. The reading on the scale is the applied force (FA)
  3. Add 100g of mass to the wood block. Determine the entire weight of the block and mass. (this is trial #1)
  4. Repeat steps 2 and 3 until reaching 1000g
  5. Repeat all steps for felt on wood
  6. Create a graph of Ff vs. FN.  Hint Ff = µFN
  7. Determine the slope of the graph and its meaning.
  8. Determine the Percent error.

Forces:

Free Body Diagrams and NET Forces

Name:______Date:______

Description of Motion:NET Force? YES or NO

______

______

______

______

______

______

  1. Free-body diagrams for four situations are shown below. For each situation, determine the net force acting upon the object.

2. Free-body diagrams for four situations are shown below. In each case, the net force is known. However, the magnitudes of some of the individual forces are not known. Analyze each situation individually to determine the magnitude of the unknown forces.

Newton’s Second Law

F= ma

Name:______Date:______

  1. What acceleration will result when a 12-N net force is applied to a 3-kg object? A 6-kg object?
  1. A net force of 16 N causes a mass to accelerate at the rate of 5 m/s2. Determine the mass.
  1. An object is accelerating at 2 m/s2. If the net force is tripled and the mass of the object is doubled, what is the new acceleration?
  1. An object is accelerating at 2 m/s2. If the net force is tripled and the mass of the object is halved, what is the new acceleration?

Newton’s Second Law

F= ma

Name:______Date:______

  1. Sidney Crosby strikes a 1 kg hockey puck wit his stick. The puck is accelerated at 5 m/s2. With what force (in newtons) did Crosby strike the puck?
  1. What acceleration will result when a 12-N net force is applied to a 4-kg object? A 8-kg object?
  1. A net force of 20 N causes a mass to accelerate at the rate of 5 m/s2. Determine the mass.
  1. An object is accelerating at 2 m/s2. If the net force is doubled and the mass of the object remains the same, what is the new acceleration?

Under Pressure

  1. Distinguish between force and pressure.
  1. Which produces more pressure on the ground, a person standing up or the same person lying down? Explain.
  1. In attempting to perform the “experiment” shown in the picture below, would it be wise to start with just a few nails and work your way upward to more nails?

  1. The massiveness of the block plays a critical role in the experiment (above). Which provides more safety, a less massive or more massive block?
  1. Pressure is the amount of force per unit area. Write the equation below. Add this equation to your equation sheet.
  1. The unit for pressure is a “pascal”. Why a pascal? What is one pascal equal to?

Pressure Problems

  1. The atmospheric pressure at sea level is about 1.0 x 105Pa. What is the force at sea level that air exerts on the top of a typical office desk, 152 cm long and 76 cm wide?
  1. A woman weighs 495 N and wears shoes that touch the ground over an area of 412 cm2.
  1. What is the average pressure in KPa that her shoes exert on the ground?
  2. How does the pressure change when she stands on only one foot?
  3. What is the pressure if she puts all her weight on the heel of one shoe with the area of the high heel of 2.0 cm2?
  1. A car tire makes a contact with the ground on a rectangular area of 12 cm by 18 cm. The car’s mass is 925 kg. What pressure does the car exert on the ground? What pressure does the ground exert on the car? (Reminder… there are four tires on car!!)
  1. A lead brick, 5.0 x 10.0 x 20.0 cm, rests on the ground on its smallest face. What pressure does it exert on the ground? (Lead has a density of 11.8 g/cm3)
  1. Same situation as above. If the brick rests on its largest face, what pressure does it exert?
  1. A 0.75 kg physics book with dimensions 24 cm by 20 cm rests on a table.
  1. What force does the book apply to the table?
  2. What pressure does the book apply to the table?
  1. Why are sharp ice skate important to hockey players?
  1. In a Tornado, the pressure can be 15% below normal atmospheric pressure. Sometimes a tornado can move so quickly that this pressure drop can occur in one second. Suppose a tornado suddenly occurred outside your front door, which is 182 cm high and 91 cm wide. What force would be exerted on the door? In what direction would the force be exerted? (Standard atmosphere is 1.01325 x 105 Pascals)

1