DAY 30

#Goals: SWBAT...

1. Draw correct FBD's, with appropriate vector magnitude and direction.

2. Support classmates with helpful tutoring

3. Use FBD's to find Net Force

4. Describe the relationship between mass, net force, and acceleration

Warm-Up (4min)

I'm sick, so I won't be talking loud today. Keep your voices low.

Read the scenario below, then answer the questions at the end

1. You apply a force to the left to push a 89g matchbox car across the floor. Draw a free body diagram showing the scenario AFTER you release the car.
2. If asked to find the Force of Friction, would you need to do any unit conversions? Why/Why not?

CLASSWORK

1. #030A: The process of determining the value of the individual forces acting upon an object involve an application of Newton's second law (Fnet=m•a) and an application of the meaning of the net force. If mass (m) and acceleration (a) are known, then the net force (Fnet) can be determined by use of the equation.

Fnet = m • a

If the numerical value for the net force and the direction of the net force is known, then the value of all individual forces can be determined. Thus, the task involves using the above equations, the given information, and your understanding of net force to determine the value of individual forces.

Your Turn to Practice

To gain a feel for how this method is applied, try the following practice problems. The problems progress from easy to more difficult. Once you have solved a problem, click the button to check your answers.

Practice #1

Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces.

A rightward force is applied to a 6-kg object to move it across a rough surface at constant velocity. The object encounters 15 N of frictional force. Use the diagram to determine the gravitational force, normal force, net force, and applied force. (Neglect air resistance.)

Practice #3

A rightward force is applied to a 10-kg object to move it across a rough surface at constant velocity. The coefficient of friction between the object and the surface is 0.2. Use the diagram to determine the gravitational force, normal force, applied force, frictional force, and net force. (Neglect air resistance.)

Practice #4

A rightward force is applied to a 5-kg object to move it across a rough surface with a rightward acceleration of 2 m/s/s. The coefficient of friction between the object and the surface is 0.1. Use the diagram to determine the gravitational force, normal force, applied force, frictional force, and net force. (Neglect air resistance.)

Practice #5

A rightward force of 25 N is applied to a 4-kg object to move it across a rough surface with a rightward acceleration of 2.5 m/s/s. Use the diagram to determine the gravitational force, normal force, frictional force, net force, and the coefficient of friction between the object and the surface. (Neglect air resistance.)

At Home Learning (HW)

1. Complete the problems from 30A. Need help? The answers to all five practice problems are here: http://www.physicsclassroom.com/class/newtlaws/Lesson-3/Finding-Individual-Forces

NGSS Standard (this is what we're learning with this unit)

Analyze data to support the claim that Newton’s second law of motion describes the mathematical relationship amongthe net force on a macroscopic object, its mass, and its acceleration. [Clarification Statement: Examples of data could include tables or graphs of position or velocity as a function of time for objects subject to a net unbalanced force, such as a falling object, an object sliding down a ramp, or a moving object being pulled by a constant force.] [Assessment Boundary: Assessment is limited to one-dimensional motion and to macroscopic objects moving at non-relativistic speeds.]