Forces And Motion Discussion Question

Forces And Motion Discussion Question

PHYS 1401 Lab-02: Forces and Motion
Name: _____________________________

Objectives
· To understand the relationship between applied force, net force, acceleration, and mass for one-dimensional motion.

· To understand how the forces exerted on an object on an inclined plane affect the object’s motion.

Use the PhET simulation Forces and Motion: Basics. https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion-basics_en.html

This simulation allows you to push on different objects with a certain horizontal force and see the resulting motion. You can control the mass of the object by using different objects or by stacking up to three objects on top of one another. Click and drag on the objects to move them to the desired location. You can also adjust Friction between None and Lots by using the slider provided on the right side of the window.

While the object is being pushed, you can see the directions and values of the various horizontal forces acting on it, as well as the value of the sum of these forces. In addition, you can display the mass, speed, and acceleration of the object.

Activity-1
1. Open up PHET simulation Forces and Motion: Basics

https://phet.colorado.edu/sims/html/forces-and-motion-basics/latest/forces-and-motion-basics_en.html

2. Click “Net Force”

3. Place 2 people that are the same size the same distance away from the cart.

4. Make a prediction about the movement of the cart.

5. AFTER you have observed the actual movement, click on the sum of the forces box at the top right-hand corner of the simulation. Record the number in the data table.

Tasks

Predicted Movement

Actual Movement

(none, left, right)

Sum of Forces

(0, x-left, x-right)

Same size, same placement on rope.

Same size, different placement on rope.

Different size, same placement on rope.

Different size, different placement on rope.

Fill in the following table with the values of the forces (some boxes have more than one possible correct answer). Capture and insert the screenshot when completing the last three rows.

Situation

Force to the right (N)

Force to the left (N)

Net force

(N, left or right)

100 N

200 N

150 N, right

100 N, left

Question-1: What causes objects to move or stay still?

Question-2: Balanced forces cause a change in motion. True or false? How do you know this?

Question-3: Unbalanced forces cause a change in motion. True or false? How do you know this?

Question-4: What do you think would be the best way to divide up your friends for the game of tug-of-war? Be sure to explain your reasoning.

Activity-2
Start the simulation. Double-click on Acceleration to get started.

Adjust the Applied Force by moving the slider bar (which is at the center bottom of the main window) left or right. Release the slider to stop applying a force on the object. You can also adjust the Applied Force in increments of 50 Newtons by clicking on the left and right arrows above the slider bar. Apply a force on the crate and watch the resulting motion.

Feel free to play around with the simulation and investigate what happens when you click on either the figure or the object, while they are in motion. When you are done, hit reset (round button with a circular arrow, to the right) and set Friction to None prior to beginning Step-1. Make sure that Forces, Sum of Forces, Values, Masses, Speed, and Acceleration are all selected.

Step-1: Choose the 200 kg refrigerator. Set the applied force to 400 N (to the right). Be sure friction is turned off.

Question-5: What is the net force acting on the refrigerator?

Question-6: What is the acceleration of the refrigerator 4 s after the person begins pushing on it with a force of 400 N?

Question-7: What is the acceleration of the refrigerator 30 s after the person begins pushing on it with a force of 400 N?

Question-8: What can you say about the speed of the refrigerator? Does it remain constant, decrease or increase?

Step-2: Adjust the mass of the refrigerator by stacking different objects on top of it.

Question-9: If the mass of the refrigerator is increased (with the Applied Force held constant), what happens to the acceleration?

Step-3: Click the reset button and adjust the Friction back to None. With the 50kg crate selected, use the slider to apply a force of about 100N. After 3 s have elapsed, release the slider so that the Applied Force is now zero, and the figure is standing still.

Question-10: What happens to the crate after the Applied Force (force exerted) goes to zero?

Step-4: Click on the crate to bring it to a stop, then replace it with the refrigerator. Use the slider to apply a force of about 400 N. After 2 s have elapsed in the simulation, decrease the Applied Force (force exerted) slowly back to zero. Try to do this adjustment in roughly 2 s.

Question-11: While the Applied Force (force exerted) is decreasing, the velocity is ____________.

Explain your observation in few sentences.

Step-5: Click on the reset button, and stack one 50kg. crate on top of the other, so that the total mass is 100kg. The Friction should be set to halfway between None and Lots. Adjust the Applied Force (force exerted) slowly and determine when the crates begin accelerating.

Question-12: What is the minimum force that must be exerted on the crates so that they accelerate?

Question-13: If the Applied Force (force exerted) on both crates with the current Friction setting is 350N, the acceleration is ____________.

Step-6: Click the reset button, and set the Applied Force (force exerted) to 350 N. After about 3 s, set the Applied Force back to zero.

Question-14: After the applied force (force exerted) is turned off, what happens to the crate?

Activity-3
For this tutorial, use the PhET simulation: The Ramp. https://phet.colorado.edu/sims/cheerpj/the-ramp/latest/the-ramp.html?simulation=the-ramp

This simulation allows you to place a variety of objects on an inclined ramp and look at the resulting forces and motion.

Select an object to place on the ramp by clicking on any object under the Choose Object section of the right panel. To change the ramp angle, you can adjust the Ramp Angle slider bar in the right panel, or you can click on the ramp and drag it up or down. To turn off friction, you can click on the Frictionless option that is above the Position setting. You can have the person pushing on the object by setting an Applied Force that is nonzero (you can type in a value in the Applied Force box on the left, drag the big vertical slider bar to the left of the parallel-force graph, or you can click and drag on the object). While the simulation is running, a graph shows the parallel forces (i.e., the components of the forces along the ramp) as a function of time.

Feel free to play around with the simulation. When you are done, click Reset before beginning Activity-3.

Step-1: The first thing you will investigate is static friction. The force of static friction is the parallel force exerted on a stationary object by the ramp. This force is always directed opposite the direction the object would slide if there were no friction.

Select the crate as the object for the ramp. Then, slowly increase the ramp angle. The individual forces acting on the crate are shown.

Question-15: What happens to the force of friction (the red vector in the picture or the red plot in the graph) before the crate starts to slide down the ramp. As the ramp angle increases, the force of static friction ________________.

Question-16: What is the maximum ramp angle that still allows the crate to remain at rest? (Make sure the coefficient of friction is 0.7.)

Question-17: With the crate stationary on a horizontal ramp, the force of static friction is ___________.

Step-2: Click on the More Features tab near the top of the window. You can adjust the coefficient of friction to a value less than 0.7.

Question-18: Determine the maximum angle that still allows the crate to remain at rest. If the coefficient of friction is less than 0.7, what happens to this angle?

Step-3: The mass of the crate can also be adjusted by clicking on the More Features tab and then using the slider bar in the right panel.

Question-19: How does the maximum angle for which the crate can remain at rest on the ramp depend on the mass of the crate?

Step-4: Click Reset, and then adjust the ramp angle to 15. Compare the force of static friction when there is no applied force to when there is an applied force of 100 N (pushing up the ramp).

Question-20: How do the two forces of static friction compare?

Question-21: For a stationary crate (with a coefficient of friction of 0.7) on the 15 ramp, can the force of static friction ever be zero?

a) No

b) Yes, but only for a specific applied force directed down the ramp.

c) Yes, but only for a specific applied force directed up the ramp.

Explain your reasoning.

Step-5: Slowly adjust the applied force (pushing both up and down the ramp) until the crate begins to move.

Question-22: Determine the minimum strength of the pushing force needed to accelerate the crate up the ramp and the minimum strength of the pushing force needed to accelerate the crate down the ramp. How do these two minimum strengths compare to each other?

Question-23: What are the factors which affect static friction of an object on a ramp.

2

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