physic

GROUP # _______

PHYS 110 Lab #4: Forces

The big idea:

Interactions between objects cause changes in motion

When objects interact, they can affect each other’s motion. Often, this is by pushing or pulling on each other—which we call forces. All interactions go both ways—if a bat pushes on a baseball, then the baseball pushes on the bat. We often only care about one of the objects, which we call the OBJECT. The other object it interacts with is the AGENT of the force. That means the force we care about is always AGENT on OBJECT.
Whether the object will keep doing what it was doing before (such as sitting still or moving with a constant speed) or if it will change its motion (by speeding up, slowing down, or changing direction) depends on if there’s a total unbalanced force acting on it from all the other agents it interacts with.
For each of the scenarios depicted, decide if the forces acting on each object are balanced or unbalanced. Circle your decision:

Cart going down a frictionless inclined track.

balanced unbalanced

Ball in free fall

balanced unbalanced

Box attached to parachute at terminal velocity

balanced unbalanced

Cart going down an inclined track at constant speed
(track has friction).

balanced unbalanced

Balanced vs unbalanced forces

Consider these three scenarios and discuss if the motion would be constant or accelerating (circle the word).

SCENARIO A: car with a mass pulling on it and friction between the cart and the track

Constant Accelerating

SCENARIO B: cart with a mass pulling on it and no friction:

Constant Accelerating

SCENARIO C: car with mass pulling on it, no friction, and a spring pulling on its other side

Constant Accelerating

For each scenario, draw a force diagram for the cart in the spaces below:

SCENARIO A:

SCENARIO B:

SCENARIO C:

Scenario A
Watch the video for scenario A, where a string is attached to the cart and also a 5 g hanger (with an additional 20 g on it). The cart is given a small tap towards the pulley side of the track, where the hanger & mass are pulled down by gravity. Observe its motion.

Is the motion constant zero velocity, constant (non-zero) velocity, or changing velocity?

Sketch the motion diagram for the motion as it moves toward the pulley (the pulley is on the right as in the diagram of Scenario A. Indicate the direction of the acceleration vector, if it’s not zero.

QUESTION 1: Is the motion you observed consistent with the force diagram you drew? If it is, explain why. If not, draw a new force diagram here and explain what change(s) was necessary.

Scenario B

Place a non-friction cart on the track and attach the string with the same 20 g on the mass hanger as shown in the diagram of Scenario B. Let go of the cart and observe its motion.

Is the motion constant zero velocity, constant (non-zero) velocity, or changing velocity?

Sketch the motion diagram for the motion as it moves toward the pulley (the pulley is on the right as in the diagram of Scenario B. Indicate the direction of the acceleration vector, if it’s not zero.

QUESTION 2: Is the motion you observed consistent with the force diagram you drew? If it is, explain why. If not, draw a new force diagram here and explain what change(s) was necessary.

Scenario C
Increase the hanging mass to 80 g. Place the cart near the end of the track far from the pulley and attach one end of the spring to the screw on the cart and the other to the end stop on the track, as shown in the diagram for Scenario C. Let go of the cart and observe its motion immediately after release.

Is the motion constant zero velocity, constant (non-zero) velocity, or changing velocity?

Sketch the motion diagram for the motion as it moves toward the pulley (the pulley is on the right as in the diagram of Scenario A. Indicate the direction of the acceleration vector, if it’s not zero.

QUESTION 3: Is the motion you observed consistent with the force diagram you drew? If it is, explain why. If not, draw a new force diagram here and explain what change(s) was necessary.

QUESTION 4: Explain the differences in the motion of each scenario by considering the motion models we’ve developed in class. Which model, or , would be appropriate to find the distance covered by the cart for each scenario?

QUESTION 5: Explain the differences in the motion of each scenario by considering balanced or unbalanced forces. Which equation, or , would be appropriate to find the acceleration of the cart for each scenario?

Forces as Interactions

Watch the video of the two force probes interacting with each other. Using the screenshot of the data, record the force felt by each probe at three different moments in time. Estimate the values based on the graph.

Force from probe A (N)

Force from probe B (N)

Time #1:

Time #2:

Time #3:

QUESTION 6: For your scenarios, were the interaction forces the same or different? Explain using both data from the graphs and the shapes of the graphs.

Jumping
You will investigate what happens when a person jumps while on top of a force measuring platform.
We will break up the motion into a few different stages:
Standing still (person is stationary on the platform before the jump)
During the “push off” (person jumping while feet are still in contact with the platform immediately before getting in the air)
In the air (jumping while feet are not in contact with the platform)
Landing (person jumping while feet are still in contact with the platform immediately after being in the air)
Standing still (person is stationary on the platform after the landing)

First, the person stands on the force platform without moving for a few seconds.

Draw a force diagram for the person standing still on the force platform.

List all of the forces on the person as “Fagent on object”.

Is there a net unbalanced force on the person? Are they accelerating? If so, indicate the direction of both the net force and the acceleration on your force diagram above. If not, write “a=0” and “Fnet=0” on your force diagram.

QUESTION 7: What is the magnitude of the force of the gravity on the person standing on the platform? Explain, using Newton’s 2nd Law and your experimental evidence.

The force platform directly measures the forces – pushes and pulls – on the platform
. From your list of “Fagent on object” forces in the above force diagram, which forces are acting on the platform (platform as object), and therefore are measured directly by the platform?

It’s possible that the platform measures a force on the person indirectly, if it measures the Third Law pair of any of the forces in your diagram above. From your list of “Fagent on object” forces in the above force diagram, which forces are created by the platform pushing on something else (platform as agent), and therefore are measured indirectly by the platform?

Watch the video of the person jumping on the force sensing platform.
Sketch the data provided onto the graph below, and mark each of the important motion milestones (standing still, push off, in the air, landing, standing still)

During the Push Off

Draw a motion diagram for the part of the jump where the person is in the process of pushing off the scale. Indicate an acceleration vector, if the acceleration is not zero.

Draw a force diagram for the person for the part of the jump where the person is in the process of pushing off the scale. If the net force is not zero, indicate a net force vector next to the diagram.

List the forces on the person as “Fagent on object” in this box.

QUESTION 9: During the “Push Off” motion, is the normal force on the person greater than, less than or equal to the force of gravity on the person? Use your diagrams above and data (numbers from your graph) to support your answer.

While in the Air

Draw a motion diagram for the part of the jump where the person is in the air.
Indicate an acceleration vector, if the acceleration is not zero.

Draw a force diagram for the person for the part of the jump where the person is in the air. If the net force is not zero, indicate a net force vector next to the diagram.

List the forces on the person as “Fagent on object” in this box.

QUESTION 10: During the “In Air” motion, is the normal force on the person greater than, less than or equal to the force of gravity on the person? Use your diagrams above and data (numbers from your graph) to support your answer.

Landing

Draw a motion diagram for the part of the jump where the person is the process of landing. Indicate an acceleration vector, if the acceleration is not zero.

Draw a force diagram for the person for the part of the jump where the person is in the process of landing. If the net force is not zero, indicate a net force vector next to the diagram.

List the forces on the person as “Fagent on object” in this box.

QUESTION 11: During the “Landing” motion, is the normal force on the person greater than, less than or equal to the force of gravity on the person? Use your diagrams above and data (numbers from your graph) to support your answer.

QUESTION 13: Explain why a person jumps. Be sure to describe the person’s acceleration and what that tells you about the net force on the person for each stage of the jump (standing still, during the push off, in the air, landing, and standing still again). Be sure to include Newton’s 2nd and 3rd Laws of Motion in your explanation.

Lab #5: Forces Group Worksheet | Page 1 of 3
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