# INTRODUCTION

The goal of this lab is to understand the relationship between net forces, acceleration, and mass. This lab also introduces you to kinetic and potential energy.

You will investigate these concepts through a series of problems and a simulation. Read over the entire lab procedure below, then complete each task and record your findings in the data section at the end of the lab.

Upon completion, save this document as a PDF with your name in the filename in the form “Yournamehere-forces.pdf” and upload to the assignment box on online campus.

Newton’s Second Law of Motion expresses the relationship between the acceleration of an object, the mass of the object, and the net (or total) force acting on it. In equation form this is given as

Fnet = ma

Fnet is the sum of the forces acting on the object. If two forces are acting in the same direction, then they add together to produce the sum. If, however, the forces are in opposite directions, then we take the difference in the forces and assign the direction of the larger force as the direction of the sum. For example, in the figure below, the net force on block A is 200 N to the right, while the net force on block B is 50 N to the left.

The acceleration of block A would be:

Fnet = ma

200 = 50a

a = 4 m/s² to the right

Similarly, the acceleration of block B would be 2 m/s² to the left.

## PROCEDURE

A. Net force, acceleration, and mass calculations

Below there are a series of figures showing the forces acting on a box. You will determine the acceleration of the box (magnitude and direction), the direction and magnitude of any unknown forces acting on the box, or the mass of the box, whichever quantity is unknown. You will record your answers on the last page, Part A.

1. The 50 kg box has a 200-N force acting on it. There is a 40 N friction force acting on the box. Determine the acceleration (magnitude and direction) of the box, note that friction always acts in the direction opposite to motion.

2. The 20 kg box shown is accelerating to the left at 5 m/s². There is a 5N friction force acting on the box. Determine the applied force (magnitude and direction).

3. The box shown accelerates to the right at 2.50 m/s². The applied force acting on the box has a value of 20 N and the friction force has a value of 2.5 N. Determine the mass of the box.

4. The 6.5 kg box shown has acceleration to the left of 3.50 m/s². There is an applied force of 50 N acting on the box. Determine the magnitude of the friction force, and the direction acting on the box.

## B. Projectile Motion Simulation

Software Requirements: This simulation is HTML5 based.

1. Go to the website Projectile Motion Simulation. (https://www.walter-fendt.de/html5/phen/projectile_en.htm)

2. Set up the applet so that the Initial height = 0, and initial speed = 10m/s, mass = 10kg.

3. Start the angle of inclination at 15°.

4. Press start. Record the horizontal distance and maximum height in the data sheet.

5. Press reset and change to 25°. Repeat step 4, recording horizontal distance and maximum height. Repeat by increasing 10° each trial until you reach 75°, recording each data point. Note: you will have to press “reset” after each round.

6. Change the applet to show “Energy” rather than position, you have the option in the green box.

7. Press the “slow motion” box. Watch what happens to the kinetic and potential energy as the ball moves up and then back down. Describe what you observe.

LAB 3 – Forces and Motion

A. Net force, acceleration, and mass calculations (from part A page 2)

B. Projectile Motion Simulation (Part B page 3)

1. At what angle does the projectile go the greatest distance? Is this the optimal angle for throwing, hitting, or kicking a ball to get the maximum distance when we factor in air resistance? Explain your answer.

2. What is the maximum height (don’t forget units!)?

3. What are your observations regarding the kinetic and potential energy? Watch what happens to the kinetic and potential energy as the ball moves up and then back down. Describe what you observe.

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