Forces and Newton’s Laws

Forces and Newton’s Laws Google: “PHET Forces and Motion Basics” and click on the first link. Click on the play button on the image to begin the simulation. Part I - Newton’s First Law Choose the “Motion” window to start the simulation Make sure the boxes that say “Force”, “Values” and “Speed” are checked! a. Apply a force of 50 N right to the box. Describe the motion of the box using physics terms (i.e. velocity, acceleration, displacement). Refer to the speedometer in your answer. - velocity of the box is increasing to the right as the speedometer shows an increasing number acceleration is constant and to the right displacement is increasing to the right b. Reset the scenario (don’t forget to check forces, speed again). Apply a force of 50 N to the right for about 5 seconds then reduce the applied force to zero (the man should stop pushing). Don’t reset the scenario. Describe the motion of the box. Refer to the speedometer in your answer. - the velocity of the box is constant, as the speedometer shows the same number acceleration is zero displacement is increasing to the right c. Apply a force of 50N to the left. Describe the motion of the box. - velocity of the box is increasing to the left as the speedometer shows an increasing number Acceleration is constant and to the left displacement is increasing to the left d. Explain the exact steps needed to make the box come to a stop. - apply a force in the opposite direction of the motion (force can be smaller, equal to, or larger than original force) until the speedometer shows 0 m/s Summary Newton’s First Law of Motion States “An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.” Explain how your observations in a - d support this Law. - When there is no force applied to the box, the box does not move (object at rest stays at rest) When there is a constant force applied to the box, the box moves with increasing velocity When there is a force applied for a short amount of time to the box, the box continues to move with constant velocity (object in motion tends to stay in motion) Part II - Newton’s Second Law a. Reset the sim, don’t forget to check force, values and speed again. Remove the box and place a garbage can on top of the skateboard. Using your timer/phone, measure the amount of time it takes to reach maximum speed using a force of 50 N. Try this again with forces of 100N, and 200N. Applied Force (N) Time To Max Speed (s) 1 minute and 37 sec = 97 seconds 50 40 seconds 100 20 seconds 200 b. Reset the sim, check force, values, speed and the masses boxes this time. Set the applied force to 200 N Right. Using your timer/phone measure the amount of time it takes to reach maximum speed. Repeat with two crates, one crate and a garbage can, and a refrigerator. Record your findings! Object – Mass (kg) Time To Max Speed (s) 50 10 seconds 100 20 seconds 150 30 seconds 200 40 seconds Open up the “Graphical Analysis” Software on the laptop. Create a graph of time vs mass. Sketch this graph in the space below. Recall how acceleration is related to time. Note: In this case the final speed of all the trials is the same. Manipulate your time data to get an appreciation for how the acceleration of the mass changes in each trial. Sketch an acceleration vs mass graph using this data in the space below (use GA software to help you) Summary Newton’s Second Law states “The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.” Explain how your observations in both a and b support this Law. - The heavier the object (i.e the larger the mass), the longer it takes to accelerate to a certain speed The larger the force applied, the larger the acceleration. Part III - Friction’s Effects The behavior of the skateboard in Part I and part II were not very realistic because friction was not present. At the bottom of the screen is a simulation that includes friction. Select this simulation. a. Set friction to “none”. Notice how the screen changed. Why do you think the app designers did that? - the ground changes to ice because different surfaces have different amounts of friction b. Make sure that only speed box is checked. i. Apply a force to get the box to about half of it’s maximum speed, then remove the force. ii. While the box is moving, move the friction slider to 1/2 way. What happened to the box? - The box slowed down until it came to rest Summary Is friction a force? What evidence do you have? - Friction must be a force because it caused the box to slow down and stop, which meant that the friction force was acting in the opposite direction of the motion Part IV - Back to Newton’s Second Law Reset the Friction app. Make sure Forces and Speed are checked. a. Apply a force of 50 N. Describe the movement of the box. - The box does not move as the friction force counters the applied force b. Apply a force of 100 N. Describe the movement of the box. - The box does not move as the friction force counters the applied force c. Apply a force of 150 N. Describe the movement of the box. The box begins to move as the applied force is larger than the friction force d. Check the box that says “Sum of Forces”. Repeat procedures a, b, and c. What was different about c? - - The friction force was less than the applied force Summary Newton’s Second Law states “The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.” Explain how your observations relate to the underlined portion of this Law (hint, you might want to look up the definition of the word “net”). The net force can be split up into two components: horizontal and vertical. The net force is the sum of all of the forces in the horizontal or vertical direction. When the box is not moving, the sum of the friction and applied force is zero, so the net force is zero. when the box is moving, the sum of the friction and applied force is greater than zero, so the net force is greater than zero. Part V: Friction in Detail a. Reset the app. Check the force and speed box. Apply 50 N of force to the crate. Slowly click (in 50 N increments) to increase the amount of applied force until the crate starts to move/accelerate. Note the range of force values when the crate first started to move. Record this range below (e.g. starts moving somewhere between 0 and 50 N). - Starts moving at around 125 N b. Reset the app. Repeat the steps in “a”. Once you have just made the crate move do not adjust the force any higher. Allow the block to move with this constant force for 2.0 seconds. Now reduce the force by 50 N. How does the crate behave immediately after reducing the force by 50 N? Did it keep accelerating? Move at a constant speed or decelerate? Does this behaviour surprise you based on your knowledge of Newton’s 1st and 2nd Law. Why do you think the crate behaves this way? - The box decelerates, which is not surprising because the friction force is the only force acting in the horizontal direction which slows it down c. Reset the app. Check the speed, force, sum of forces and values boxes. Adjust the value of the applied force using single Newton increments to determine the exact force required to make the crate move. Mentally note the value of this force so you can record it after in the space below. Now keep increasing the value of the applied force in large 50 N or small single Newton increments. What happens to size of the friction force? What is the value of the friction force now? Why does the friction force behave this way? - There are two different friction forces, the static friction force, and the kinetic friction force. The static friction force is always larger than the kinetic friction force. This means that the box needs more force to overcome the static friction at first, then the kinetic friction is in play after the box starts to move. d. Repeat step c and after the block has accelerated for 3.0 seconds reduce the applied force to 0 N. What happens to the size of the friction force after the applied force goes to 0 N? How does the crate behave? - The friction force stays the same until the box stops moving, to which then it disappears. e. Reset the app. Check the forces, values and speed boxes. Place another crate on top of the first. Determine the minimum amount of force required to move the crates. Determine the minimum amount of force required to keep the blocks moving at a constant speed. Record these below: - minimum force required to move the crates: 251 N minimum amount of force required to keep the blocks moving at a constant speed: 188 N i. How do these values compare to the values found for one crate? What does this tell you about the affect of mass on friction? - The force required to move the crates is about double that of a single crate Mass and friction are directly related, the larger the mass, the more friction is experiences f. Reset the app. Check the force and values boxes. Apply 50 N of force to the crate. What is the value of the force of friction now? Record this. Increase the applied force and note how the friction force behaves. Record this behaviour in the space below. - Force of friction is 50 N The force of friction increases to 125 N, to which then the box starts moving and the force of friction drops to 94 N Summary Make four or more general statements about the force of friction based on your observations in parts a – f. - Force of friction scales with applied force to a certain point Once box starts moving, the friction force decreases Friction force is directly proportional to mass Different surfaces have different amounts of friction