Forces and Motion

 Motion & Force

Vocabulary:

motion, reference point, force, newton, friction, gravity, net force

Motion: An object is in motion if its position changes when compared to another object.

Reference point: A reference point is a place or object used for comparison to determine whether something is in motion.

Force: A  force is a push or pull that can change the motion of an object.

Newton: In the International System of  Units (SI), the unit for the strength of a force is called a newton (N), after the scientist Sir Isaac Newton.

Friction: Friction is a contact force that two surfaces exert on each other when they rub against each other.

Contact forces: Contact forces are those applied only when one object actually touches another.

Noncontact Force: A noncontact force is a force applied to an object whether it touches the object or not. 

Ex. Gravity- a force that pulls objects toward each other as a result of their masses.

Q. What are three examples of noncontact forces?

Ans: magnetism, gravity, electrical forces.

Net Force: The net force on an object is the combination of all the forces acting on that object. 

Balanced forces: If two forces acting on an object are equal in strength and opposite in direction, they are balanced forces.

Ex. A single book resting on a shelf has two forces acting on it. The downward force of gravity is equal in strength and opposite in direction to the upward force of the shelf on the book. 

When the forces on an object are unbalanced, there is a nonzero net force acting on the object.

Q. What sources of information might you use to determine the relative motion of Earth compared to other planets in the solar system?

Ans: We can use observational data from telescopes and spacecraft, data from astronomical databases, and models that simulate the solar system's dynamics. 

Q. Determine Differences:

1. What is the difference between a contact force and a noncontact force?

Ans: Contact forces are those applied only when one object actually touches another, a noncontact force touches the object or not.

Q. Apply Scientific Reasoning:

2. A child is riding in a wagon. What reference point might have been used if an observer said the child was not moving?

Ans: wagon itself

Q. Calculate:

3. Two children fight over a toy. One pulls with a force of 8N to  the right. The other pulls with a force of 6N to the left. What is the strength and direction of the net force on the toy?

Ans :              Fnet = F1  + F2                              F1 = +8N

                               = 8N - 6N                              F2  = -6N

                               = 2N

Direction of the net force will be right.

Q. Identify Criteria:

Q. A cow is grazing in a field. Under what conditions does the cow have relative motion?

Ans : A cow is grazing in a field has relative motion when its position changes with respect to any other object or point of reference.

                        

Q. Synthesize Information: 

5. One man pushes on the front of a cart while another man pushes on the back of the cart. The cart begins to move forward. What are three things you know about these two applied forces?

Ans: The force applied by the man pushing from the front is greater than the force from the man pushing from the back.

Quest:

Identify:

Q. What are the forces that act upon amusement park rides? Why is it important for engineers to understand how motion and forces affect the rides that design?

Ans: The main forces on amusement park rides are gravity, friction

Because engineers need to rider safety and create an exciting experience.

Q. A girl picks up a bag of apples that are at rest on the floor. How does the force the girl applies compare to the force of gravity acting on the apples?

Ans: When the girl is holding the apples still, the force she applies is equal in magnitude and opposite in direction to the force of gravity.

Lesson-2

Speed, Velocity, & Acceleration:

Objectives:

 Determine the Average Speed of an object

- from Calculations.

- using distance-versus-time graphs.

Text evidence to compare

- Velocity, Speed,  & Acceleration.

Interpret Graphs

- to determine acceleration.

Q. How do you determine speed from calculations and distance-versus-time graphs?

Q. How is velocity related to speed and acceleration?

Q. How can you interpret graphs to determine acceleration?

Vocabulary:

Speed = The speed of an object is the distance the object moves per unit of time.

Instantaneous Speed = The speed is a particular instant in time is called instantaneous speed.

Slope = The angle of a line on a graph is called slope. 

Velocity= The speed at which an object travels in a given direction is called velocity.

Acceleration

Average

Variable

Q. How does the snow help the sled move down the hill?

Ans: It reduces friction between the sled and the ground.

Q. How would you describe the difference in the motion of the sled from when the people first start pushing to when the sled is halfway down the hill?

Ans: The sled starts off slowly, and halfway down the hill it is moving much more quickly.

Reading Check:

Explain: How does instantaneous speed differ from average speed?

Ans: Instantaneous speed is speed at a particular moment. Average speed is an average over a certain amount of time, given by the total distance divided by total time.

Average Speed

Q. A racer at the Daytona 500 zips around the track. It travels the first 80 km in 0.4 hours. The next 114 kilometers take 0.6 hours. The following 80 kilometers take 0.4 hours. Calculate the racecar's average speed.

Solution:    Average speed = Total distance / Total time

                                            = (80km + 114km + 80km)/ (0.4h +0.6h +0.4h)

                                           = 274km/1.4h

                                           = 195.7 km/h

Reading Check

Q. How can understanding velocity help to prevent a mid-air collision?

Ans: Knowing the velocities of the planes can help determine whether they are heading toward one another and how quickly they are moving.

Calculate:

On landing, the plane touches the runway with a speed of 65 m/s. The figure shows the speed of the plane after 1 second. Calculate the acceleration of the plane during its landing.

Acceleration = (60 m/s - 65 m/s) / 1 s

                     = -5 m/s²

3. Translate Information:

Q. What does a negative value for acceleration mean here?

Ans: It indicate that the airplane is decelerating, or slowing down.

Reading Check:

Summarize:

Q. How are the speed, velocity, and acceleration of a  moving object related?

Ans: Acceleration is the rate which the object's velocity changes. A change in velocity can involve a change in the object's speed, direction, or both.

Lesson 2 Check:

1. Relate Change:

Q. What three changes in motion show that an object is accelerating?

Ans: It speeds up, slows down, or changes direction.

2. Calculate:

Q. What is the average speed of a train that covers 80 km in 1 h, 200 km in 2h, and 420km in 4h?

Ans: (80km + 200km + 420km)/ (1h +2h+4h)

 = 700 km/7h

= 100 km/h

3. Evaluate your Claim:

Q. A ball is pushed from a stop and rolls 6m in 2 s. Student A says the average speed of the ball is 3m/s. Student B says the average speed of the ball is 1.5 m/s². Which student is correct? Explain your answer.

Ans: Student A is correct. Speed is distance divided by time. Student B found the ball's acceleration, not its speed.

4. Interpret Data: 

Q. A student graphed distance versus time for an object that moves 14m every 2 s. What is the slope of the line on the graph? Explain.

Ans: The speed of the object is 7 m/s, so the slope of the line is 7 m/s. The slope of the line equals the speed on this type of graph.

5. Apply Scientific Reasoning:

Q. If the line on a distance-versus-time graph and the line on a speed-versus-time graph are both straight lines going through the origin, can the two graphs be displaying the motion of the same object? Explain.

Ans: No, A straight line through the origin on a distance-versus-time graph shows an object moving at constant speed and a straight line through the origin on a speed-versus-time graph shows an object changing speed or accelerating.