Forces. Net force

Force, Net Force

In physic, force is a push or a pull that act over an object. Force can be capable of changing the velocity of the object.

Forces are measured in “Newtons” or “N”, in honor of Sir Isaac Newton. According to Mr. Newton, an object will only accelerate if there is “unbalanced force” acting upon it. 

Force has size and direction. Size refers to the magnitude or value of the force and direction refers to the direction in which the force is applied.

Sometimes over the same object acts different forcers. The combination of all forces acting on an object is called net force.

When the forces acting over an object have the same direction, the value of the net force wills the result of addition of all the forces; and the direction will be the same of the forces acting over the object. Let see an example.

When the forces acting over an object have different directions, the value of the net force wills the result of the subtraction of all the forces acting over the object. The direction will be the same direction of the force with the major value. Let see an example.

We are in presence of Balanced Forces, when the net force acting over an object is zero. The velocity is constant.

We are in presence of Unbalanced Force, when the net force acting over an object is different to zero. Unbalanced forces acting on an object changes its speed and/or direction of motion.

Free-body diagrams for four situations are shown below. For each situation, determine the net force acting upon the object. 

Represents and calculate the net force in each one of the following situations.

1. A boy pulls a wagon with a force of 6 N east as another boy pushes it with a force of 4 N east.

2. Mr. Smith and his wife (Mrs. Smith) were trying to move their new chair. Mr. Smith pulls with a force of 30 N while Mrs. Smith pushes with a force of 25 N in the same direction.

3. The Colts team is playing tug of war. Mrs. Larson’s homeroom pulls with a force of 50 N. Ms. Mitko’s homeroom pulls with a force of 45 N in the opposite direction. What is the net force? And who won?

Linear graph. Distance versus time. Speed versus time

Distance versus time graphs. Speed versus time graphs.

Distance versus time graphs and speed versus time graphs are linear graphs. In linear graphs we can analyzed how varies one magnitude with respect another variable. Let see the steps to construct a good linear graph.   

1. Have data ready to plot as points on a graph. If you haven't already done so, create a table/chart in which one column is the time and one column is the distance. Fill in the time paired with the distance traveled (the data may be presented to you in paragraph form, in a list or in a table included in the question).

2. Draw two perpendicular lines that intersect on the graph paper, leaving a margin of three or four boxes between the axes and the edges of the paper. Draw arrows at the ends of the lines. Note that the x axis is the horizontal line and the y axis is the vertical line.

Axis [ak-sis], (plural ax·es [ak-seez]) -- one of two (or three) reference lines used in the coordinate system to locate a point (x,y) in a plane (or in space using a third axes, z, plotting (x,y,z) points).

3. Label the axes. x is normally independent but y depends on x. Time will be graphed on the x-axis, because time does not depend on distance (distance is dependent), because the "distance covered does depend on the time" -- how long you spend traveling at a certain rate -- which will be measured on the y-axis. Label the x-axis "Time" (t), and write the units (usually seconds, or s) in parentheses under "Time". Make the y-axis, using "Distance" (d) and the unit of distance often meters, m, or kilometers (km) or miles, etc.).

4. Begin plotting (graphing points) based on your (time, distance) data pairs (t,d) = (x,y); remember we are substituting t for x, and d for y. Continue using your data and graphing the points until you've finished with all your data; for example (2,5) means place a point/small dot using x = t = 2 and y = d = 5.

How? Look across the x-axis for +2, then go straight up +5 units, place your dot there. That is the (x,y) point for (t,d) = (2,5). Note: x is measured across and y is measured up and down.

5. After all the points have been plotted, take a straight edge (preferably a ruler) and connect the dots from the lowest X axis measurement to the highest measurement.

Remember a good graph contains the:

• o    Title,
• §  Include the data and object that is being studied -- e.g., "The Time/Distance Ratio of a Tennis Ball"
• o    Labeled axes,
• o    Numbered scale on each axis.

These steps will be the same any time that we want to draw a linear graph. The changes will be relating with the magnitudes we need to represent.

In linear graphs the axis are not only provided information also the slope (line that connects all the point in the graphic) is a good source of information. In distance versus time graph the slope indicates the speed of the object; and in a speed versus time graph the slope indicates the acceleration of the object.

For calculate slope we are going to use the following formula

slope=rise/run

rise is how much change the y axis from one point to the next point.

run is how much change the x axis from one point to the next point.

In a distance versus time graph slopes equal to zero means that the object does not move, because it is not changing its distance, the object is in rest.

In speed versus time graph slopes equal to means that the object moves at a constant speed, the speed does not change.

Acceleration

Acceleration

In everyday, the word acceleration is often used to describe a state of increasing speed. For many Americans, their only experience with acceleration comes from car ads. When a commercial shouts "zero to sixty in six point seven seconds" what they're saying here is that this particular car takes 6.7 s to reach a speed of 60 mph starting from a complete stop. This example illustrates acceleration as it is commonly understood, but acceleration in physics is much more than just increasing speed.

In Physics Acceleration is the rate of change of velocity with time. These means that any changes in the velocity of an object results in acceleration: increasing speed, decreasing speed (also called deceleration) or changing direction.  Thus, a falling apple accelerates, a car stopping at a traffic light accelerates, and an orbiting planet accelerates. Acceleration occurs anytime an object's speed increases, decreases, or changes direction.

Average acceleration is determined over a "long" time interval. The word long in this context means finite — something with a beginning and an end. The velocity at the beginning of this interval is called the initial velocity (vi) and the velocity at the end is called the final velocity (vf). Average acceleration is a quantity calculated from measurements.

a=(vf-vi)/t

where

a represents acceleration

vf represents the final velocity of the object

vi represents the initial velocity of the object

t represents the time that takes change the velocity.

Units

Calculating acceleration involves dividing velocity by time — or in terms of units, dividing meters per second [m/s] by second [s]. Dividing distance by time twice is the same as dividing distance by the square of time. Thus the SI unit of acceleration is the meter per second squared (m/s²) of kilometers per hour squared (km/h²)

Now you have some problems to solve, remember uses the algorism for solve problems. In some cases you will need to clearway the formula or use the triangle.

1.    A helicopter’s speed increases from 25 m/s to 60 m/s in 5 seconds. What is the acceleration of this helicopter?

2.    As she climbs a hill, a cyclist slows down from 25 mi/hr to 6 mi/hr in 10 seconds. What is her deceleration?

3.    After traveling for 6.0 seconds, a runner reaches a speed of 10 m/s. What is the runner’s acceleration?

4.    A car accelerates at a rate of 3.0 m/s2. If its original speed is 8.0 m/s, how many seconds will it take the car to reach a final speed of 25.0 m/s?

5.    A motorcycle traveling at 25 m/s accelerates at a rate of 7.0 m/s2 for 6.0 seconds. What is the final speed of the motorcycle?

6. A roller coaster car rapidly picks up speed as it rolls down a slope. As it starts down the slope, its speed is 4 m/s. But 3 seconds later, at the bottom of the slope, its speed is 22 m/s. What is its average acceleration?

7.  A cyclist accelerates from 0 m/s to 8 m/s in 3 seconds. What is his acceleration? Is this acceleration higher than that of a car which accelerates from 0 to 30 m/s in 8 seconds?

8. A car advertisement states that a certain car can accelerate from rest to 70 km/h in 7 seconds. Find the car’s average acceleration.

9. A lizard accelerates from 2 m/s to 10 m/s in 4 seconds. What is the lizard’s average acceleration?

10. A runner covers the last straight stretch of a race in 4 s. During that time, he speeds up from 5 m/s to 9 m/s. What is the runner’s acceleration in this part of the race?

11. You are traveling in a car that is moving at a velocity of 20 m/s. Suddenly, a car 10 meters in front of you slams on it’s brakes. At that moment, you also slam on your brakes and slow to 5 m/s. Calculate the acceleration if it took 2 seconds to slow your car down.

12.  A ball is dropped from the top of a building. After 2 seconds, its velocity is measured to be 19.6 m/s. Calculate the acceleration for the dropped ball.

13. A car starting from rest accelerates at a rate of 8.0 m/s/s. What is its final speed at the end of 4.0 seconds?

14. A cyclist accelerates at a rate of 7.0 m/s2. How long will it take the cyclist to reach a speed of 18 m/s?

15. A skateboarder traveling at 7.0 meters per second rolls to a stop at the top of a ramp in 3.0 seconds. What is the skateboarder’s acceleration?

16. If a Ferrari, with an initial velocity of 10 m/s, accelerates at a rate of 50 m/s/s for 3 seconds, what will its final velocity be?

17. Falling objects drop with an average acceleration of 9.8 m/s2. If an object falls from a tall building, how long will it take before it reaches a speed of 49 m/s?

18. Josh rolled a bowling ball down a lane in 2.5 s. The ball traveled at a constant acceleration of 1.8 m/s2 down the lane and was traveling at a speed of 7.6 m/s by the time it reached the pins at the end of the lane. How fast was the ball going when it left Tim’s hand?