# what is force

## forces acting in opposite directions

When two forces act on an object in opposite directions, like the book on the table, the net force is equal to the difference between the two forces. In other words, one force is subtracted from the other to calculate the net force. If the opposing forces are equal in strength, the net force is zero. Thats what happens with the book on the table. The upward force minus the downward force equals zero (20 N up - 20 N down = 0 N). Because the forces on the book are balanced, the book remains on the table and doesnt move. In addition to these downward and upward forces, which generally cancel each other out, forces may push or pull an object in other directions. Look at the dogs playing tug-of-war in Figure 13.4. One dog is pulling on the rope with a force of 10 newtons to the left. The other dog is pulling on the rope with a force of 12 newtons to the right. These opposing forces are not equal in strength, so they are unbalanced. When opposing forces are unbalanced, the net force is greater than zero. The net force on the rope is 2 newtons to the right, so the rope will move to the right.

## forces acting in the same direction

Two forces may act on an object in the same direction. You can see an example of this in Figure 13.5. After the man on the left lifts up the couch, he will push the couch to the right with a force of 25 newtons. At the same time, the man to the right is pulling the couch to the right with a force of 20 newtons. When two forces act in the same direction, the net force is equal to the sum of the forces. This always results in a stronger force than either of the individual forces alone. In this case, the net force on the couch is 45 newtons to the right, so the couch will move to the right. You Try It! Problem: The boys in the drawing above are about to kick the soccer ball in opposite directions. What will be the net force on the ball? In which direction will the ball move?

## defining force

Force is defined as a push or a pull acting on an object. Examples of forces include friction and gravity. Both are covered in detail later in this chapter. Another example of force is applied force. It occurs when a person or thing applies force to an object, like the girl pushing the swing in Figure 13.1. The force of the push causes the swing to move.

## force as a vector

Force is a vector because it has both size and direction. For example, the girl in Figure 13.1 is pushing the swing away from herself. Thats the direction of the force. She can give the swing a strong push or a weak push. Thats the size, or strength, of the force. Like other vectors, forces can be represented with arrows. Figure 13.2 shows some examples. The length of each arrow represents the strength of the force, and the way the arrow points represents the direction of the force. How could you use an arrow to represent the girls push on the swing in Figure 13.1?

## si unit of force

The SI unit of force is the newton (N). One newton is the amount of force that causes a mass of 1 kilogram to accelerate at 1 m/s2 . Thus, the newton can also be expressed as kgm/s2 . The newton was named for the scientist Sir Isaac Newton, who is famous for his law of gravity. Youll learn more about Sir Isaac Newton later in the chapter.

## combining forces

More than one force may act on an object at the same time. In fact, just about all objects on Earth have at least two forces acting on them at all times. One force is gravity, which pulls objects down toward the center of Earth. The other force is an upward force that may be provided by the ground or other surface. Consider the example in Figure 13.3. A book is resting on a table. Gravity pulls the book downward with a force of 20 newtons. At the same time, the table pushes the book upward with a force of 20 newtons. The combined forces acting on the book or any other object are called the net force. This is the overall force acting on an object that takes into account all of the individual forces acting on the object. You can learn more about the concept of net force at this URL: .

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

Force can cause a

``````a. stationary object to start moving.

b. moving object to change speed.

c. moving object to change direction.

-->  d. all of the above
``````

Examples of forces include

``````a. motion.

-->  b. friction.

c. acceleration.

d. two of the above
``````

If gravity pulls you down toward the center of Earth with a force of 500 N, how much upward force does the ground exert on you?

``````a. 0N

b. 50 N

-->  c. 500 N

d. none of the above
``````

In the following sketch, what is the net force acting on the box?

``````a. 5 N to the right

b. 5 N to the left

-->  c. 15 N to the right

d. 15 N to the left
``````

Which diagram represents balanced forces?

``````-->  a. a

b. b

c. c

d. d
``````

Which pair of forces in question 5 differ from each other in both strength and direction?

``````a. a

-->  b. b

c. c

d. d
``````

Mass is a measure of the force of gravity on an object.

``````a. true

-->  b. false
``````

Most objects have at least two forces acting on them at all times.

``````-->  a. true

b. false
``````

If opposing forces are unequal in strength, the net force is less than zero.

``````a. true

-->  b. false
``````

The SI unit for weight is the newton.

``````-->  a. true

b. false
``````

When two forces act on an object in the same direction, the net force equals zero.

``````a. true

-->  b. false
``````

When forces act in opposite directions on an object, they are subtracted to yield the net force.

``````-->  a. true

b. false
``````

Every sport involves forces.

``````-->  a. true

b. false
``````

Forces are always balanced when they act on an object in the same direction.

``````a. true

-->  b. false
``````

Whenever an object is stationary, it has no forces acting on it.

``````a. true

-->  b. false
``````

Two forces acting in the same direction always result in a stronger force.

``````-->  a. true

b. false
``````

combination of all the forces acting on an object

``````a. force

b. unbalanced forces

-->  c. net force

d. applied force

e. newton

f. gravity

g. balanced forces
``````

force that a person or thing exerts on to an object

``````a. force

b. unbalanced forces

c. net force

-->  d. applied force

e. newton

f. gravity

g. balanced forces
``````

push or pull acting on an object

``````-->  a. force

b. unbalanced forces

c. net force

d. applied force

e. newton

f. gravity

g. balanced forces
``````

forces that produce a net force of zero

``````a. force

b. unbalanced forces

c. net force

d. applied force

e. newton

f. gravity

-->  g. balanced forces
``````

example of a force

``````a. force

b. unbalanced forces

c. net force

d. applied force

e. newton

-->  f. gravity

g. balanced forces
``````

SI unit for force

``````a. force

b. unbalanced forces

c. net force

d. applied force

-->  e. newton

f. gravity

g. balanced forces
``````

forces that produce a net force greater than zero

``````a. force

-->  b. unbalanced forces

c. net force

d. applied force

e. newton

f. gravity

g. balanced forces
``````

## diagram questions

No diagram questions associated with this lesson