LR247.1.2: Forces

Karl is the keeper on his soccer team. He often has to kick and block the ball to keep it out of his goal. Sometimes, he just has to tap it off to the side. Other times, he has to kick straight ahead with a lot of force. In science, a force is a push or a pull. Every force has both a strength and a direction. The size of a force is measured in newtons (N), and arrows are often used to show the direction of a force.

Forces can act on objects directly or indirectly. When a force is exerted on an object through contact or touch, this is a direct force called a contact force. For example, when a person pulls open a door or pushes down on a keyboard, he is applying contact force.

When a force acts on an object indirectly without touching the object, this is called a non-contact force. Gravity and magnetism are non-contact forces because they can act on objects without touching them.

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Every force has both __________and __________.

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For each image, decide if the arrow describes a contact force or a non-contact force?

When this man sits in this chair, neither the man nor the chair moves. Why is that the case?

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If each of the eight rowers pulls in the same direction with a force of 50 N, the net force they apply to the boat is __________

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These two dogs are pulling on a toy in opposite directions. The yellow lab pulls with a force of 70 N to the left. The black lab pulls with a force of 100 N to the right. Describe the net force on the toy. Enter any numbers as numerals instead of words. The toy has a net force of__________N to the __________.

Check for Understanding:

1. Every force has both a strength and a direction.

2.  Net force is the combination of all forces on an object.
•  A nonzero net force is created when two opposite forces act on an object, and the object accelerates.
•  A zero net force is created when two opposite forces have the same strength.

3.  If the net force on an object is balanced, the object will not accelerate.

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Which of these surfaces would be easiest to push a sled over?

Remember, forces can either be contact forces or non-contact forces. One type of contact force is called friction. Friction is the force that two surfaces exert on each other when they rub against each other. Friction is affected by the type of surfaces involved and how hard the surfaces push together. The previous problem showed that ice is an easier surface to push a sled over than grass or asphalt. This is because ice is smoother than grass or asphalt. In general, smoother surfaces produce less friction than rougher surfaces.

However, if someone sat on the sled, the sled would be harder to push on all three surfaces. That is because the extra weight from the person sitting on the sled would cause the sled to push downward with more force. That extra downward force would increase the friction between the sled and the surface beneath it. In that case, it would still be easiest to push the sled over ice, but it would be more difficult than pushing an empty sled over ice.

Friction is a primary reason that moving objects on Earth will eventually stop unless more force is added. For example, when a soccer player kicks a ball down the field, the force of friction between the surface of the ball and the grass slows the ball down and brings it to a stop. However, when a satellite is in orbit around Earth, there is no surface for the satellite to rub against, and therefore there is no friction acting on the satellite. The satellite moves around the Earth at a constant speed because there is no friction to slow it down.

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Friction is a type of __________.

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The amount of friction between two surfaces depends on __________ and __________.

Friction is different depending on the surfaces involved.

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When Michael drives his car over asphalt,__________friction is occurring between his tires and the road.
When Michael drives his car through a puddle,__________friction is occurring between his tires and the road.
When Michael applies the brakes on his car,__________friction is occurring between his tires and road.
When Michael’s car is stopped at a red light,__________friction is occurring between his car and the air.
When Michael drives down the road,__________friction is occurring between his car and the air.

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Pedro is rearranging the furniture in his living room. He is trying to slide his couch across the floor, but it will not move. Which type of friction is preventing Pedro from sliding the couch?

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Select the floor that will be the most difficult for Pedro to move his couch over.

Check for Understand:

Friction is a contact force that occurs when surfaces rub against each other.

 Friction depends on the types of surfaces involved and how hard the surfaces push together.

 Static friction occurs between objects at rest. Sliding friction occurs when surfaces slide across each other.

 Rolling friction occurs when an object rolls over a surface. Fluid friction occurs when a solid object moves through a fluid.

 Rolling friction and fluid friction are easier to overcome than sliding friction.

Stonehenge is a prehistoric structure in the United Kingdom that was completed about 3,500 years ago. The massive stones that were used to build Stonehenge are far too heavy for people to have carried, and many historians believe that the stones were rolled to the site on logs. 

The universe would look incredibly different if gravity did not exist, and yet people often do not think about gravity. Gravity is a force that pulls objects toward each other. It allowed the planets to form. It keeps the atmosphere around Earth, and it is the reason a person falls back to the ground when she jumps instead of flying off into space.

The force of gravity acts between all objects in the universe that have mass, from the smallest grain of sand to the largest stars. This is called the law of universal gravitation. According to the law of universal gravitation, a person is attracted to the Earth, the Moon, other stars, and all objects around him. All of those objects are also attracted to the person and to each other. However, people normally do not notice the pull of gravity between small objects because those forces are extremely small compared to the force of Earth’s attraction.

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Which of these objects attract others through the force of gravity?

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In order for gravity to act on an object, the object must __________.

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The__________states that gravity acts on all objects in the universe that have__________.

Gravitational force is not the same between all objects. The strength of gravitational attraction depends on two things: the mass of the objects and their distance between each other. If an object has more mass, then the strength of gravitational force between it and other objects will be greater. Remember, mass is a measurement of how much matter is in an object. Mass is different from weight. Weight is actually the force exerted by an object when gravity pulls on it. Weight increases with an object’s mass. This is because objects with greater mass exert more force when attracted to Earth by gravity.

As the distance between two objects decreases, the gravitational force between them increases. As the distance between the two objects increases, the gravitational force between them decreases. People on Earth feel the gravitational pull of Earth because they are so close to it. However, if a person were to travel far out into space, she would no longer notice Earth’s gravitational pull.

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This picture shows an imaginary solar system in which all the planets are the same size. Click the planet that would experience the least gravitational attraction from the Sun at the center of this solar system.

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__________is a measurement of how much matter an object has.__________
is the force exerted by an object when gravity pulls on it.

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Which object experiences a greater gravitational attraction to the Earth, a car or a bicycle?

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A scientist is building a rocket to send to Mars. In order to reach Mars, the rocket will need to overcome Earth’s gravitational pull. What can she do to the rocket to lessen Earth’s gravitational pull on the rocket?

Checking for Understand:

 The force of gravity works between all objects in the universe that have mass. It is an attractive force that pulls objects together.

 Objects with more mass have greater gravitational force between themselves and other objects.

 Objects that are closer to each other experience more gravitational attraction than objects that are farther apart.

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What is the definition of force?

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Net force is __________.

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Friction is __________.

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What is gravity?

 Every force has a strength and a direction. Contact forces act directly on objects, while non-contact forces act indirectly.

 Net force is the combination of all forces acting on an object. If the forces on an object are balanced, there is zero net force on the object, and it will not accelerate.

 Friction is a contact force that two surfaces exert on each other when they rub against each other. There are four types of friction: sliding friction, static friction, fluid friction, and rolling friction.

 Gravity is a force that pulls objects toward each other. The law of universal gravitation states that gravity affects all objects in the universe that have mass.

 The strength of gravitational attraction between two objects depends on the mass of each object and how far apart the objects are.

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Neha is walking her dog on a leash. Her dog sees a squirrel and pulls forward on the leash with a force of 150 N. Neha pulls backward on the leash with a force of 130 N. The net force acting on the leash is__________N.

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In each scenario, select whether the force of gravity between objects A and B increases and decreases.

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Select the picture or pictures that show fluid friction occurring.

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What strength would the pushing force of the player on the right have to be in order for the two players to have zero net force?

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Tyler is trying to roll a ball as far as he can. What could he do to decrease the friction on the ball?
Check all that are true (choose 2).

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As Sophie pedals her bike down the street, which type of friction is generated between her body and the air?

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Which of the following objects exert gravitational attraction on other objects?

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Gravitational attraction keeps the Earth orbiting around the Sun. Which of the following changes would increase the gravitational attraction between the Earth and the Sun?
Check all that are true (choose 2).

LR247.1.2: Forces

When this man sits in this chair, neither the man nor the chair moves. Why is that the case?

Which of these surfaces would be easiest to push a sled over?

Pedro is rearranging the furniture in his living room. He is trying to slide his couch across the floor, but it will not move. Which type of friction is preventing Pedro from sliding the couch?

Select the floor that will be the most difficult for Pedro to move his couch over.

Which of these objects attract others through the force of gravity?

In order for gravity to act on an object, the object must __________.

This picture shows an imaginary solar system in which all the planets are the same size. Click the planet that would experience the least gravitational attraction from the Sun at the center of this solar system.

Which object experiences a greater gravitational attraction to the Earth, a car or a bicycle?

A scientist is building a rocket to send to Mars. In order to reach Mars, the rocket will need to overcome Earth’s gravitational pull. What can she do to the rocket to lessen Earth’s gravitational pull on the rocket?

What is the definition of force?

Net force is __________.

Friction is __________.

What is gravity?

In each scenario, select whether the force of gravity between objects A and B increases and decreases.

Select the picture or pictures that show fluid friction occurring.

What strength would the pushing force of the player on the right have to be in order for the two players to have zero net force?

Tyler is trying to roll a ball as far as he can. What could he do to decrease the friction on the ball?Check all that are true (choose 2).

As Sophie pedals her bike down the street, which type of friction is generated between her body and the air?

Which of the following objects exert gravitational attraction on other objects?

Gravitational attraction keeps the Earth orbiting around the Sun. Which of the following changes would increase the gravitational attraction between the Earth and the Sun?Check all that are true (choose 2).

When this man sits in this chair, neither the man nor the chair moves. Why is that the case?

Which of these surfaces would be easiest to push a sled over?

Pedro is rearranging the furniture in his living room. He is trying to slide his couch across the floor, but it will not move. Which type of friction is preventing Pedro from sliding the couch?

Select the floor that will be the most difficult for Pedro to move his couch over.

Which of these objects attract others through the force of gravity?

In order for gravity to act on an object, the object must __________.

This picture shows an imaginary solar system in which all the planets are the same size. Click the planet that would experience the least gravitational attraction from the Sun at the center of this solar system.

Which object experiences a greater gravitational attraction to the Earth, a car or a bicycle?

A scientist is building a rocket to send to Mars. In order to reach Mars, the rocket will need to overcome Earth’s gravitational pull. What can she do to the rocket to lessen Earth’s gravitational pull on the rocket?

What is the definition of force?

Net force is __________.

Friction is __________.

What is gravity?

In each scenario, select whether the force of gravity between objects A and B increases and decreases.

Select the picture or pictures that show fluid friction occurring.

What strength would the pushing force of the player on the right have to be in order for the two players to have zero net force?

Tyler is trying to roll a ball as far as he can. What could he do to decrease the friction on the ball?Check all that are true (choose 2).

As Sophie pedals her bike down the street, which type of friction is generated between her body and the air?

Which of the following objects exert gravitational attraction on other objects?

Gravitational attraction keeps the Earth orbiting around the Sun. Which of the following changes would increase the gravitational attraction between the Earth and the Sun?Check all that are true (choose 2).

Tyler is trying to roll a ball as far as he can. What could he do to decrease the friction on the ball?
Check all that are true (choose 2).

Gravitational attraction keeps the Earth orbiting around the Sun. Which of the following changes would increase the gravitational attraction between the Earth and the Sun?
Check all that are true (choose 2).

Tyler is trying to roll a ball as far as he can. What could he do to decrease the friction on the ball?
Check all that are true (choose 2).

Gravitational attraction keeps the Earth orbiting around the Sun. Which of the following changes would increase the gravitational attraction between the Earth and the Sun?
Check all that are true (choose 2).