What Is a Force?

from Let's Do Science 3 – Text Book B

Balanced and Unbalanced Forces

What Is a Force? The tennis player uses push forces to throw and hit the ball. tion ceIs a Forc a Forc s yer uses d throw and

Pushes and Pulls es and Pull and Pull

Whenever an object is in motion, it is never an object an objec because a force acted on the object, cause a force ace a force a causing it to move. A forceausing it to movng it to mo is a push or a pull. a pull. A push is when you press something A push is w away from you. You use a push when away from you jump in the air. You use a push you jum when you throw or hit a ball. The push when force starts the ball moving. forc

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A pull is when you tug pull is when you tug something closer to you. omething closer to you. You use a pull when you ou use a pull when you open a drawer or drag pen a drawer or drag a suitcase. suitcase.

The pull causes the he pull causes the objects to move. bjects to move.

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There can be more than There can be mo one force acting on an one force acting object. When you use object. When you a bow and arrow, you a bow and arrow push and pull at the push and pull at same time. same time.

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How are forces used to move the golf ball differently? What effects do forces have on the motion of the golf ball?

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ndo forc olf ball? Forces can have different strengths. an h ve dif A large, strong force will move an ge, ng fo object fur object further and faster than a ther a smaller, softer force. maller, s fte

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Larger, stronger forces are required to move Compare the logs in the truck to the tree in the cart. Which would require a greater force to move? Why? to move c Did You Know? heavier objects. We can measure the strength of a force using ng The force required to move the truck is much e truck is much a spring balance called d greater than the force required to pull the cart. red to pull the ca a Newton meter.

AB Activity 11.6

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Think Deeply

Forces can affect the motion ffect the motion the moton of objects in different ways. n different ways n different way When a player kicks the ball a player kicks th ayer kicks th off the ground, it starts the e ground, it star round, it sta ball moving. moving. ving.

How does the strength of a kick affect the distance the ball travels? Plan and conduct an investigation with your friends to find out. Communicate your findings to other groups. ga ut. ngs

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How do forces change the motion of a soccer ball during a match?

Players kick the ball as they run, moving the g t ball in a zigzag motion.

Passing the ball from player to player yer causes the ball to speed up, slow down and w down and own and change direction.

To prevent a goal, the a goal, the oal, the goalkeeper uses a push per uses a push es a push force which stops the which stops the h stops the motion of the ball. Activity 11.7AB on of the ball. the ball.R t

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Contact Forces

There are many different types of forces f forces ces acting around us. Some forces occur occur ur when objects are touching. They are called hey are calledare called contact forces. Pushes and pulls from people, animals and eople, animals ae e, animals machines are example of contact forces. e of contact forcof contact fo In a baseball game, the pitcher is in contact e, the pitcher is ie pitcher is i with the ball when he pushes it forward. en he pushes it fopushes it f To change the direction of the ball, there is direction of theection of the contact between the bat and the ball.een the bat andthe bat an

A tow truck needs to be in contact with the car to affect its motion.

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Judo is a contact sport. Players make contact as they use pushes and pulls to affect each other’s motion.

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Discuss the contact forces you observe in daily life. How do the contact forces affect motion?R ob

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Take a book and slide it along the classroom clas floor. What do you observe? When you slide a book, you use a push se a push force to set it in motion. The book slows book slows down and comes to a stop due to a contact p due to a cont force called friction.. Friction is a force that at opposes motion. opposes mot It occurs when the surfaces of objects rub surfaces of obj together. Rough surfaces produce more friction than ces produce mo smooth surfaces. Friction is also greater rfaces. Friction iic when surfaces press harder together. rfaces press harh Sometimes it is useful to reduce friction. A slide has a smooth surface to reduce friction. A snowboard has a smooth surface to help it move quickly on snow.

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As a force that opposes motion, we can increase friction to help things to slow down or stop. The soles of shoes are often rough. This helps to increase the friction between the shoe and the What are some examples of when it is useful to increase friction? Try This! Have you ever seen someone rub their hands together when it is cold? Try it now. What do you observe? What can you infer about friction?cat ease rease is helps to lps to oe and the he catioon This! n you ever s someone rub th ogether when Try it now. W observe? W infer aboca ground and stops you from slipping. g. A bicycle has rubber brake pads to increase friction when it presses against the wheel rim. This allows the cyclist to slow down or stop moving. ds rim. w

AB brake pads Activities 11.8 – 11.9 g brake pa brake al 1.8 – 1 R

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Some forces can push and pull objects without touching. These forces can act at a distance and are called non-contact forces. Non-contact Forces So ta Nonuca es ca es uch ce ontact ontact Some force me force without tou without at a distanc non-conta non-con on-co -co cat Place a magnet on a piece of paper and sprinkle iron filings over the magnet. What do you observe? The pattern of the iron filings is the result of magnetic force. A magnetic force can act at a distance. A magnet is an object with a magnetic force. A magnet can pull or attract objects made of certain metals such as iron and nickel. A magnet cannot attract objects made of plastic, wood, rubber or glass.

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e a maPlace a d sprinkland and magnet. W ma The pattern result of m force can a

j rubber or glass

A magnet has two poles – a North pole and a South pole. When the like poles of two magnets are brought together, they push or repel each other. When unlike poles are brought together, they pull or attract each other. Many objects make use of magnets. Some doors used magnets to keep them closed. A maglev (magnetic levitation) train has magnets that repel it from the track. This reduces friction and allows the train to travel at fast speeds. Magnets are also used for medical imaging and storing data on computer disks. Unlike poles attract each other. Like poles repel each other. Activity 11.10AB Go Online! A maglev train uses magnets to hover above the tracks. This allows it to reach speeds of more than 400 kilometers per hour! Watch a maglev train in action on the NGScience website. QuickCode: P8T1 Engineer It! Design, model and build a useful tool that uses magnetic force. Describe how the tool works and how it uses magnetic force. and ul tool magnetic cribe how orks and s force. gal tation tati track. This ack. Th n to travel at fast travel at fa so used for medical ed for med on computer disks. omputer dis eg vity 11.10 education magnets. Som gnets. So them close hem close n has nhas act catioon Engineer I Design, mo build a use that uses m orce. Desc the tool how it mag odel a eful eful m

magnetic levitation train

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As the boy moves down the slide his hair stands on end! What force is causing this to happen?

nWhat happen? Electrical forces are created when re created whene created when there are unbalanced electrical nced electricalced electrical charges inside an object. an object. hargeses repee l eac ges attract eachtract eac astic slide causeslid us e an imbalance alance his body. The che repelledelle each h otot forcer wasw stro haira stand on

Opposite charges repel each other. Like charges attract each other.

The plastic slide caused the boy to have an imbalance of like charges in his body. The charges in his hair repelled each other. The electrical force was strong enough to make his hair stand on end.

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You can create and observe the electrical force yourself. Rub a balloon through your hair. Slowly pull the balloon away from your head. What do you observe? Rubbing the balloon through your hair created two opposite static charges. The charges attract one another and make your hair stand up. Notice that your hair and the balloon do not need to be touching to affect each other. Electrical forces are non-contact forces and can act at a distance. Activity 11.11AB Regal through yo ough y charges. The ch arges. The make your hair sta ke your hair our hair and the ball hair and the touching to affect e hing to affe forces are non-cont are non-co distance. ance. Re ActiviR education 119

Think Deeply

Gravity on the Earth’s moon is only one-sixth of the Earth’s gravity. With a classmate, talk about what it would be like to walk, jump and play different sports on the moon.

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When skydivers jump out of an airplane, they are pulled towards the surface of the Earth. What force is pulling them down?

All objects have an invisible force that pulls on at pulls o other objects. The force is called gravitational gra itati n force, or gravity.

The strength of an object’s gravitational The strehe force depends on it mass. force d Mass is the amount of matter something is made of. amoa T The gravitational force of objects with a small mass, such as a basketball, is too weak to observe. Compared to a basketball, the Earth has a much greater mass. The strong gravitational force of the Earth affects everything on its surface.

It is the force of gravity that pulls you down when you jump in the air. It is the force that keeps everything on the Earth’s surface. Gravitational force also depends on distance. The closer objects are to each other, the greater the gravitational force. Amazing Fact! The object with the most mass in our solar system is the Sun. The gravitational force of the Sun keeps all of the planets and other bodies in the solar system in orbit around it. cation n with the m ur solar syste e grav of the Sun ke e planets a bodies in the so orbit around itcati