Forces and Motion

A force is a push or pull that makes it possible to move an object. A force can also be used to change the direction that an object is moving. The amount of force put on an object will determine how far and fast the object will move. Nothing moves without some kind of force!

If you pull or push something, you are the force that is creating movement. The more you push or pull, the farther and faster an object will go. But the heavier an object is, or the more friction working against its movement, the harder it will be to move it.

What causes objects to move?

• weight

• speed

• force

• friction

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The weight of the object will affect how far and fast it will move. Think about moving a box across a wood floor. If the box were empty and you gave it a push, it would easily move across the floor. If the same box were full of books you would need more force (or physical energy) to move it across the floor.

If you add weight to an object, it will ________ to move it.

• need only friction

• not need any force

• be impossible

• need more force

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Friction is when two objects rub against each other. Rub your hands together, and you'll feel friction. A lot of friction makes it harder for movement to happen. Whenever two surfaces are rubbed together, there is friction. This zoomed-in image shows why rough objects have more friction than smooth objects.

Friction is also the force made by brakes on a bicycle or a car in order to stop it. The wheels are spinning, but when the brakes touch the wheel, the force of friction slows them down. If the brakes push hard enough, the wheels stop completely.

A ball travels farther on ice than on grass

What causes friction?

• something traveling through the air

• surfaces rubbing against each other

• a surface being cut in half

• an object standing still

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The reason skis and surfboards glide smoothly and quickly over snow or water is because there is little friction, making the movement very smooth and fast. And the reason a soccer player wears cleats is because cleats are the opposite of smooth, so they keep the soccer player from slipping on grass. But imagine skis with cleats. That wouldn't work. Sometimes friction works for you, and sometimes it works against you.

In which of these situations is there the most friction?

• a mover drags a couch over a cracked sidewalk

• an ice skater glides across the ice on skates

• a bowler rolls a round ball down a smooth, wooden lane

• a rower pulls a boat over the water to the shore

When you kick a ball or throw a rock, it doesn’t fly through the air forever. Eventually, it returns to the ground. This is also because of force: the force of gravity. Gravity is the force that keeps everything on the surface of the Earth. Gravity keeps us from flying off into space. It pulls us toward the Earth.

Which of these is an effect of gravity?

• a cup placed on a table won't float away

• you can throw a ball or a rock up

• the brakes on a bike can make it stop

• liquid water can become a gas

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