Forces as agents of change

What unit is used to measure force in the International System of Units (SI)? Which famous scientist is this unit named after?

Mateo sees a stationary ball on the playground and kicks it: the ball rolls away, as shown in the figure.

What has changed in the ball as a result of the force of the kick: its shape or its state of motion? Explain in your own words.

Isabella squeezes a ball of soft clay with her hands and squashes it. What effect does the force applied by her hands produce? Has the speed of the clay changed or its shape?

Complete the sentence with the missing words: "Forces can change the $\dots$ of an object (deforming it) or change its $\dots$: starting its motion, accelerating it, slowing it down, or deflecting it."

In a tennis match, the ball comes toward Emma and she hits it back with her racket toward the opposite side of the court, as shown in the figure.

What two effects does the force of the racket produce on the ball? Look at its speed and its direction.

Daniela stretches the rubber band of a slingshot and, on releasing it, the stone is shot out.

1. a) What effect does Daniela's force produce on the rubber band when she stretches it?
2. b) What effect does the rubber band produce on the stone when released?

Emma pushes a refrigerator with all her strength to move it to a different spot and fails to move it even a single centimeter, as shown in the figure.

1. a) Is Emma applying a force on the refrigerator?
2. b) If it does not move, what other force must be acting between the refrigerator and the floor? Explain.

State the main effect produced by the force in each case: deforming the object, changing its speed, or changing its direction.

1. a) You squeeze a clothespin and it opens.
2. b) A gust of wind deflects a falling leaf.
3. c) A runner sprints in the final meters and goes faster and faster.

We squeeze a spring and a ball of modeling clay with the same force, and then stop squeezing.

1. a) What happens to each of them after we stop squeezing?
2. b) What is the difference between the two deformations?
3. c) Which of the two materials is elastic?

A classmate claims: "If an object does not change its shape or its motion, it means that no force is acting on it." Think of a picture hanging on the wall, which is stationary.

1. a) Is your classmate right?
2. b) State the forces acting on the picture.
3. c) Write down the corrected sentence.

Mateo pushes the classroom door with his hand to close it. Does Mateo need to touch the door to push it? What are forces like this, which require contact between objects, called?

You drop an eraser and it reaches the ground without anyone pushing it down. What force pulls it to the ground? Does this force act by touching the eraser or without touching it?

Ethan holds the string of his kite on a windy day so that it doesn't fly away, as shown in the figure.

Name a force acting on the kite, state who or what exerts it, and what effect it produces on the kite.

Classify these forces as contact forces or non-contact forces, and briefly justify each answer:

1. a) The wind pushes the sail of a boat.
2. b) A magnet attracts a paperclip.
3. c) Gravity keeps us attached to the ground.
4. d) You push a grocery cart.

A baker kneads a ball of bread dough, squeezing it over and over with her hands.

1. a) What effect does the force produce on the dough?
2. b) Why do we say it is a contact force?

A sailboat crosses a lake, as shown in the figure. The forces acting on it are the wind, which pushes the sail; the water, which slows down the hull; and gravity, which pulls the boat downward.

State for each of the three forces whether it is a contact or non-contact force and what effect it produces on the boat.

Mateo is riding a bicycle and pulls the brake: the brake pads squeeze the rim and the bike eventually stops.

1. a) What force slows down the wheel?
2. b) What effect does this force have on the speed of the bike?

Emma lets go of an inflated, untied balloon and it flies around the room, pushed by the escaping air.

1. a) What object pushes the balloon?
2. b) Is it a contact force or a non-contact force?
3. c) What effect does it produce on the balloon?

A gymnast falls onto a trampoline: as she lands she depresses it and, a moment later, she is launched upward.

1. a) What effect does the gymnast produce on the trampoline?
2. b) What effects does the trampoline then produce on the gymnast?
3. c) Are these contact forces or non-contact forces? Justify your answer.

Gravity cannot be seen, but we all know it exists. Explain how it is possible to detect an invisible force by looking at its effects, and give two everyday examples where we "discover" a force by what it does.

In a volleyball match, Emma jumps and hits the ball, which goes up, follows a curved path, and lands on the opponent's side of the court, as shown in the figure.

1. a) What forces act on the ball while Emma is touching it?
2. b) And while it flies through the air, when no one is touching it?
3. c) Classify all these forces as contact forces or non-contact forces.

Air cannot be seen, but it pushes. Design a simple experiment using household objects (for example, a balloon or a plastic bag) to demonstrate to your younger brother that air can exert a force. Explain what you would set up, what effect would be observed, and what force produces it.

You push a sled with the same force, first on ice and then on grass, as shown in the figure.

1. a) On which surface does the sled gain more speed?
2. b) What force opposes the motion in each case?
3. c) Explain why the effect of the same push is so different on the two surfaces.