Exercises: Forces as agents of change

Marcos sees a stationary ball in 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.

Aitana 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?

In a tennis match, the ball comes towards Lucía and she hits it back with her racket towards 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.

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

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

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

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."

Iker 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 supermarket trolley.

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

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

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 sailing boat 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 downwards.

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

Hugo 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?

Carla pushes a fridge with all her strength to move it to a different spot and fails to move it even a single centimetre, as shown in the figure.

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

Nerea lets go of an inflated, un-tied 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?

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 peg and it opens.
2. b) A gust of wind deflects a falling leaf.
3. c) A runner sprints in the final metres and goes faster and faster.

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

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.

Leo hits a table tennis ball and a football, both stationary on the ground, with the same force.

1. a) Which of the two will move off faster?
2. b) Which property of objects explains the difference?
3. c) What would you have to do for the football to move off as fast as the table tennis ball?

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

1. a) On which surface does the sledge 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.

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, Vega 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 Vega 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.

We squeeze a spring and a ball of Plasticine 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.

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.