Forces as agents of change
You kick a ball that is lying still and it rolls away. You squeeze a ball of clay and it ends up flattened. In both cases the same thing has happened underneath: something has pushed or pulled an object. We call this a force.
What a force is
You push a trolley to make it move, stretch an elastic band or squeeze the brakes on your bicycle: in all these actions your muscles act on an external object. What do all these everyday situations have in common, and how does physics define them?
What unit is used to measure force in the International System of Units? Which famous scientist is this unit named after?
What unit is used to measure force in the International System of Units? Which famous scientist is this unit named after?
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…
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.
The two effects of a force
You step on an empty drinks can and it gets crushed; you hit a ball that is lying still and it shoots through the air. What two different changes can a force cause when it acts on any object?
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…
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?
Daniela stretches the elastic band of a catapult and, on releasing it, the stone is shot out. a) What effect does Daniela's force produce on the elastic band when she…
Daniela stretches the elastic band of a catapult and, on releasing it, the stone is shot out.
- a) What effect does Daniela's force produce on the elastic band when she stretches it?
- b) What effect does the elastic band produce on the stone when released?
We squeeze a spring and a ball of Plasticine with the same force, and then stop squeezing. a) What happens to each of them after we stop squeezing? b) What is the…
We squeeze a spring and a ball of Plasticine with the same force, and then stop squeezing.
- a) What happens to each of them after we stop squeezing?
- b) What is the difference between the two deformations?
- c) Which of the two materials is elastic?
Contact forces and forces at a distance
To push a table you have to press against it physically, but if you drop your keys they fall to the floor by themselves, and a magnet attracts paper clips before touching them. How is it possible for some forces to require touching the object while others act across empty space?
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…
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?
Classify these forces as contact forces or non-contact forces, and briefly justify each answer: a) The wind pushes the sail of a boat. b) A magnet attracts a paperclip.…
Classify these forces as contact forces or non-contact forces, and briefly justify each answer:
- a) The wind pushes the sail of a boat.
- b) A magnet attracts a paperclip.
- c) Gravity keeps us attached to the ground.
- d) You push a supermarket trolley.
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. a) What…
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.
- a) What forces act on the ball while Vega is touching it?
- b) And while it flies through the air, when no one is touching it?
- c) Classify all these forces as contact forces or non-contact forces.
How much a body changes with the same force
You push a sledge with the same force, first on ice and then on grass, as shown in the figure. a) On which surface does the sledge gain more speed? b) What force opposes…
You push a sledge with the same force, first on ice and then on grass, as shown in the figure.
- a) On which surface does the sledge gain more speed?
- b) What force opposes the motion in each case?
- c) Explain why the effect of the same push is so different on the two surfaces.
Leo hits a table tennis ball and a football, both stationary on the ground, with the same force. a) Which of the two will move off faster? b) Which property of objects…
Leo hits a table tennis ball and a football, both stationary on the ground, with the same force.
- a) Which of the two will move off faster?
- b) Which property of objects explains the difference?
- c) What would you have to do for the football to move off as fast as the table tennis ball?