Put the verbs in brackets into the correct tense.

A: What 1) ….. (you / to do) when I 2) ….. (to call) at eight, Burt?

B: I 3) ….. (to work) in the garden because the wind 4) ….. (to blow down) the fence during the night.

A: Oh, 5) ….. (you / to manage) to fix it?

B: Yes, I ….. (to do ) it eventually, but it 7) ….. (to be) very hard work. I 8) ….. (to ask) my neighbor to help in the end. Why 9) ….. (you / to call) me?

A: I 10) ….. (to want) to tell you about the factory. It 11) ….. (to close down) yesterday.

B: I know. The company 12) ….. (to have) problems for a long time before they finally 13) ….. (to decide) to close down the factory.

A: I 14) ….. (to hope) they would change their minds about it, though. It 15) ….. (to be) a part of the town for years.

B: Well, at least everyone who worked there 16) ….. (now / to find) a new job. That’s good news.

UNIT 7

FACTS ABOUT FORCES

LEAD-IN

1. What is force? Give examples of forces acting on people and objects around us.

2. What kinds of forces do you know? How are they related to motion?

READING

3. Read the text and give the definitions of:

- force;

- motion;

- Newton’s three laws of motion;

- net force

Facts about Forces

Force is a word that has lots of different meanings and different uses. For instance, there is electrical force, the force of gravity, and the force of magnetism. Nuclear force holds atoms together. Another kind of force is mechanical force. The word force means a push or a pull, or to be a little bit more scientific, it means anything that causes the shape or motion of an object to change. The amount of force is measured in units called newtons, named after the scientist Sir Isaac Newton (definition of a Newton: the force required to accelerate a one kilogram object by one meter per second squared).

Force and Motion.Force and motion describe everyday things that are happening all the time. Hundreds of times every day, you use force and motion. Did you just pick up a pencil? – Force and motion. Did you turn a page? – Force and motion. Force and motion are also parts of a complicated branch of science, called physics. Motion means moving something from one place to another. In fact, the word motion is a form of the word move.

Newton's Three Laws of Motion.Sir Isaac Newton is well known for his three laws of motion as well as for other scientific breakthroughs. Here are Sir Isaac Newton's three laws of motion.

Law 1 - An object moving in a straight line will continue moving in a straight line, unless acted on by an outside force. Also, an object at rest will stay at rest. The word for this is inertia.

Law 2 - Force will cause a change in the motion of an object. The change in motion depends on the amount of force and the mass of the object. There is a formula for this F=ma (force equals mass times acceleration).

Law 3 - For each action, there is an equal and opposite reaction.

Force is a vector, which means that it has both direction and magnitude. When several forces act on an object, the forces can be combined to give a net force.

If multiple forces combine to give a net force that is zero, then the object will not accelerate; the object will either remain motionless or continue moving at a constant velocity. For example, if a person pushes a shopping cart with a force equal in magnitude to the force of friction that opposes the cart's motion, the forces will cancel, giving a net force of zero. As a result, the cart will move down the aisle with a constant velocity. If the person suddenly stops pushing the cart, the only force acting on the cart is the frictional force. The velocity of the cart drops to zero.

Date: 2015-12-11; view: 1939