Newton's Laws

Newtons Three Laws

(Remember a Law is the summary of the theory.)

1) An object that isn't being pushed or pulled by an unbalance force stays still or keeps moving in a straight line at constant speed.

ie. If you let go of a shopping cart it keeps going until it crashes into something.

2) Forces make thing accelerate. The bigger the force and the smaller the mass, the greater acceleration.

ie it's easier to make a lightweight bike accelerate than a ten-ton truck.

3) For every force acting on a body, the body reacts with a force that is equal in magnitude but opposite in direction at the same point of contact. (Do not confuse this with balanced forces.)

ie. I push down on the floor I am standing on and the floor pushes up with the same force

Newtons Three Laws

1) An object that isn't being pushed or pulled by an unbalance force stays still or keeps moving in a straight line at constant speed.

ie. If you let go of a shopping cart it keeps going until it crashes into something.

Newtons Three Laws

2) Forces make thing accelerate. The bigger the force and the smaller the mass, the greater acceleration.

ie it's easier to make a lightweight bike accelerate than a ten-ton truck.

Newtons Three Laws

3) For every force acting on a body, the body reacts with a force that is equal in magnitude but opposite in direction at the same point of contact. (Do not confuse this with balanced forces.)ie. I push down on the floor I am standing on and the floor pushes up with the same force

ie. a rocket pushes gas out its engines and the gas pushes back on the rocket and lifts it into space.

Units of Force

From Newton's 2nd Law

F α a

F = ma = (kg)(m/s2) = kg* m s2

= N (Newton)

Force is a vector quantity

Note:

Mass is not the same as weight.

Force = weight

In the USA Weight is measured in pounds. This is a measure of force. People use it as if it were a mass.

Be careful!

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted on by an unbalanced force.

Why Newton's 1st law isn't obvious.... friction

https://www.youtube.com/watch?feature=fvwp&v=LvVf_-krLVw&NR=1&safe=active

Inertia- the resistance of an object to any change in its motion.

The more Inertia an object has the more mass an object has.

Inertia and Mass

http://www.youtube.com/watch?v=e30roNZthw4&safe=active

1) Imagine a place in the cosmos far from all gravitation and frictional influences. If you throw a rock, what happens to the rock? Will it:

a) keep going at a constant velocity and in the direction it was thrown.b) gradually slow down

2) A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

3) Mutt and Jeff are arguing in the cafeteria. Mutt says that if he flings Jello with a greater speed it will have a greater inertia. Jeff argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with? Explain.

4) Supposing you were in space in a weightless environment, would it require a force to set an object in motion?

6) Ben Tooclose is being chased through the woods by a bull moose that he was trying to photograph. The enormous mass of the moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

5) Fred spends most Sunday afternoons at rest on the sofa, watching pro football games and eating large quantities of food. What effect, if any, does this practice have upon his inertia? Explain.

1) Look at the following video. (ouch!) Briefly explain how this demonstrates Newton's first Law.

http://cheezburger.com/8054137088

Homework:

Read pages 5-7 in your workbook

Questions 1-8 on pages 28 and 29

This can be handed in on Engrade. (Please) Save a tree!

Physics 30S Project: Newton’s Laws of Motion

a) Find 3 cartoons (Not cartoons that are physics illustrations!) that illustrate each of Newton’s Laws. (1 cartoon for each law) b) In words describe which of the law that the cartoon shows.c) Comment on whether the comic artist is using the law correctly or is defying the law.

Due date: Project value: 15 marks

Cartoon Project Example