Uniform Motion

The stretch or compression of a spring can be used to measure a force. If the cart is at rest, a force is needed to make the cart speed up.

If you stop pulling the cart, what will it do if there is no friction between the cart and track?

Canonical Spaceship

Observation

If opposite thrusters fire, what is the net force?

Poll

Suppose that the spaceship is speeding up as it moves to the right, when all of its thrusters turn off. The spaceship will

1. immediately stop

2. slow down until it comes to a stop and then remain stopped.

3. slow down until it stops, changes direction, and speeds up while moving to the left.

4. continue to move to the right with the same velocity as when the thrusters were turned off.

Poll

In all possible cases that you could investigate, which statement best describes the results of your investigations?

1. If the net force on the spaceship is zero, it will move in a straight line with a constant speed or it will be at rest and will remain at rest.

2. If the net force on the spaceship is zero, then it will remain at rest, or if it is moving, it will slow down until it is at rest.

3. The net force on the spaceship is zero only when it is at rest.

Poll

Moving objects left the traces shown at left. The dots were laid down at equal time intervals. For which object(s) is the net force on the object zero?1) A2) B3) C4) D5) A and D6) A and B7) A, B, and D

Newton’s first law

A

In the absence of interactions, an object will remain at rest or move in a straight line with a constant speed (i.e. uniform motion).

Initialdirection 4

1

2

3

A spacecraft initially drifts in the direction shown. Exhaust jets on the rocket (1, 2, 3, and 4) may be fired singly or in combination to alter the rocket’s motion. If exhaust jet 2 is turned on at instant t1 and

turned off at instant t2, which diagram most accurately shows the spacecraft’s path?

A. B. C.

D.

Initialdirection 4

1

2

3

A space module initially drifts in the direction shown. Exhaust jets on the module (1, 2, 3, and 4) may be fired singly or in combination to alter the module’s motion. Starting with the original motion, what jet or combinationof jets would you have to turn on if you wish the module’svelocity to end up directed toward the top of the screen?

A. 2

B. 4

C. 2 and 3

D. 2 and 4

E. 3 and 4

Interactions

A force is one side of an interaction. When two objects interact, they exert forces on each other.

A B

Newton’s third law says that these forces are equal in magnitude and opposite in direction.

Demo

A B

Force is a measure of the strength of the interaction.

What if they have different masses?

The carts exert equal and opposite forces on each other, as is true in any interaction.

Newton’s third law does not depend on their masses or motion. It just tells us about the “two-sided” nature of an interaction.

Poll

If Cart A has twice the mass of Cart B, which cart exerts a larger magnitude force on the other during the collision?

A B

1. Cart A

2. Cart B

3. neither, because they exert equal magnitude forces on each other.

Poll

If Cart A is moving with twice the speed as Cart B when they make a head-on collision, which cart exerts a larger magnitude force on the other during the collision?

A B

1. Cart A

2. Cart B

3. neither, because they exert equal magnitude forces on each other.

Poll

A certain ball is in free-fall. As it falls, Earth exerts a constant downward force of 5 N on the ball. What is the force of the ball on Earth?

1. zero; (the ball exerts a negligible, almost zero, force on Earth)

2. a small, non-zero, upward force.

3. 5 N, upward

4. an upward force that is greater than 5 N.

Demo

A B

Force is a measure of the strength of the interaction.

The carts exert equal and opposite forces on each other, as is true in any interaction.

Which cart has a bigger acceleration as a result of the collision?

Inertial Mass

A B

m is inertial mass.

For a given interaction, a smaller mass will have a larger acceleration.

Net Force

A B

A

It’s the net interaction that determines motion.

Newton’s second law

A B

A

It’s the net interaction that causes an object to change velocity.

Identifying forces

To identify all forces acting on an object, ask yourself the following:

1. What objects are “touching” the given object?

2. What objects exert a force through “action at a distance” such as a gravitational, electrostatic, or magnetic force?

Gravitational Force (i.e. weight)

Near the surface of Earth:

Example

A girl pulls her red wagon in the +x direction with a force of 50 N at an angle of 30 with respect to the horizontal. List the forces that are exerted on the wagon?

Example

A girl pulls her red wagon in the +x direction with a force of 50 N at an angle of 30 with respect to the horizontal. If the wagon moves at a constant velocity, what is the net force on the wagon?

Summary

1. Newton’s first law: (equilibrium)If the net force on an object is zero, then the object will remain at rest or will move with a constant speed in a straight line (uniform motion).

2. Newton’s second law: (accelerating motion)The net force on an object will cause an object to accelerate with an acceleration equal to the net force on the object divided by its mass.

3. Newton’s third law: (interactions)When two objects interact, they exert equal magnitude forces on each other, in opposite directions. (This says nothing about their motion or the results of these forces.)

The object or objects that exert forces on the book in the situation shown here are ___________ .

A. the table and gravity

B. the table and the Earth

C. only gravity

D. only the Earth

E. the Earth and gravity

If the hand is causing blocks A and B to accelerate, the force that A exerts on B is _______ equal and opposite to the force that B exerts on A.

A. always

B.

sometimes

C. never

If the hand is causing blocks A and B to accelerate, the force that A exerts on B is _______ equal to the force that the hand exerts on A.

A. always

B.

sometimes

C. never