Acceleration and Momentum

Apr 21, 2023 General Science Chapter 3 1

Acceleration and Momentum

Chapter 3


Review

Newton’s first law Weight of one kilogram on Earth’s

surface Gravity


Newton’s second law

A net force acting on an object causes the object to accelerate in the direction of the force.

The acceleration is affected by two things size of force mass of object


Force

If the force on an object increases, acceleration will increase for that object and if force decreases the acceleration will decrease. IF THE MASS REMAINS CONSTANT


Mass

A more massive object will take a larger force to give it the same acceleration as a smaller massed object.


Equation for Newton’s 2nd law

onacceleratimassforce ×=

€

F = m • a

maF =

We can also use the “magic” triangle


Units on force

Last chapter we learned that force has units of Newtons.

From Newton’s second law equation,

2s

mkgN ×=


Example

How much force is needed to accelerate a 70-kg rider and the 200-kg motorcycle the rider is on at 4 m/s2?

F = 270 kg x 4 m/s2

1080 N


You try

How much force is needed to accelerate a 60-kg person and the 500-kg car the person is in at 6 m/s2?

F = 560 kg x 6 m/s2

3360 N


You try

A 63-kg skater pushes off from a wall with a force of 300 N. What is the skater’s acceleration?

300 N = 63 kg • a a = 300 N ÷ 63 kg 4.76 m/s2


Review #1

What is Newton’s 2nd Law of Motion? What equation do we use with the 2nd

Law of Motion? What 2 things will affect acceleration? What are the units for Force? True or False: A more massive object

will take less force to accelerate?


Acceleration due to gravity

Near Earth’s surface, gravity causes all falling objects to accelerate at a rate of 9.8 m/s2, regardless of their mass.

Acceleration due to gravity is represented by the letter g.

Does this number seem familiar?


Weight

The weight of an object is the force of gravity acting on it.

maF = mgW =

( )( )2m/s 8.9kg 1=W

N 8.9=W


Reality check

Do all objects really fall with the same acceleration?


Air resistance

The force air exerts on a moving object.

Acts in the opposite directions to which an object is moving.

For falling objects, air resistance pushes up while gravity pulls down.


Air resistance

Depends on the speed, size, and shape of the object.

The larger the object, the more air resistance affects it.

The faster an object is moving, the more air resistance affects it.


Terminal velocity

As an object falls, air resistance gradually increases until it equals the pull of gravity.

At this point, the object stops accelerating and moves with a constant velocity – called its terminal velocity.


Example

A ball is dropped from the roof of a 50-m tall building. It hits the ground 3.2 s later. What is its velocity right before it hits the ground?

a = v ÷ t OR v = a • t v = 9.8 m/s2 x 3.2 s 31.36 m/s


Review #2

Define air resistance. What is air resistance dependent

upon? What is terminal velocity? What is the acceleration of gravity

near the surface of the earth? How do we determine the weight of

an object?


Projectiles

Anything that’s shot or thrown through the air.

Will always follow a curved path.


Projectile motion

Can be split into independent vertical and horizontal motions.


The horizontal part

Once the object is released, there is no force acting on it horizontally.

If there is no force, it must maintain a constant horizontal velocity.


The vertical part

There is a force acting – gravity The object is pulled downward with

a constant acceleration of 9.8 m/s2.


Circular motion

Objects moving in circles are constantly accelerating.

This acceleration is towards the center of the circle or curve.

It is called centripetal acceleration. Towards the center of a circle or a

curved path.


Centripetal force

The force that causes an object to move in a curved or circular path.


Weightlessness

To be truly weightless, an object would have to be free from gravity.

To feel weightless, something has to be in free fall along with its surroundings.


Weightlessness

How does a scale measure weight? If it can’t push back, it would read

zero. You would feel weightless.


Review #3

What is a projectile? What are the 2 components of a

projectile? What does centripetal mean? Why are objects weightless during

free fall?


Newton’s 3rd law

To every action, there is an equal and opposite reaction.

When one object exerts a force on a second object, the second object exerts a force on the first object that is equal in size and opposite in direction.


Examples

Released balloon Firing a gun Jumping out of a boat rockets


Balanced forces?

Action-reaction pairs of forces are not balanced forces.

The forces act on different objects. To be balanced forces, the equal

and opposite forces must act on the same object


Momentum

A property a moving object has because of its mass and velocity

velocitymassmomentum ×=

mvp =


Momentum

Has units of kg∙m/s Has direction, because velocity has

direction


Momentum

Which has a higher momentum, a 50-kg dolphin swimming at 16.4 m/s or a 6300-kg elephant walking at 0.11 m/s?

Dolphin – (50 kg)(16.4 m/s) = 820 kg∙m/s Elephant – (6300 kg)(0.11 m/s) = 693

kg∙m/s The dolphin has higher momentum.


Example

Which has higher momentum, a 65-kg person running at 9 m/s or a 60-kg person running at 11 m/s?

1st person – (65 kg)(9 m/s) = 585 kg∙m/s 2nd person – (60 kg)(11 m/s) = 660

kg∙m/s The 60-kg person has higher momentum.


Law of conservation of momentum

The total amount of momentum of a group of objects does not change unless outside forces act on the objects.


Momentum transfer

Momentum can be transferred from one object to another.

Think about playing pool The cue ball transfers its

momentum to the ball it hits. The momentum lost by the cue

ball is gained by the other ball.


Example

A 5-kg object with a velocity of 6 m/s strikes a motionless 10-kg ball. The 5 kg object stops moving. What is the velocity of the 10 kg ball after the collision?

Momentum of object before collision: (5 kg)(6 m/s) = 30 kg∙m/s


Example continued

When the object stops, it loses all its momentum.

The momentum it lost is transferred to the ball.

Momentum of the ball after collision: (10 kg)(velocity) = 30 kg∙m/s Velocity = 3 m/s


Example continued

Notice that the velocity was not conserved, but momentum was.


Review #4

What is Newton’s 3rd Law of Motion?

Define momentum. What units does momentum have? What is the law of conservation of

momentum?


Chapter 3 Review

State all 3 Laws of Motion What 2 things affect acceleration on

an object What is air resistance dependent

upon? How are the Law of Inertia and the

Law of conservation of momentum related?

Acceleration and Momentum

Documents

Transcript of Acceleration and Momentum