Force and Motion Notes

Unit 4 Force and Motion

Learning Goals (TEKS): Force, motion, and energy. The student knows that there is a relationship between force, motion, and energy. The student is expected to:

8.6B differentiate between speed, velocity, and acceleration

8.6A demonstrate and calculate how unbalanced forces change the speed or direction of an object’s motion

8.6C investigate and describe applications of Newton’s law of inertia, law of force and acceleration, and law of action-reaction such as in vehicle restraints, sports activities, amusement park rides, Earth’s tectonic activities, and rocket launches

Sir Isaac Newton (1643 – 1727)

One of the world’s greatest scientists

Developed the 3 Laws of Motion

His ideas are still correct and very much in use today!

How is motion described?

What is your current position?

Position is the location of an object

If you decide to move around, your position changes each time you move.

Motion is an object’s change in position relative to reference point

Whenever your position changes you are in motion

A reference point is used to determine if an object has moved or not

A point about Reference Points

A place or object used for comparison to determine if something is in motion

Ideally, that reference point should itself not be moving. But that is impossible, because the ground we walk on is part of a moving tectonic plate. Our planet Earth spins on its axis, and orbits around our Sun. Our solar system in turn orbits around the center of the Milky Way Galaxy. Our galaxy moves in relation to other galaxies. In fact, no reference point that we could choose is in fact fixed. But we can assume that a reference point, such as a starting line on a running track, is fixed for purposes of measuring motion.

What is a Force?

A force is a push or pull

When one object pushes or pulls another object, the first object is exerting a force on the second object

Forces are described by strength and direction

And are drawn with arrows

Forces are measured in Newtons

SI unit of force

Symbol: N

Measured by using a spring scale

Forces occur in pairs

Balanced Forces(No change in motion)

No Acceleration!

Unbalanced Forces

(Change in Motion)

Acceleration!

Net Force

We calculate forces using net force.

The overall force on an object after all the forces acting on it are determined is called the net force.

Net Force = 3N

Balanced Forces and Motion

Are equal forces acting on one object in opposite directions.

Equal forces acting on an object will not change the object’s motion.

The net force is 0 (zero).

Net force = 0 (push)

Net force = 0 (pull)

Unbalanced Forces

The forces acting on an object are not equal

When a net force greater than 0 acts on an object

the forces are UNBALANCED. One or more force is stronger than others, the object moves in

the direction of the greatest force

Change in Motion occurs

Unbalanced Forces and Motion

Can cause an object to: start moving

stop moving

change direction or speed.

When two forces act in the same direction, they are added together.

When forces act in opposite directions they are subtracted. If one force is greater than the other force, the overall force is

in the direction of the greater force.

All of these are considered

“change in motion”

Unbalanced Forces -Calculation Practice

225 N

Unbalanced

Unbalanced

13 N

Balanced

0 N

=

Bill Nye

Calculating Force

Force = mass x acceleration

Don’t forget the units!

Force problems practice

Get out calculators and try these 3 problems:

1. The mass of a large car is 1000 kg. How much force would be required to accelerate the car at a rate of 3 m/sec2?

2. A 50 kg skater pushed by a friend accelerates 5 m/sec2. How much force did the friend apply?

3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec2?

Force problems answers

Get out calculators and try these 3 problems:

1. The mass of a large car is 1000 kg. How much force would be required to accelerate the car at a rate of 3 m/sec2? (1000g x 3 m/sec2 = 3,oooN)

2. A 50 kg skater pushed by a friend accelerates 5 m/sec2. How much force did the friend apply? (50kg x 5m/sec2 = 250N)

3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec2? (1000kg x 9.8 m/sec2 = 9,800N)

A few “motion” terms:

Speed - "how fast an object is moving." Speed can be thought of as the rate at which an object covers a certain distance.

Velocity – It is speed in a specific direction. Also known as: the rate at which an object changes its position.

Acceleration – a measure of how quickly speed changes

Means a change in velocity

It doesn’t always mean to speed up!!