Magnets and Electricity Study Guide

SECTION F: CHAPTER 2
MOTION
Grade 4: SCIENCE
STUDY GUIDE

Review from textbook, pages F36-F59

VOCABULARY

acceleration A change in the sped or direction of an object's motion

force A push or pull

frame of reference The things around you that you can sense and use to describe motion

friction A force that keeps objects that are touching each other from sliding past each other easily

gravity A force that pulls all objects toward each other

motion A change of position

newton The metric, or Système International (SI) unit of force

position A certain place

relative motion
A motion that is described based on a frame of reference

speed A measure of an object's change in position during a unit of time; for example, 10 meters per second

weight A measure of the force of gravity upon an object

CRITICAL THINKING

1. What is a force?

A force is a push or a pull.

2. What happens to a door is you push on one side and someone else pushes with the same amount of force on the other side?

If both are pushing with equal force the door will not move.

3. Give two different frames of reference from which to describe a roller-coaster ride.

Possible answers would be: the ground, the roller-coaster car, a view from the ferris wheel, or a plane flying overhead.

4. Describe the forces you would use in catching, dribbling, and shooting a basketball. how could another player cause the forces on the basketball to be balanced?

Catching is a pull, dribbling is a series of pushes, and shooting is a push. Gravity pull the ball through the net. Friction between shots and floor enables motion. The forces on the basketball are balanced whenever someone is holding the ball still.

5. What is relative motion?

Relative motion is motion described from a frame of reference.

6. What is needed to make an object change its motion?

A force is needed to make an object change its motion.

7. What is needed to given an object a greater acceleration?

To give an object greater acceleration you need more force, or the same force for a longer time.

8. Describe the force that results when two forces act on an object in the same direction.

The force that results is in the same direction as the individual forces and is equal to the individual forces added together.

9. Describe the force that results when tow forces act on an object from opposite directions and the force on the left is greater than the force on the right.

The force that results moves in the direction of the force on the left and will be equal in size to the force on the left minus the force on the right.

10. What is a spring scale used to measure and what unit of measurement does it use?

A spring scale measures the amount of force pulling on it and the unit of measure is newtons.

11. What is gravity?

Gravity is force that pulls all objects toward each other.

12. Which exerts a greater gravitational force - an object with a large mass or an object with a small mass?

An object with a large mass exerts more gravitational force.

13. Why would you weigh less on the moon than you do on Earth?

The force of gravity is less on the moon.

14. What is friction?

Friction is a force that keeps objects that are touching from easily sliding past each other.

15. How does he texture of two surfaces affect the amount of friction between them?

Rough surfaces have more friction than smooth surfaces.

16. How is friction between machine parts often reduced?

Friction can be reduced by adding oil to the parts.

Sir Isaac
Newton's
Laws of Motion:

Newton's First Law of Motion:

An object at rest or traveling in uniform motion will remain at rest or traveling in uniform motion unless acted upon by a net force."

This law is also called the Law of Inertia or Galileo's Principle.

An object may be acted upon by many forces and maintain a constant velocity so long as these forces are balanced. For example, a rock resting upon the Earth keeps a constant velocity (in this case, zero) because the downward force of its weight balances out the upward force (called the normal force) which the Earth exerts upwardly on the rock. Only unbalanced forces induce acceleration, or a change in the velocity or an object. If you push someone, he or she will accelerate in the direction of the unbalanced force which you have provided (called the applied force). Likewise if you roll a ball along the floor, the unbalanced force of friction will decelerate the ball from some positive velocity to rest.

Before Galileo, people agreed with Aristotle that a body's natural state was at rest, and that movement needed a cause. This is understandable, since in everyday experience, moving objects eventually stop because of friction (except for celestial objects, which were deemed perfect). Moving from Aristotle's "A body's natural state is at rest" to Galileo's discovery was one of the most profound and important discoveries in physics.

There are no true examples of the law, as friction is usually present, and even in space gravity acts upon an object, but it serves as a basic axiom for Newton's mathematical model from which one could derive the motions of bodies from elementary causes: forces. Another way to put it is, "An object in motion tends to stay in motion, an object at rest tends to stay at rest until a force acts upon it".

Newton's Second Law of Motion:

The rate of change of momentum of a body is equal to the resultant force acting on the body and is in the same direction.

Newton's second law of motion pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables – the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. As the net force increases, so will the object's acceleration. However, as the mass of the object increases, its acceleration will decrease. Newton's second law of motion can be formally stated as follows:

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

In terms of an equation, the net force is equal to the product of the object's mass and its acceleration.
Fnet = m * a

Newton's Third Law of Motion:

All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction.

Third law follows from conservation of momentum law.

As shown in the diagram opposite, the skaters' forces on each other are equal in magnitude, and opposite in direction. Although the forces are equal, the accelerations are not: the less massive skater will have a greater acceleration due to Newton's second law. If a basketball hits the ground, the basketball's force on the Earth is the same as Earth's force on the basketball. However, due to the ball's much smaller mass, Newton's second law predicts that its acceleration will be much greater. Not only do planets accelerate toward stars; but, stars accelerate toward planets.

The two forces in Newton's third law are of the same type, e.g., if the road exerts a forward frictional force on an accelerating car's tires, then it is also a frictional force that Newton's third law predicts for the tires pushing backward on the road.

Measuring Instruments

Spring scale

Tape Measure

Stopwatch

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Sir Isaac Newton's Laws of Motion:
Newton's First Law of Motion: An object at rest or traveling in uniform motion will remain at rest or traveling in uniform motion unless acted upon by a net force." This law is also called the Law of Inertia or Galileo's Principle. An object may be acted upon by many forces and maintain a constant velocity so long as these forces are balanced. For example, a rock resting upon the Earth keeps a constant velocity (in this case, zero) because the downward force of its weight balances out the upward force (called the normal force) which the Earth exerts upwardly on the rock. Only unbalanced forces induce acceleration, or a change in the velocity or an object. If you push someone, he or she will accelerate in the direction of the unbalanced force which you have provided (called the applied force). Likewise if you roll a ball along the floor, the unbalanced force of friction will decelerate the ball from some positive velocity to rest. Before Galileo, people agreed with Aristotle that a body's natural state was at rest, and that movement needed a cause. This is understandable, since in everyday experience, moving objects eventually stop because of friction (except for celestial objects, which were deemed perfect). Moving from Aristotle's "A body's natural state is at rest" to Galileo's discovery was one of the most profound and important discoveries in physics. There are no true examples of the law, as friction is usually present, and even in space gravity acts upon an object, but it serves as a basic axiom for Newton's mathematical model from which one could derive the motions of bodies from elementary causes: forces. Another way to put it is, "An object in motion tends to stay in motion, an object at rest tends to stay at rest until a force acts upon it".
Newton's Second Law of Motion: The rate of change of momentum of a body is equal to the resultant force acting on the body and is in the same direction. Newton's second law of motion pertains to the behavior of objects for which all existing forces are not balanced. The second law states that the acceleration of an object is dependent upon two variables – the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object. As the net force increases, so will the object's acceleration. However, as the mass of the object increases, its acceleration will decrease. Newton's second law of motion can be formally stated as follows: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. In terms of an equation, the net force is equal to the product of the object's mass and its acceleration. Fnet = m * a
Newton's Third Law of Motion: All forces occur in pairs, and these two forces are equal in magnitude and opposite in direction. Third law follows from conservation of momentum law. As shown in the diagram opposite, the skaters' forces on each other are equal in magnitude, and opposite in direction. Although the forces are equal, the accelerations are not: the less massive skater will have a greater acceleration due to Newton's second law. If a basketball hits the ground, the basketball's force on the Earth is the same as Earth's force on the basketball. However, due to the ball's much smaller mass, Newton's second law predicts that its acceleration will be much greater. Not only do planets accelerate toward stars; but, stars accelerate toward planets. The two forces in Newton's third law are of the same type, e.g., if the road exerts a forward frictional force on an accelerating car's tires, then it is also a frictional force that Newton's third law predicts for the tires pushing backward on the road.

acceleration		A change in the sped or direction of an object's motion
force		A push or pull
frame of reference		The things around you that you can sense and use to describe motion
friction		A force that keeps objects that are touching each other from sliding past each other easily
gravity		A force that pulls all objects toward each other
motion		A change of position
newton		The metric, or Système International (SI) unit of force
position		A certain place
relative motion		A motion that is described based on a frame of reference
speed		A measure of an object's change in position during a unit of time; for example, 10 meters per second
weight		A measure of the force of gravity upon an object