 Newton’s Laws of Motion

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Newton’s Laws of Motion

Newton’s Laws of Motion
1. An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by an unbalanced force. 2. Force equals mass times acceleration (F = ma). 3. For every action there is an equal and opposite reaction.

Newton’s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force.

What does this mean? An object will “keep doing what it was doing” unless acted on by an unbalanced force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

Some Examples from Real Life
A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion. Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

Newton’s First Law is also called the Law of Inertia
Inertia: tendency of an object to resist changes in its state of motion The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).

Real World Connection This law explains why you fly forward in a car when someone slams on the brakes. Because of Inertia, your body wants to keep moving at the same speed as the car.

More Examples from Real Life
A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them. On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever?
Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.

Force equals mass times acceleration. F = ma
Newton’s Second Law Force equals mass times acceleration. F = ma Acceleration: a measurement of how quickly an object is changing speed.

What does F = ma mean? Force is directly proportional to mass and acceleration. Imagine a ball of a certain mass moving at a certain acceleration. This ball has a certain force. Now imagine we make the ball twice as big (double the mass) but keep the acceleration constant. F = ma says that this new ball has twice the force of the old ball. Now imagine the original ball moving at twice the original acceleration. F = ma says that the ball will again have twice the force of the ball at the original acceleration.

Formula chart says F=ma, m is mass in kg, a is acceleration in m/s2.
Q: The frog leaps from its resting position at the lake’s bank onto a lily pad. If the frog has a mass of 0.5 kg and the acceleration of the leap is 3 m/s2, what is the force the frog exerts on the lake’s bank when leaping? (A) 0.2 N (B) 0.8 N (C) 1.5 N (D) 6.0 N Formula chart says F=ma, m is mass in kg, a is acceleration in m/s2. So, .5 kg x 3 m/s2= 1.5 N

What does F = ma say? F = ma means that the force of an object comes from its mass and its acceleration. Something very massive (high mass) that’s changing speed very slowly (low acceleration), like a glacier, can still have great force. Something very small (low mass) that’s changing speed very quickly (high acceleration), like a bullet, can still have a great force. Something very small changing speed very slowly will have a very weak force.

For every action there is an equal and opposite reaction.
Newton’s Third Law For every action there is an equal and opposite reaction.

What does this mean? For every force acting on an object, there is an equal force acting in the opposite direction. Right now, gravity is pulling you down in your seat, but Newton’s Third Law says your seat is pushing up against you with equal force. This is why you are not moving. There is a balanced force acting on you– gravity pulling down, your seat pushing up.

Q: The hands of a swimmer pushing backward against water represent an action force. What is the reaction force? A. The swimmer’s body moving forward? B. The water pushing against the swimmer’s hands C. The swimmer’s body pushing against the water. D. The water moving backward from the swimmer.

Momentum The product of an object’s mass and its speed. A force applied to an object causes a change in its momentum. p(momentum)= m(mass) x v(velocity) p = mv common unit for momentum (kg x m/s)

Q: A ball moving at 30 m/s has a momentum of 15 kg·m/s
Q: A ball moving at 30 m/s has a momentum of 15 kg·m/s. The mass of the ball is — A. 45 kg B. 15 kg C. 2.0 kg D. 0.5 kg Formula Page says that Momentum = Mass x Velocity So, 15 kg.m/s = M x 30 m/s solving for M it is:

Velocity Graphs V = distance time
Velocity (v) is the slope (rise over run) of a position (d) vs. time (t) graph

Q: The diagram represents the total travel of a teacher on a Saturday
Q: The diagram represents the total travel of a teacher on a Saturday. Which part of the trip is made at the greatest average speed? A. Q B. R C. S D. T

Q: The picture shows the position of a ball every 0
Q: The picture shows the position of a ball every 0.25 second on a photogram. Using a ruler, determine the velocity of the ball. A. 3.5 cm/s B cm/s C cm/s D cm/s

Use the ruler on the side of the chart and the equation for velocity
Use the ruler on the side of the chart and the equation for velocity. The answer was H. Measure from the center of ball 1 to the center of ball 2 and multiply by 4.

Acceleration When an object’s speed changes over time it is accelerating (or decelerating) A = vfinal – vinitial / time Units for acceleration m/s/s or m/s2

Acceleration Graphs Acceleration (a) is the slope of a velocity (v) vs. time (t) graph Positive Acceleration Negative Acceleration NO Acceleration Velocity (m/s) Velocity (m/s) Velocity (m/s) Time (s) Time (s) Time (s)

Think about it . . . What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force. Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

Teresa runs in one direction at 1. 5 meters per second (m/s)
Teresa runs in one direction at 1.5 meters per second (m/s). She hen turns around and runs in the opposite direction at 2.0 m/s. The entire trip takes 5.0 seconds (s). What is Teresa’s average acceleration, in meters per second squared (m/s2)? A m/s2 B m/s2 C m/s2 D m/s2

Work Work: application of a force to an object that results in the movement of the object over a certain distance. W = F x d The work done by forces on an object = changes in energy for that object. Work and Energy are measured in Joules 1 Joule = 1 Newton • meter

Q: How much work is performed when a 50 kg crate is pushed 15 m with a force of 20 N?
A. 300 J B. 750 J C. 1,000 J D. 15,000 J Use the formula Work = Force x distance Force of 20 N x 15 meters = 300 Joules Answer:

Q: If a force of 100 newtons was exerted on an object and no work was done, the object must have —
A. accelerated rapidly B. remained motionless C. decreased its velocity D. gained momentum Work = Force x Distance Work = 0 Force = 100 N so 0 J = 100 N x d distance must be 0 It did not move!

Work Example: The teacher pushes on the wall until she is exhausted.
A book falls off the table and hits the floor. The waiter carries a tray of food. A rocket accelerates through space. Is Work Being Done? No. The wall did not move. Yes, gravity applied a force and moved the book in the direction of the floor. No. The force to hold the tray is not applied in the direction of the motion. Yes. The force of the rocket thrust is causing the rocket to move.

Friction A force that opposes, or works against, motion of two objects that are touching.

Friction Friction causes an object to slow down and stop.
Since the amount of energy stays constant, the energy becomes heat.

Universal Law of Gravity
Gravity varies depending on two factors: 1) the mass of the object doing the pulling, and 2) the distance from the center of that object

On Earth gravity = 9.8 m/s/s
For every second that an object falls its speed increases by 9.8 m/s

Weight= Mass (m) X gravity (g)
Unit of mass = kg Unit of acceleration = m/s/s Unit of weight = Newton 1 Newton= about ¼ pound

Review Newton’s First Law:
Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law: Force equals mass times acceleration (F = ma). Newton’s Third Law: For every action there is an equal and opposite reaction.

Vocabulary Inertia: Acceleration: Velocity: Force:
the tendency of an object to resist changes in its state of motion Acceleration: •a change in velocity •a measurement of how quickly an object is changing speed, direction or both Velocity: The rate of change of a position along a straight line with respect to time Force: strength or energy