GRAVITY

Anything that has mass is attracted by the force of gravity GRAVITY= is an attractive force between any two objects.

GRAVITY Every one of you is attracted to each other. Look at the person sitting next to you, in front of you or behind you. You ARE attracted to them.

Gravity depends on the mass of the objects and the distance between them. This force increases as the mass of either object increases or as the objects move closer to each other.

Only the Earth is both close enough and has a large enough mass that you can feel its gravitational attraction. The sun is too far away. To exert a noticeable gravitational attraction on you.

The Law of Universal Gravitation For thousands of years people everywhere have observed the stars and the planets in the night sky.

Gradually data was collected on the motion of the planets. Issac Newton used some of this data to formulate the LAW OF UNIVERSAL GRAVITATION in F = G(m 1 m 2 ) d 2

F = G(m 1 m 2 ) d 2 Gravitational force =(constant)x (mass 1)x(mass 2) (distance) 2 In this equation G is a constant called the universal gravitational constant, and d is the distance between the two masses. The law of universal gravitation enables the force of gravity to be calculated between any two objects.

B. Gravity Who experiences more gravity - the astronaut or the politician? less distance more mass Which exerts more gravity - the Earth or the moon?

If you drop a bowling ball and a feather at the same time, which would hit the ground first? BOWLING BALL

Suppose the effects of air resistance are small enough to be ignored. When all forces except gravity are acting on an object the object is said to be in FREE FALL

Close to Earth’s surface, the acceleration of a falling object in free fall is about 9.8 m/s 2. This acceleration is given the symbol g. This is sometimes called THE ACCELERATION OF GRAVITY

By Newton’s second law of motion, the force of Earth’s gravity on a falling object is the object’s mass times the acceleration of gravity. Force of gravity (N) = mass (kg) x acceleration of gravity (m/s 2 ) F = mg

The gravitational force on a sky diver with a mass of 60 kg would be? F = 60kg x 9.8 m/s 2 F = 588 N

WEIGHT Even if you are not falling the force of Earth’s gravity still is pulling you downward. If you are standing on a floor the Net Force on you is zero. Force of gravity pulls you downward The floor exerts an upward force on you

Whether you are standing, jumping or falling, Earth exerts a gravitational force on you. The gravitational force exerted on an object is called the objects WEIGHT.

The weight of an object in Earth is equal to the force of Earth’s gravity on the object. Weight can be calculated from this equation. Weight (N) = mass (kg) x acceleration of gravity (m/s 2) W = mg

Weight and Mass are NOT the same. Weight is a FORCE MASS is the amount of matter an object contains. Weight will increase as mass increases

B. Gravity Accel. due to gravity (g) –In the absence of air resistance, all falling objects have the same acceleration! –On Earth: g = 9.8 m/s 2 elephant feather Animation from “Multimedia Physics Studios.”Multimedia Physics Studios

The weight of an object is the gravitational force between the object and Earth. Weight of an object can change depending on the gravitational force. GRAVITY = 9.8 M/S 2

Why does an astronaut float in space?

The shuttle orbits the Earth at an altitude of about 400km. The force of Earth’s gravity is about 90% as strong as it is at Earth’s surface An astronaut at with a force of 780N on Earth would be 700 N in space

A space shuttle is orbit is in free fall, but is falling around Earth, rather that straight downward. Everything in the shuttle is falling around the Earth at the same rate.

Your weight would be different on the moon. Gravity is 1.6 m/s 2. You would weigh only about 1/6 of what you weigh on Earth.

Objects in the shuttle seem to be floating because they are all falling with the same acceleration.

Why do ast. float

Ast. In space

Think about how you measure your weight. You stand on a scale. You are at rest and the Net Force on you is zero.

The scale supports you and balances your weight by exerting an upward force. The dial on the scale shows the upward force exerted by the scale which is your WEIGHT. Scale exerts upward force = your weight

Suppose you stand on the scale in an elevator that is falling. If you and the scale were in free fall, then you would no longer push down on the scale. The scale would read “0” Even though the force of gravity hasn’t changed.

Free-fall rides are really made up of three distinct parts: the ride to the top, the momentary suspension, and the downward plunge. In the first part of the ride, force is applied to the car to lift it to the top of the free-fall tower. The amount of force that must be applied depends on the mass of the car and its passengers.

After a brief period in which the riders are suspended in the air, the car suddenly drops and begins to accelerate toward the ground under the influence of the earth's gravity. Just as Galileo and Newton explain in their theories of free fall, the least massive and most massive riders fall to the earth with the same rate of acceleration.

What happens to the gravitational force as objects get farther apart? A) It decreases. B) It increases. C) It stays the same. D) It fluctuates.

You exert a gravitational force on __________. A) everything B) nothing C) only objects near you D) only objects on Earth

Answer : B How does weight relate to mass? A) Weight is the same as mass. B) Weight is mass times acceleration due to gravity. C) Weight is mass divided by acceleration due to gravity. D) They are unrelated.

Answer : A How much does a 10-kg block weigh? A) 98 pounds B) 9.8 N C) 9.8 pounds D) 98 N

Answer : B What happens to the weight of astronauts when they are in orbit? A) They become weightless. B) It remains the same. C) They gain weight. D) They lose weight.

Answer: A A net force of 2500 N acts on an African male elephant with a mass of 7000 kg. What is the acceleration of the African male elephant? A) 0.36 m/s2 B) 0.30 m/s2 C) 0.25 m/s2 D) 0.22 m/s2

Answer : C A physics book with a mass of 2.8 kg is pushed along a table with a net force of 1 N. What is the book's acceleration? A) 0.28 m/s2 B) 0.32 m/s2 C) 0.36 m/s2 D) 0.28 m/s2

If you have ever tossed a ball to someone, you have probably noticed that thrown objects don’t always travel in straight lines. They curve downward.

That is why quarterbacks, dart players and archers aim above their targets.

Anything that is thrown or shot through the air is a PROJECTILE. Earth’s gravity causes projectiles to follow a curved path.

Horizontal and Vertical Motions When you throw a ball, the force exerted by your hand pushes the ball forward. This force gives the ball HORIZONTAL MOTION

After you let go of the ball, no force accelerates it forward. So, its HORIZONTAL VELOCITY is constant, if you ignore air resistance..

However, if you let go of a ball, gravity can pull it downward giving it VERTICAL MOTION

Now the ball has constant horizontal velocity, but increasing vertical velocity. Gravity exerts an unbalanced force on the ball, changing the direction of its path from only forward to forward and downward. The ball appears to travel in a curve, even though the horizontal and vertical motions are completely independent of each other.

If you were to throw a ball as hard as you could from shoulder height in a perfectly horizontal direction, would it take longer to reach the ground than if you dropped a ball from the same height? YES NO

They would both reach the ground at the same time. Both will travel the same vertical distance in the same amount of time. The one thrown horizontally travels a greater horizontal distance.

CENTRIPETAL FORCE When an object enters a curve, even if its speed does not change, it is accelerating, because its direction is changing.

When something goes around a curve, the change in the direction of the velocity is toward the center of the curve. CENTRIPETAL ACCELERATION= acceleration toward the center of a curved path.

According to the second law of motion, when the ball has centripetal acceleration, the direction of the NET FORCE on the ball also must be toward the center of the curved path.

CENTRIPETAL FORCE= The NET FORCE exerted toward the center of a curved path.

When a car rounds a curve on a highway, a centripetal force must be acting on the car to keep it moving in a curved path.

The centripetal force is the frictional force, or the traction, between the tires and the road surface.

If the road is slippery and the fictional force is small, The centripetal force might not be large enough to keep the car moving around the curve.

Anything that moves in a circle, such as the people on an amusement ride or a park ride is doing so because a centripetal force is accelerating it toward the center.

Many amusement park rides rely on circular motion to confuse your senses, to apply strong forces to your body, and to suspend your body in highly unusual positions... all in the name of funforces

Many flat rides starts out flat on the ground. When you and the riders are in your places the ride begins to rotate faster and faster until it reaches a certain speed. Due to Newton's first law of motion inertia tries to keep you moving in a straight line, which would be away from the ride. (NOTE: The path that inertia wants you to move in is actually tangential to the circular path of the ride. We will talk about this later.) Fortunately, you don't fly out because the wall prevents you from doing so.Newton's first law of motion

If there were no harness or walls to confine you, as the ride spins it would become harder and harder to hold on. If you let go your body will follow the path that it was on the moment you let go

GRAVITY CAN BE A CENTRIPETAL FORCE Imagine whirling an object tied to a string above your head. The string exerts a centripetal force on the object that keeps it moving in a circular path Earth’s gravity exerts a centripetal force on the Moon that keeps it moving in a nearly circular orbit.

On earth, what is the weight of a large screen TV that has a mass of 75 kg? A.345 N B.455 N C.735 N C = 735 N

Find the acceleration of gravity on Mars if a person with a mass of 60 kg weighs 22.4 N on Mars A.3.7 m/s/s B.4.2 m/s/s C.6.7 m/s/s a = 3.7 m/s 2

Find the force exerted by a rope on a 10kg mass that is hanging from the rope. A.98 N B.56 N C.47 N A = 98 N

Answer : B What causes thrown objects to travel in a curved path? A) gravity B) Earth's gravitational pull and the inertia of the object C) air resistance D) momentum

Answer : D What happens to the acceleration due to gravity of an object in projectile motion? A) It increases. B) It decreases. C) It fluctuates. D) It stays the same.

Answer : A In which direction is the centripetal force? A) toward the edges of the circle B) along the curve C) upward D) toward the center of the circle

Answer : A If you swing a large washer on a string in a circle, the net force is _______. A) toward your hand B) away from your hand C) in the direction of motion of the weight D) toward the ground