Everyday Physics 3. Force of Gravity
Force of Gravity
•We will explore one force that can change the velocity of an object• GRAVITY
•Everything that has mass is affected by gravity
•It is the most common force we have to deal with – it’s what keeps us on earth and the earth revolving around the Sun.
Weight and gravity
•All objects exert an attractive force on each other – Universal Law of Gravity
•Your weight is the attractive force that the earth exerts on you- it’s what makes things fall!
•All objects are pulled toward the center of the earth by gravity.
•The sun’s gravity is what holds the solar system together.
•It is a non-contact force no touching required!
What does your weight depend on?
•The weight of an object depends on its mass and the magnitude of the acceleration due to gravity.
•Weight points toward the earth’s center
•Sometimes down is up!
Click here to Download
Acceleration due to Gravity•When an object falls its speed increases as it descends.
•Recall that acceleration is the rate of change of velocity
•Experiments show that when objects fall, its speed increases at the rate of about per second
Example – a falling object
time
How to calculate weight
•Weight = mass x acceleration due to gravity
•Or w = m x g (mass times g)
•In this formula m is given in kilograms (kg) and g 10 meters per second per second (m/s2), then w comes out in force units – Newtons (N)
Means approximately equal to
example
•What is the weight of a 100 kg object?
•w = m x g = 100 kg x 10 m/s2 = 1000 N
You weigh more on Jupiter and less on the moon
•The value of g depends on where you are, since it depends on the mass of the planet
•On the moon g 1.6 m/s2 (1/6) g on earth, so your weight on the moon is only (1/6) your weight on earth
•On Jupiter, g 23 m/s2 2.3 g on earth, so on Jupiter you weigh 2.3 times what you weigh on earth.
Get on the scale: How to weigh yourself
m
weight
spring force
mass
Free Fall
•Free fall is the motion of an object that falls under the action of force of gravity only
•Galileo showed that all objects (regardless of mass) fall to earth with the same acceleration g = 10 m/s2
•This is only true if we remove the effects of air resistance.
•We can show this by dropping two very different objects inside a chamber that has the air removed.
Galileo’s experiments
•To test this we must drop two objects from the same height and measure the time they take to fall.
•If H isn’t too big, then the effects of air resistance are minimized
H
Aluminum Platinum
The two ball bearings have the same diameter,
but the platinum ball has 8 times more mass
than the aluminum ball
On the other hand . . .
•If you drop an object from a small height it falls so quickly that it is difficult to make an accurate measurement of the time
•We can show experimentally that it takes less than half a second for a mass to fall 1 meter.
Click here to Download
What did Galileo learn from the inclined plane experiments?
•He measured the time it took for different masses to fall down the inclined plane.
•He found that different masses take the same time to fall down the inclined plane.
•Since they all fall the same distance, he concluded that their accelerations must also be the same.
•By using different distances he was able to discover the relation between time and distance.
•How did Galileo deal with friction?
How did Galileo measure the time?
•Galileo either used his own pulse as a clock (he was trained to be a physician)
•Or, a pendulum.
Projectile motion
A projectile is an object that is thrown or struck or shot and then travels under the influence of gravity
Projectile Examples
•Tennis ball
•Golf ball
•Football
•Softball
•Soccer ball
•bullet
•Hockey puck or ball
•Basketball
•Volleyball
•Arrow
•Shot put
•Javelin
These are all examples of things that are projected, then go off under the influence of gravity
Not projectiles
•Jet plane
•Rocket
•Car (unless it looses contact with ground)
•catapult (before rock leaves)
•slingshot (before rock leaves sling)
The key to understanding projectile motion is to realize that gravity acts vertically
gravity affects only the vertical motion, not the horizontal motion
force of gravity
Demonstration
•We can see that the horizontal and vertical motions are independent
•The red ball falls vertically
•The yellow ball was given a kick to the right.
•They track each other vertically step for step and hit the ground at the same time
Galileo’s inclined plane experiments
NO!
YES
In the absence of gravity a bullet would follow a straight line forever.
With gravity it FALLS AWAY from that straight line!
Hitting the target – aim high, not directly at the target BULLSEYE!
Shoot the Monkey
Projectile motion – key points
1)The projectile has both a vertical and horizontal component of velocity
2)The only force acting on the projectile once it is shot is gravity (neglecting air resistance)
3)At all times the acceleration of the projectile is g = 10 m/s2 downward
4)The horizontal velocity of the projectile does not change throughout the path
Key points, continued
5)On the rising portion of the path gravity causes the vertical component of velocity to get smaller and smaller
6)At the very top of the path the vertical component of velocity is ZERO
7)On the falling portion of the path the vertical velocity increases
More key points
8)If the projectile lands at the same elevation as its starting point it will have the same vertical SPEED as it began with
9)The time it takes to get to the top of its path is the same as the time to get from the top back to the ground.
10)The range of the projectile (where it lands) depends on its initial speed and angle of elevation
Maximum Range
•When an artillery shell is fired the initial speed of the projectile depends on the explosive charge – this cannot be changed
•The only control you have is over the angle of elevation.
•You can control the range (where it lands) by changing the angle of elevation
•To get maximum range set the angle to 45°
The ultimate projectile: Putting an object into orbit
• Imagine trying to throw a rock around the world.
• If you give it a large horizontal velocity, it will go into orbit around the earth!