Chapter 10Chapter 10

Forces and MotionForces and Motion

Gravity Gravity Gravity is a force of attraction that acts Gravity is a force of attraction that acts

between bodies that have a mass. between bodies that have a mass. Gravity was discovered by Sir Isaac Newton.Gravity was discovered by Sir Isaac Newton. Gravity acts on all objects the same. If you Gravity acts on all objects the same. If you

drop a bowling ball and a tennis ball at the drop a bowling ball and a tennis ball at the same time, they will both hit the ground at same time, they will both hit the ground at the same time because gravity is a constant.the same time because gravity is a constant.

The more matter something has the more The more matter something has the more gravitational pull it has. Our moon is much gravitational pull it has. Our moon is much smaller than Earth and only has about 1/6 smaller than Earth and only has about 1/6 the amount of gravity .the amount of gravity .

Jupiter’s gravity is about 2.5 times stronger Jupiter’s gravity is about 2.5 times stronger than Earth’s because of its size. That is why than Earth’s because of its size. That is why people weigh different weights on different people weigh different weights on different planets.planets.

How to Measure GravityHow to Measure Gravity Weight is a measure of how much Earth’s Weight is a measure of how much Earth’s

gravity pulls down on an object.gravity pulls down on an object. Mass is the amount of matter in an object, Mass is the amount of matter in an object,

and mass does not change from place to and mass does not change from place to place.place.

While your weight would change on the While your weight would change on the moon, your mass would not change.moon, your mass would not change.

Mass is measured with a balance. Mass is measured with a balance. Weight is measured with a scale.Weight is measured with a scale. Mass and weight are two different things, so Mass and weight are two different things, so

we have to use two different units for them.we have to use two different units for them. Mass is measured in g or kg. Weight is Mass is measured in g or kg. Weight is

measured in Newtons (N). measured in Newtons (N). A 10g weighs 1N on Earth. So if a rock had a A 10g weighs 1N on Earth. So if a rock had a

mass of 30g, it would weigh 3N.mass of 30g, it would weigh 3N.

SpeedSpeed Speed is the measure of how far you move Speed is the measure of how far you move

in a given time. To find speed you need to in a given time. To find speed you need to know time and distance. know time and distance.

If you rolled a ball down a hill that was 100 If you rolled a ball down a hill that was 100 meters long and it took 20 seconds to meters long and it took 20 seconds to reach the bottom, its speed is 5 m/s. So reach the bottom, its speed is 5 m/s. So the equation for velocity equals, v=d/t or the equation for velocity equals, v=d/t or distance over time. distance over time.

The speed of the ball throughout the trip The speed of the ball throughout the trip changed. It started off slow and got faster changed. It started off slow and got faster as it rolled down.as it rolled down.

So, this calculation gives you an average So, this calculation gives you an average speed and describes the movement for the speed and describes the movement for the entire trip down the street. entire trip down the street.

StoppingStopping

Gravity speeds you up as you go down a Gravity speeds you up as you go down a hill, but what makes you stop. Friction.hill, but what makes you stop. Friction.

Friction is the push or pull that opposes Friction is the push or pull that opposes motion between two surfaces that are motion between two surfaces that are touching each othertouching each other

If you are roller blading, you might use the If you are roller blading, you might use the rubber stopper to cause friction against rubber stopper to cause friction against the street to help you stop.the street to help you stop.

H.W. Pg. 274, ques. 1 & 2. Page 288, 1-10.H.W. Pg. 274, ques. 1 & 2. Page 288, 1-10.

Newton’s Three Laws of MotionNewton’s Three Laws of Motion Before we talk about the laws, we have to Before we talk about the laws, we have to

learn about force, which is a push or a learn about force, which is a push or a pull. Gravity is a force, as well as friction. pull. Gravity is a force, as well as friction. Everytime you turn a page or open a door, Everytime you turn a page or open a door, you are using force to do so.you are using force to do so.

Law #1: Inertia- An object at rest tends to Law #1: Inertia- An object at rest tends to stay at rest, and a moving object moves in stay at rest, and a moving object moves in a straight line until it is acted upon by an a straight line until it is acted upon by an outside force.outside force.

In other words, things don’t stop or start In other words, things don’t stop or start by themselves. The reasons things stop is by themselves. The reasons things stop is because of friction.because of friction.

Newton’s Second LawNewton’s Second Law This law involves force, mass and This law involves force, mass and

acceleration.acceleration. Law #2: Acceleration depends on the Law #2: Acceleration depends on the

mass of an object and the force pushing or mass of an object and the force pushing or pulling the object. pulling the object.

Force = mass x acceleration, F=mXaForce = mass x acceleration, F=mXa Acceleration = force/mass, a=F/mAcceleration = force/mass, a=F/m It takes less force to accelerate something It takes less force to accelerate something

with a small mass. This is why large trucks with a small mass. This is why large trucks need more powerful motors to get them need more powerful motors to get them going from a complete stop. If you want to going from a complete stop. If you want to go faster, you have to use more force. Like go faster, you have to use more force. Like having to pedal harder on a bike.having to pedal harder on a bike.

Newton’s Third LawNewton’s Third Law

Law #3: For every action there is an Law #3: For every action there is an equal and opposite reaction.equal and opposite reaction.

If you push on a wall, the wall is If you push on a wall, the wall is pushing back at you at the same pushing back at you at the same rate.rate.