Review SlideReview Slide

Newton’s Second LawNewton’s Second Law

Newton’s Second Law of Motion The acceleration of an object is directly

proportional to the net force acting on it and inversely proportional to its mass.

F = ma

Force and Acceleration Force and Acceleration

The hard-thrown ball has a greater The hard-thrown ball has a greater change in change in velocityvelocity, and the change , and the change occurs over a shorter period of time.occurs over a shorter period of time.

Recall that acceleration is the change Recall that acceleration is the change in velocity divided by the in velocity divided by the timetime it it takes for the change to occur. takes for the change to occur.

So, a hard-thrown ball has a greater So, a hard-thrown ball has a greater accelerationacceleration than a gently thrown than a gently thrown ball. ball.

FrictionFriction

• Recall that when an object Recall that when an object slowsslows down it is accelerating.down it is accelerating.

• By Newton’s second law, if the By Newton’s second law, if the skateboard is skateboard is acceleratingaccelerating, there , there must be a net force acting on it.must be a net force acting on it.

• The force that slows the skateboard The force that slows the skateboard and brings it to a stop is and brings it to a stop is frictionfriction..

Sliding FrictionSliding Friction

• You ask a friend to help you move You ask a friend to help you move the box.the box.

Pushing together, the box moves. Pushing together, the box moves. Together you and your friend have Together you and your friend have exerted enough force to break the exerted enough force to break the microweldsmicrowelds between the floor and between the floor and the bottom of the box.the bottom of the box.

GravityGravity Gravity

force of attraction between any two objects in the universe

increases as... mass increases distance decreases

If the mass of either of the objects increases, the gravitational force between them increases

If the objects are closer together, the gravitational force between them increases

Earth’s Gravitational Earth’s Gravitational Acceleration Acceleration

When all forces except gravity acting When all forces except gravity acting on a falling object can be ignored, the on a falling object can be ignored, the object is said to be in object is said to be in free fallfree fall..

Close to Earth’s surface, the Close to Earth’s surface, the acceleration of a falling object in free acceleration of a falling object in free fall is about fall is about 9.8 m/s9.8 m/s22..

This acceleration is given the symbol This acceleration is given the symbol gg and is sometimes called the and is sometimes called the acceleration of acceleration of gravitygravity. .

Earth’s Gravitational Earth’s Gravitational Acceleration Acceleration

Close to Earth’s surface, the acceleration Close to Earth’s surface, the acceleration of a falling object in free fall is about of a falling object in free fall is about 9.8 9.8 m/sm/s22..

This acceleration is given the symbol This acceleration is given the symbol gg and is sometimes called the acceleration and is sometimes called the acceleration of gravity. of gravity.

By Newton’s second law of motion, the By Newton’s second law of motion, the force of Earth’s gravity on a falling object force of Earth’s gravity on a falling object is the object’s mass times the is the object’s mass times the accelerationacceleration of gravity. of gravity.

WeightWeight

The gravitational force exerted on an The gravitational force exerted on an object is called the object’s object is called the object’s weightweight

Because the weight of an object on Because the weight of an object on Earth is Earth is equalequal to the force of Earth’s to the force of Earth’s gravity on the object, weight can be gravity on the object, weight can be calculated from this equation: calculated from this equation:

GravityGravity WeightWeight

the force of gravity on an objectthe force of gravity on an object

MASSalways the same

(kg)

WEIGHTWEIGHTdepends on gravitydepends on gravity

(N)(N)

W = mgW:weight (N)m: mass (kg)g: acceleration

due to gravity (m/s2)

Projectile MotionProjectile Motion

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

Earth’s Earth’s gravitygravity causes projectiles to causes projectiles to follow a curved path. follow a curved path.

Centripetal ForceCentripetal Force

According to the second law of According to the second law of motion, when a ball has centripetal motion, when a ball has centripetal acceleration, the direction of the net acceleration, the direction of the net force on the ball also must be toward force on the ball also must be toward the the centercenter of the curved path. of the curved path.

The net force exerted toward the The net force exerted toward the center of a curved path is called a center of a curved path is called a centripetal forcecentripetal force. .

Newton’s Third LawNewton’s Third Law Newton’s Third Law of MotionNewton’s Third Law of Motion

When one object exerts a When one object exerts a forceforce on a second object, the second on a second object, the second object exerts an object exerts an equalequal but but oppositeopposite force on the first. force on the first.

Action and Reaction Action and Reaction

When a When a forceforce is applied in nature, a is applied in nature, a reaction force occurs at the same reaction force occurs at the same time.time.

When you jump on a trampoline, for When you jump on a trampoline, for example, you exert a example, you exert a downwarddownward force on the trampoline.force on the trampoline.

Simultaneously, the trampoline Simultaneously, the trampoline exerts an equal force exerts an equal force upwardupward, , sending you high into the air. sending you high into the air.

Action and Reaction Action and Reaction Forces Don’t Cancel Forces Don’t Cancel

According to the third law of motion, According to the third law of motion, action and reaction forces act on action and reaction forces act on differentdifferent objects. objects.

Thus, even though the forces are Thus, even though the forces are equalequal, they are not balanced , they are not balanced because they act on different because they act on different objects.objects.

MomentumMomentum

MomentumMomentum quantityquantity of motion of motionp = mv

p: momentum (kg·m/s)m: mass (kg)v: velocity (m/s)m

p

v

• Use the unit Use the unit kg·m/skg·m/s

Conservation of MomentumConservation of Momentum

Law of Conservation of Law of Conservation of MomentumMomentum The total momentum in a group The total momentum in a group

of objects doesn’t change unless of objects doesn’t change unless outside outside forcesforces act on the objects. act on the objects.

pbefore = pafter