Conceptual Physics

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What is a net force? How do you calculate net force?

Net force is the sum of all forces acting on an object. \n Add all forces acting in the same direction and subtract those in the opposite direction.

When all the forces acting on an object are balanced, the object will ________

continue doing what it was doing, stay at rest, or keep moving at a constant speed

When there are unbalanced forces acting on an object, the object will _____

accelerate

An object experiences a rightward force of 5 N, leftward force of 3 N, an upward force of 10 N, and a downward force of 10 N. What is the net force of the object?

2 N to the right

An object experiences a rightward force of 5 N, leftward force of 5 N, an upward force of 10 N and a downward force of 10 N. What is the net force of the object?

0 N

Name vector qualities

displacement, velocity, force, acceleration, momentum, and impulse

An object experiences 16 N of upward motion, 16 N of downward motion, 21 N of rightward motion, and 42 N of leftward motion. What is the net force? Are the forces balanced or unbalanced? Will it accelerate if it's in motion?

21 N leftward, unbalanced forces, it will accelerate

An object experiences 13 N of upward motion and 13 N of downward motion. What is the net force? Are the forces balanced or unbalanced? Is it possible that the object is in motion at a constant velocity?

0 N, balanced forces, it is possible that the object is in motion at a constant velocity

What does Newton's first law explain?

inertia: why a stationary object stays still or accelerates \n why a moving object maintains constant speed or accelerates

Newton's First Law is also known as the law of ______

inertia

How are mass and inertia connected

more mass = more inertia

Newton's second law explains:

why an object accelerates with a particular acceleration value \n f = ma

Newton's second law is also known as the law of _____

acceleration

The acceleration of an object is ______ proportional to the net force

directly

The acceleration of an object is ______ proportional to the mass

inversely

f = ma \n A sled has a mass of 20 kilograms. It speeds up with an acceleration of 3 m/s 2 . What is the net force acting \n on the sled?

60 N

\n

m = f/a \n Trevor is accelerating down a water slide with an acceleration of 2 m/s 2 . The net force exerted on her is 80 \n Newtons. What is Trevor's mass?

40 kg

f/m = a \n A net force of 180,000 Newtons makes a 20,000 kilogram rocket accelerate up. How much is the \n acceleration?

9 m/s2

Suppose a cart is being pushed by a certain net force. If the net force is doubled, by how much does the \n acceleration change?

the acceleration doubles

Suppose a cart is being moved by a certain net force. If a box is dumped into the cart, so its mass is doubled, by how much does the acceleration change?

the acceleration is halved

If a loaded truck can accelerate at four m/s 2 and loses its load so it is only half as massive, what acceleration can it attain for the same driving force?

8 m/s2

Newton's 3rd law explains why:

Every force is accompanied by a reaction force

Newton's 3rd of law of motion is also known as the law of _____

interaction

One example of Newton's Third Law of Motion

A swimmer pushes back on the water to pull themselves forward

If a bicycle and a massive truck have a head-on collision, upon which vehicle is the impact force greater? \n Which vehicle undergoes the greater change in acceleration?

equal impact force, bicycle change is bigger

When a rifle is fired, how does the size of the force of the rifle on the bullet compare to the force of the \n bullet on the rifle? - How do the accelerations of the rifle and bullet compare?

force is equal, bullet’s acceleration is greater

Why is it easier to walk on a carpeted floor than on a smooth, polished floor?

there is more friction

If the force of friction acting on a sliding crate is 100 N, how much force must be applied to maintain a \n constant velocity? - What will be the net force acting on the crate? - What will be the acceleration?

100 N of force, 0 N of net force, 0 acceleration

Free-falling objects are under the sole influence of what force?

gravity

In the absence of ________________________________________________ and other external forces, all objects free \n fall towards Earth with the same acceleration value, despite ________________________________________________.

air resistance, differences in mass

If a freely falling object were somehow equipped with a speedometer, its speed reading would increase \n each second by:

9\.8 m/s2

An object falls from a skyscraper. Ignoring air resistance, what will its velocity be after 6 seconds of \n falling?

6 x 9.8 m/s2 = 58.8 m/s

What three factors affect air resistance?

air density, object speed, object cross-sectional area

What letter is used to represent momentum? What are the units of momentum?

momentum = p

units = kg . ms

What determines the direction of momentum? Are the direction of an object's momentum and the \n direction of the net force on an object always the same?

direction of momentum = direction of motion

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No, for example when an object is slowing down

What is the law of momentum conservation?

For an isolated system, total momentum before collision = total momentum after

The momentum of an object depends on the object's _________________________ and ___________________________.

mass and velocity

What is momentum?

The product of mass and velocity, the tendency of an object to keep moving

If an object has a velocity of 0 m/s, what is its momentum?

0

Calculate the momentum of a 1200 kg car with a velocity of 25m/s.

1200 x 25 = 30,000 kg.m/s

The parking brake on a 1200 kg automobile has broken, and the vehicle has reached a momentum of 7800 \n kg.m/s. What is the velocity of the vehicle?

v = p/m \n 6.5 m/s = 7800/1200

A school bus that is traveling at 11.1 m/s has a momentum of 152625 kg.m/s. What is the mass of the bus?

p/v = m

13750 kg

What is the formula for impulse? What are the units of impulse?

force x time = impulse \n Newton x seconds, or N-s

How does impulse relate to momentum?

impulse is the change in momentum

Given the information below, what is the velocity of the truck after it collides with the car? \n Before: Truck's mass = 3000kg Truck's velocity = 10 m/s \n car's mass = 1000kg car's velocity = 0 m/s \n \n After: Truck's mass = 3000kg Truck's velocity = ??? m/s \n car's mass = 1000kg car's velocity = 15 m/s

multiply the mass by the velocity for the car and the truck before and after, find what makes both sides equal \n \n a = truck b = car \n before after' \n MaVa + MbVb = MaVa' + MbVb' \n 3000(10) + 1000(0) = 3000(Va') + 1000(15) \n 30,000 = 3000(Va') + 15000 \n -15000 -15000 \n 15000 = 3000(Va') \n Va' = 5 m/s

A boy kicks a football with a force of 20 N. The time that the force acts is 0.3 s. Calculate the impulse on \n the ball.

force x time = impulse \n 20 x 0.3 = 6 N-S

A 5 N force is applied to a 3 kg ball to change its velocity from +9 m/s to +3 m/s. The impulse on the ball is \n ____________ N-s.

(impulse is the change in momentum, so to see the change you have to subtract the momentums) \n (momentum/p = mass x velocity, impulse = force x time) \n \n 3(9) - 3(3) = 18 N-s

A 40-kilogram mass is moving across a horizontal surface at 5.0 m/s. What is the magnitude of the net \n force required to bring the mass to a stop in 8.0 seconds?

p = mv \n mv = 40(0) - 40(5) = -200 \n mv = impulse = ft - f = mv/t = -200/8 = -25 N

How has the relationship between collision force and collision time been used in safety engineering?

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Life saving devices like airbags have been developed to extend the collision time so that collision force is lessened and less damage occurs

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