EXAM 1

𝑦=0.0427 m

Bob can throw a dart at a speed of 30 m/s. If the dart board is 2.8 m away, how far below the bullseye will the dart land if the direction of the initial velocity of the dart was horizontal and aimed at the bullseye?

Yes, acceleration equals a change in direction, therefore, when the earth is rotating, it is indeed changing direction.

Does the Earth accelerate as it goes around the Sun? Is so why?

Newton's Third Law

For every action there is an equal and opposite reaction

Newton's Second Law

Force = mass x acceleration

9.3 m/s, 10.8s

If a fast sprinter can run 40 m in 4.3 s, what is their average speed and how fast could they run 100 m?

a= -3.59 m/s2; vi= 26.8 m/s

If the braking distance for a car traveling at 60 mi/hr is 100 m, what is the acceleration of the car in m/s 2?

60mi/hr, west

Minneapolis is about 90 miles away from here. If it takes 90 minutes to drive there, what is your average velocity?

0 Au/mo

The Earth goes around the Sun once every year. If the distance covered by the Earth is 6.28 A.U., what is the average velocity of the Earth in A.U./mo.?

Ignored acceleration

The world record for the 100 m dash is less than 10 s. What did we do wrong?

E

What does Kepler's first law state? A: there is a force on the planets B: the planets can have moons going around them C: the sun is much more massive than the planets D: A and B E: A and C

A

What does keplers second law tell us about gravity? A: gravity is stronger when closer to the sun B: gravity is weaker when closer to the sun C: the strength of gravity does not depend on the distance from the sun

The massive cleaver has more inertia than the knife. That means it is more able to resist changes in motion. Once you get the cleaver started, it is much less likely to stop inside the vegetable which makes chopping less of a chore.

Why does a massive cleaver work better for cutting vegetables than an equally sharp but less massive knife?

-99.0m/s, 0m/s

With what velocity would raindrops hit the ground if they freely fell from rest at a height of 500m? What is the initial speed of the raindrops?

a: 𝐹𝑛𝑒𝑡 =12 N b: 𝑚=2.4 m/s2

a 5kg block is acted upon by three horizontal forces, as shown in the diagram: (a) what is the net horizontal force acting upon the block? (b): what is the horizontal acceleration of the block?

𝑣𝑓 =−43.4 m/s

a ball is thrown downward with an initial velocity of 14 m/s, using this approx. value of g= 10 m/s2, what is the velocity of the ball 3 seconds after it is released?

1.63s

a ball is thrown upward with an initial velocity of 16 m/s. How much time is required to reach the highest point?

(a): 𝑣𝑓 =−3.92 m/s (b): 3 m/s.

a ball rolls off a table with a horizontal velocity of 3 m/s to the right. If it takes 0.4 sec for it to reach the floor, (a) what is the downward component of the balls velocity just before it hits the floor (g=10 m/s2): (b) what is the horizontal component of the balls velocity just before it hits the floor:

The ball is moving upward so the initial velocity vector points in the upward direction. The acceleration is caused by gravity so that the acceleration vector points downward. The acceleration due to gravity never changes so the acceleration vector is a constant 9.8 m/s2 downward.

a ball thrown straight upward moves initially with a decreasing upward velocity. What are the directions of the velocity and acceleration vectors during this part of the motion? Does the acceleratioon dcrease also?

The two forces acting on the boy are the force of gravity pulling the boy downward (W) and the floor pushing up on the boy (N). These forces do not constitute an action/reaction pair. The reaction force to gravity (the Earth) pulling on the boy is the boy pulling up on the Earth. The reaction force to the floor pushing upward on the boy is the boy pushing down on the floor. Action/reaction pairs always deal with different objects.

a boy sits at rest on the floor. What two vertical forces act upon this boy? Do these forces constitute an action/reaction pair as defined by Newtons third law of motion?

(a): 12 m/s (b): 110 m

a car moving with an initial velocity of 32 m/s slows down at a constant rate of -4 m/s2: (a): what is its velocity after 5 seconds of deceleration? (b): what distance does the car cover in this time?

𝑚=8 kg

a net force of 32N acting upon a wooden block produces an acceleration of 4.0 m/s2 for the block. What is the mass of the block?

(a): 𝑡 =5.10 s (b): 𝑥 =255 m

a projectile is fired at an angle such that the vertical component of its velocity and the horizontal component of its velocity are both 50 m/s: (a) using the approximate value of (g= 10 m/s2), how long does it take for the projectile one to reach its high point? (b) what horizontal distance does the projectile travel in this time?

As the rock moves upward, gravity continuously accelerates it in the downward direction causing the rock to slow down. It is traveling at its slowest speed near the highest point it reaches. This means that the rock spends more of its time at the top 5 m of its arc than in the first 5 m, where it is moving at a higher speed.

a rock is thrown straight upward, reaching a height of 20 meters. On its way up, does the rock spend more time in the top 5 meters of its flight or the first five meters of its flight?

(a): 𝑣𝑓 =17.2 m/s (b): 𝑥 =15.1 m

A ball is thrown straight up and returns to the ground 3.5 seconds later. It spends half its time traveling upward and the other half going down. (a) With what speed was the ball thrown upward? (b) What was the maximum height of the ball above the ground? (c) Sketch graphs of the height vs. time, velocity vs. time, and acceleration vs. time for the ball while it was in the air. Make sure you have appropriate labels and numbers on your graphs.

velocity north, acceleration south

A car is moving north at 30mi/hr when the driver applies the brakes. What are the directions of the velocity and acceleration? A: both north B: both south C: velocity north, acceleration south D: velocity south, acceleration north

No, since the car is starting to move, it is changing velocity from zero to something non-zero, so therefore there must be an acceleration.

A car just starting up from a stop sign has a velocity of zero the instant it starts, must the acceleration of the car also be zero?

No, if the car were to travel backward, it would have a negative velocity and this whole graph is positive

A car moves along a straight section of road so that its velocity varies with the time as shown on the graph: (a): does the car ever go backward

(a): 24.4 m/s (b) 53.3 m

A car traveling with an initial velocity of 16 m/s accelerates at a constant rate of 2.8 m/s2 for a time of 3 seconds: (a): what is its velocity at the end of this time? (b) what distance does the car travel during this process?

displacement

the change in position of an object

instantaneous speed

the speed of an object at one instant of time

velocity

the speed of an object in a particular direction

6 m/s2

the velocity of a car increases with time, as shown in the graph: (a): what is the average acceleration between 0 seconds and 1 second?

1 m/s2

the velocity of a car increases with time, as shown in the graph: (b): what is the average acceleration between 1 second and 4 seconds?

2.25 m/s2

the velocity of a car increases with time, as shown in the graph: (c): what is the average acceleration between 0 seconds and 4 seconds?

-0.5 m

where is object at t=1 s?

-1 m

where is object at t=2 s?

-.5 m

where is object at t=3 s?

a: 𝑎 =4 m/s2 b: 𝐹𝑛𝑒𝑡 =160 N c: 𝐹𝑓 =190 N d: If the crate moves with constant velocity, then the acceleration must be zero. If there is no acceleration, then the net force must be zero as well. That will happen when the force of friction to the left balances the pull to the right. So if the force of the pull is 190 N, the crate will move with constant velocity.

A rope exerts a constant horizontal force of 350N to pull a 40kg crate across the floor. The velocity of the crate is observed to increase from 1 m/s to 9 m/s in a time of 2 seconds under the influence of this force and the frictional force exerted by the floor on the crate. (a): what is the acceleration of the crate? (b): what is the net force acting upon the crate? (c): what is the magnitude of the frictional force acting upon the crate? (d): what force would have to be applied to the crate by the rope in order for the crate to move with a constant velocity?

Yes, the hurdler is a projectile. When she leaves the ground, the only important force on her is gravity. Just after she leaves the ground, her velocity has components that are both up and forward so the velocity vector is angled upward. At the top of her arc, there is no vertical velocity so her velocity vector is completely horizontal. Just before she hits the ground, she is moving both forward and downward so her total velocity vector is angled downward. The acceleration is caused by gravity and points straight down for all points on her arc.

A sprinter jumps over a hurdle. Is she a projectile while she is in the air? Why or why not? Make a sketch of the path of the hurdler going over the hurdle. Draw and label the velocity and acceleration vectors just after she jumps, at the top of her jump, and just before she lands.

a: The net force on the book is 0, 𝑊 =7.84 N b: 𝑁=19.84 N

An 0.8kg book rests on a table. A downward force of 12N is exerted on the top of the book by a hand pushing down on the book: (a): what is the net force of the book? is it accelerating? (b): what is the magnitude of the gravitational force acting upon the book?

Newton's First Law

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

does

anytime an object changes direction it (does/does not) accelerate

vectors

displacement, velocity are (scalars/vectors).

scalars

distance, time, temperature are (scalars/vectors)?

speed

distance/time

scalars

quantities that are fully described by a magnitude (or numerical value) alone

vectors

quantities that have both a magnitude and a direction

(2x/t2)- (2v0/t)

solve the following expression for a: x=v0t + 1/2at2

same speed when they hit the ground

someone stands at the edge of a cliff and throws a ball upward. They throw another one straight down at the same speed. Which ball has the larger speed when it hits the ground? A: The one thrown upward B: The one thrown downward C: same speed when they hit the ground