concepts key - Rohrbach Science

Name: _____________________________________________________ Date: _________ Per: ___
Unit 2 Review – KEY!!
2D Motion (Projectiles)
Concept Questions
1. What is the difference between a scalar and a vector?
A vector has magnitude and direction. A scalar has magnitude only.
2. A red and a yellow ball of the exact same shape and mass, are dropped from the same height
and at the same time. The red ball is dropped from rest, but the yellow ball is given an x
velocity. Which one (if either) should hit the ground first? Explain your answer! (hint: there is a
picture of this situation on p. 74 in your book)
They should hit the ground at the same time. Even though the yellow ball has an initial xvelocity, it has no initial y-velocity because it is not moving up or down. Therefore, in the ydirection, both balls start at rest and accelerate at a rate of -9.8 m/s2.
3. An arrow is shot into the air at a 45o angle. Explain what happens to its velocity and acceleration
in the x and y direction as it travels through the air.
In the x-direction, acceleration is zero and velocity is constant.
In the y-direction, acceleration is -9.8 m/s2. The velocity in the y-direction starts out in the
positive direction, but it slows down and comes to a stop at the peak of the arrow’s flight. It
then becomes negative and speeds up as it falls down.
4. Why is the horizontal (or x) velocity of an object constant in projectile motion?
There is no acceleration in the x-direction. The only acceleration of the object is due to gravity.
Gravity pulls down (a y-direction); it does not pull left or right.
5. Define projectile motion, give an example of it, and explain why that is a good example.
Projectile motion is motion in which gravity is the only influence acting on the object. For
example, if I throw a baseball forward, the only thing acting on after it has left my hand is
6. Why do projectiles follow curved paths through the air?
In the y-direction, the ball goes up, stops, and then comes back down. In the x-direction the
ball consistently moves forward. When you combine these two motions together, the ball
traces out a curved path.
Name: _____________________________________________________ Date: _________ Per: ___
θ = 60°
Big vy
θ = 45°
7. Why do projectiles travel their greatest x displacements at 45 angles?
At 45 degrees, the initial x- and y- velocities are equal to each
other. At larger angles, the x-velocity is too small, so the
projectile does not travel forward as quickly and therefore
doesn’t make it as far. At smaller angles, the y-velocity is too
small, so the projectile hits the ground sooner, preventing it
from travelling very far.
θ = 30°
Small vy
Small vx
Big vx
8. Explain what I mean by, “x and y components of a resultant vector.” (You may want to draw a
picture to aid you in your explanation.)
The x-component of a velocity is how fast the object is moving horizontally. The y-component
is how fast the object is moving vertically. (see above picture)
9. What are some factors that might affect the projectile motion of an object (it might be useful to
think about the simulation lab)?
As initial velocity increases, the projectile travels higher and farther.
As angle increases, the projectile travels higher. It travels the farthest at an angle of 45
10. What do we know about the x- and y- velocity and acceleration at the top of a projectile’s
At the top of the motion, the x-velocity is positive, the x-acceleration is zero, the y-velocity is
zero, and the y-acceleration is -9.8 m/s2.
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