Projectile Motion Quiz 1

INSTRUCTIONS: Answer the quiz below by submitting your answers by email to melitabsarigumba@yahoo.com

1. What is a projectile?

   • any object that is thrown
   • an object that has multiple forces acting upon it
   • an object that has one the one force of gravity acting upon it
   • none of the above

2. An example of projectile motion is...

   • a golf ball being hit
   • a water fountain
   • a rocket
   • all of the above

3. The projectile will drop below the straight line of projection because...

   • the force of gravity acting upon it
   • it looses speed
   • it no longer accelerates
   • all of the above

4. The definition of range is...

   • the horizontal distance traveled of a projectile
   • how fast a projectile is moving
   • the amount of time a projectile is in air
   • none of the above

Projectile Motion Quiz 2

INSTRUCTIONS: Submit your answers to this quiz by email to melitabsarigumba@yahoo.com

1. At what part of its trajectory does a projectile have minimum speed?

the instant it is thrown
half-way to the top
at the top of its path
at the instant it hits the ground

2. A bullet is fired horizontally with a velocity of 1000. m/s. Neglecting air resistance, at the end of 1.0 s, how far is the bullet from the muzzle?

1000. m horizontally and 4.90 m vertically
1000. m horizontally
1000. m horizontally and 1000. m vertically
1000. m

3. Which statement is not true about a projectile fired at an angle other than 90^° from the ground and air resistance is not a factor?

the time to reach the highest point is the same as the time to fall to the ground
the initial vertical velocity is equal to the final vertical velocity, but in the opposite direction
the horizontal velocity does not change
the maximum range occurs when fired at a 45^° angle
at the highest point of its trajectory, both horizontal and vertical velocities are equal to zero

Neglecting air resistance, a horizontally fired projectile maintains its horizontal component of velocity because:

it is not acted upon by any forces
it is not acted upon by any horizontal forces
it has no initial vertical component of velocity
the net force acting on it is zero

Projectile Motion

A projectile is an object upon which the only force acting is gravity. There are a variety of examples of projectiles. An object dropped from rest is a projectile (provided that the influence of air resistance is negligible). An object which is thrown vertically upward is also a projectile (provided that the influence of air resistance is negligible). And an object is which thrown upward at an angle to the horizontal is also a projectile (provided that the influence of air resistance is negligible). A projectile is any object which once projected or dropped continues in motion by its own inertia and is influenced only by the downward force of gravity.

By definition, a projectile has only one force acting upon it - the force of gravity. If there was any other force acting upon an object, then that object would not be a projectile. Thus, the free-body diagram of a projectile would show a single force acting downwards and labeled force of gravity (or simply F_grav). Regardless of whether a projectile is moving downwards, upwards, upwards and rightwards, or downwards and leftwards, the free-body diagram of the projectile is still as depicted in the diagram at the right. By definition, a projectile is any object upon which the only force is gravity.

Projectile Motion and Inertia

Many students have difficulty with the concept that the only force acting upon an upward moving projectile is gravity. Their conception of motion prompts them to think that if an object is moving upward, then there must be an upward force. And if an object is moving upward and rightward, there must be both an upward and rightward force. Their belief is that forces cause motion; and if there is an upward motion then there must be an upward force. They reason, "How in the world can an object be moving upward if the only force acting upon it is gravity?" Such students do not believe in Newtonian physics (or at least do not believe strongly in Newtonian physics). Newton's laws suggest that forces are only required to cause an acceleration (not a motion).

A projectile is an object upon which the only force is gravity. Gravity acts to influence the vertical motion of the projectile, thus causing a vertical acceleration. The horizontal motion of the projectile is the result of the tendency of any object in motion to remain in motion at constant velocity. Due to the absence of horizontal forces, a projectile remains in motion with a constant horizontal velocity. Horizontal forces are not required to keep a projectile moving horizontally. The only force acting upon a projectile is gravity!

(Click here for animated discovery on projectiles)