Newton's Second Law of Motion Put to the Test
This activity uses a clever apparatus to see exactly how Newton’s First law interacts with Newton’s Second law. The activity take 20-30 minutes. Intended for ages 11 and up.
What you'll need:
- Second Law of Motion Apparatus (PH0319)
- Paper and Pencil to record your observations
- Assemble second law of motion apparatus. Remove the square nut from the aluminum dowel handle. Insert the threaded part of the handle through the hole on the side of the apparatus. Hold the square nut to the tip of the threaded section and rotate the dowel clockwise until the dowel is firmly attached to the apparatus.
- Prepare the second law of motion apparatus to be loaded. Slide the black lever toward the spring and rotate it down behind one of the two black knobs. The lever should catch so the steel dowel sticks out past the spring.
- Put the steel ball that has a hole (we’ll call this the holey ball) on the steel dowel sticking out of the apparatus. Place the steel ball with no hole (we’ll call this the solid ball) on the grey mount on the opposite side from the holey ball. The apparatus is now loaded. See the picture below which shows a correctly assembled and loaded apparatus.
Now that the apparatus is loaded let’s think through what Newton’s first two laws of motion tell us will happen when the lever is pushed up, releasing the energy in the spring. To jog your memory:
Newton’s First law: Unless acted upon by a force, an object at rest will stay at rest and an object in motion continues to move in the direction at the same speed.
Newton’s Second law: The acceleration of an object is equal to the net force on the object divided by its mass.
Take a moment to write down your prediction.
When the lever is released, which ball will reach the floor first?____________________________
- Now place the apparatus on the corner of a level surface like a table, so that each side extends off the edge. This will ensure that when the lever is released each ball will proceed unimpeded toward its final destination.
- Close your eyes and push the lever up. Listen carefully to hear which ball hits the floor first. Which ball hit first?
- If a cell phone is available, repeat the experiment while a parent or sibling records a video. Use the slow-motion capture setting if available. What happens?
By now you should be convinced that the balls hit the floor at exactly the same time.
Why does this happen?
Let’s think through the forces immediately after the steel balls have been released (holey ball) or launched (solid ball).
What force acts on the holey ball in what direction?______________________________________
What force acts on the solid ball in what direction?_______________________________________
What is the direction of the velocity of the holey ball?_____________________________________
What is the direction of the velocity of the solid ball?______________________________________
The answer to the first two questions is the force of gravity which is in the downward direction. The holey ball is at rest (remember this is immediately after the balls have been released). The solid ball is moving horizontally with constant speed.
Will a force in the downward direction change the motion in the horizontal direction? Newton’s Second law says the acceleration is proportional to force, so the acceleration has to be in the same direction as the force. So the acceleration will only be in the downward direction.
Will the motion in the horizontal direction change? Newton’s First law says that unless acted upon by a force, the motion will remain unchanged. So the motion in the direction which experiences no force (there is no horizontal force) will remain unchanged. The holey ball will have no horizontal motion. The component of the velocity of the solid ball in horizontal direction will not change.
So both balls accelerate toward the ground at the exact same rate. So they must hit the floor at the exact same time. And the horizontal position of the solid ball moves at the same speed, independent of the motion in the downward direction. The motion in each direction must be considered separately!
Here are some other things to try. See if you can figure out why the result is the same or different.
- Repeat the experiment using the other black knob.
- Add air resistance by taping a paper streamer to each ball.
- Tilt the apparatus so the initial velocity of the solid ball is not solely in the horizontal direction. You will have to get a bit clever to keep the holey ball from sliding off the steel dowel early.