Newton's First Law

Rolling soda can

A challenge quiz about Newton's First Law

Isaac Newton

Isaac Newton revolutionized physics. He discovered how to predict the orbit of a planet or the trajectory of a golf ball, using the same simple equations. But, Newton's physics contradicts many of our "commonsense" beliefs.

How does your intuition about the physical world compare to Newton's physics? Take this challenge quiz about his First Law.

The speeding trampoline

A tractor steadily moves down a road, pulling a trailer with a trampoline on it. A man on the trampoline is jumping up and down. Assume that wind resistance is not a significant factor.

Man jumping up and down on a moving trampoline

When he comes back down from a straight jump, where does he land on the trampoline surface, compared to the point from which he jumped?

a. Forward of the point from which he jumped b. At the same point from which he jumped c. Behind the point from which he jumped d. It depends on how fast the tractor is moving

Here's a video clip of the moving trampoline.

Surprised? Many people expect that while the man is hovering in flight, he should be falling behind the trailer (answer c). But he doesn't fall behind. He keeps up! (answer b)

This is an example of Newton's First Law of motion. Once the jumper has forward motion, he gets to keep it, unless there's a force that slows him down. If there's no such braking force, then no force is needed to maintain the motion. Objects have inertia, that is, they resist changes in motion, i.e., speeding up, slowing down, or changing direction. If there's no net force, the natural tendency of an object is to keep moving in a straight line at a constant speed.

Another example is a soda can that rolls to the front on the floor of a braking bus or train. The vehicle is slowing down, but the can naturally "wants" to keep going. This is why seat belts are a good idea.

Newton's First Law contradicts the commonsense notion that moving objects naturally slow down and come to rest, as if their energy "runs down." So why do we think that? Because most real-world situations have friction, which is a real braking force. To keep the tractor going, it needs an engine to keep pushing it. But that's not because it slows down by nature, but rather because a push is needed to offset the effect of friction. Our intuition is biased from living in a friction-ridden world. If the tractor were moving fast enough for air friction to be important, then the jumper would be slowed down during his jump, and then answer d starts to make sense. Also, if he uses his energy of motion to power his toaster, that will slow him down because the energy is being converted to another form.

Now here's the startling implication. Suppose that that the tractor had a closed chamber to shield the trampoline jumper from the wind. The tractor could go 1000 miles per hour. The jumper would still keep up!

The speeding coffee break

Imagine riding in a closed vehicle with a cup of coffee that you've set on a tray table. In which kinds of vehicles would the coffee cup spill or fly off the tray because of the vehicle's motion?

a. A bullet train going straight at a steady speed of 200 miles per hour b. A commercial airline flight flying straight at a steady cruising speed of 550 miles per hour c. Vehicle Earth going 66,000 miles per hour in its orbit around the sun d. A car accelerating from 0 to 60 miles per hour in 5 seconds e. A car going 40 miles per hour around a tight circle on an exit ramp f. Both d and e g. All of c, d, and e

Passengers relaxing on a air flight as if they felt they were on the ground

If you've flown on a plane, you would not pick answer b. There could be severe turbulence but that's not steady speed. We relax and drink coffee on a train or plane and don't worry about getting scalded. In fact, if the motion of the vehicle is perfectly smooth and steady, we won't be able to use the cup of coffee to detect the motion. It would behave exactly the same way as in a stationary vehicle.

The correct answer is f. Sharp accelerations and tight turns spill the coffee. Steady speed, even fast steady speed, does not.

Many might have difficulty about c. If you were going 66,000 mph, they ask, wouldn't you know it? According to Newton's physics, the answer is no. What's the misconception here? We confuse speed with acceleration. Compare the airline flight with the accelerating car. The coffee cup spills in the car but not on the plane, even though the plane goes ten times faster. That's because the car is accelerating whereas the plane is moving at a steady speed.

It's easy to confuse speed with acceleration because situations of rapid motion often bombard our senses with cues that tell us we're moving. Those cues are due to unsteady motions. An engine vibrates, or we hit a bump in the road, or we hear the air colliding with the exterior surface of the vehicle.

Again, there's a startling implication. We feel the bouncing and rocking of an airplane because of wind and air pressure variations. We cannot feel its steady 550 mph speed, nor does that speed have any physical effect on people or other things on board!

Is there any way we can detect steady motion? That brings us to the next question.

The Speeding radar cop

Car and radar cop car moving toward each other

We're driving down a road at 30 miles per hour. In the distance, we see a police car coming the opposite way. He's also driving at 30 miles per hour. When the officer is about 100 yards away, he sticks his radar gun out the window and clocks our speed. What reading does he get?

a. Zero b. 30 miles per hour c. 60 miles per hour d. None of the above

The correct answer is c, 60 miles per hour. If the traffic court judge doesn't understand the physics, we might get unfairly fined. The radar tells the cop how fast the other car is moving relative to his own. This is one example of a more general principle. We can detect our steady motion only relative to other things. Look out an airplane window at a cloud cover that hides the ground, for example, and we see the clouds going by. We see a cloud's motion relative to the plane without being able to tell how much of that motion "belongs" to the cloud rather than to the plane.

And again, if we pull down the window shades, the steady motion of the plane cannot be detected by a watching a cup of coffee, dropping a ball, or doing any other experiment!