Frequently Asked Questions: Motion

Q. When a car accelerates quickly, why are the people in the car forced backwards into the seat?
A. Inertia is the tendency for an object at rest to remain stationary. The people in the car are still when the car accelerates and they have a tendency to remain still. The overall effect is to be appear to be thrown back into the seat.

Q. Why does a moving skateboarder fly forwards when he/she collides with a gutter?
A. Inertia is the tendency for a moving object to continue moving in the same direction at the same speed. The skateboarder continues with the same movement in spite of the skateboard being left in the gutter.

Q. How can you tell the difference between a hard-boiled egg and a fresh egg by spinning each?
A. The hard-boiled egg will spin faster than the raw egg.

Q. Why is it dangerous to drop a pebble from the top of a tall building onto the footpath below?
A. Gravitational acceleration on earth is about 10 m/s2. This means that every second after the pebble has dropped its speed will increase by 10 m/s. For example, after 1 second the speed will be 10 m/s, after 2 seconds it will be falling at 20 m/s, and after 10 seconds it will be falling at 100 m/s. That means that after 10 seconds, it could travel the length of a football field in 10 seconds. At speeds like that, a tiny pebble would be like a bullet when it hit the footpath.

Q. What's a 'g'? How many g's can humans withstand before becoming ill?
A. 'g' means gravitational acceleration of 9.8 m/s2. Rollercoasters usually have a maximum of about 4g, because people tend to vomit if it is more. Fighter pilots and astronauts tend to be able to withstand higher g's but there a limit at which they may black-out.

Q. Should you walk or run in the rain to avoid getting wet?
A. You will get equally wet.