Question

1. How could you remove the placemat without removing the plate?

2. Suppose you jumped onto a bathroom scale. The scale initially indicates a high weight and then settles down to your actual weight. Is this statement true or false? Could you explain why it is true or why it is false?

3. What is the weight of a person in free fall? Could you please show your work?

Answer 1

1. If we pull the placemat really fast from beneath the plate, then we can manage to remove it without moving the plate. This is due to the law of inertia. Since plate has a certain amount of inertia, i.e. the ability to oppose a change, so if it does not experience a force for long enough, then it will not get subjected to motion. This is why the plate remains motionless.

2. This statement is true. When you jump on the scale, just before landing on the scale, you will be accelerating under gravity, i.e. an extra force is associated according to newton's second law. so the moment you land on the scale, the scale also measures this extra force along with your weight for a small period until you come to rest on the scale. After you hit the ground, you are no longer accelerating and so the scale shows your actual weight.

3. weight under free fall is zero

this can be explained by newton's second law

suppose a man of mass m is falling under gravity. the downward force due to gravity will be "mg"

and the acceleration a= - g

If we consider a normal force, N acting upward then we get from newton's second law:

N - mg = ma

but a = - g

so, N - mg = -mg

=> N = 0

i.e. weight = 0 because the normal force defines weight.