Home » Mechanics

# Category Archives: Mechanics

## Newton second law and pendulum

A pendulum is hanged in the car.

Car starts to move by constant acceleration

How to measure acceleration by angle the pendulum?

Asked Afshin

Answer

If the pendulum makes an angle θ with the vertical, then a = g tan θ

This is based on the idea of pseudo force experienced by a body in an accelerated frame of reference.

When a body is in an accelerated frame of reference, it feels as if it is acted upon by a force equal to its mass multiplied by the acceleration of the system. Please have a look at the diagram below.

Please feel free to ask further doubts on the topic via comments

## Work Energy Problem

A 2kg hammer with a speed of 40m/s strikes a 100g nail driving it into a wooden block. If there is no dissipation of energy in this process and the frictional force applied by the wooden block is 8000N, how deep would be nail penetrate into the block?

Asked Jaisy

## What is Pseudo force?

What is Pseudo force? Asked Rahul Answer: The word pseudo means false. Remmeber pseudopodia, the false legs of amoeba.(Also called fictitious force,or phantom force) An additional force due to nonuniform relative motion of two reference frames is called a *pseudo-force.* *A pseudoforce does not arise from any physical interaction between two objects, but rather from the acceleration a of the non-inertial reference frame itself. A fictitious force on an object arises when the frame of reference used to describe the object’s motion is accelerating compared to a non-accelerating frame.*

[gview file=”http://www.askphysics.com/wp-content/uploads/2013/09/pseudoforce.pdf”]

## Force – Impulse – Momentum

Hi All! I’m having mega problems with one part of a physics assignment. Looking for any help.

The details are as follows:

The diagram shows the normal force on Christine’s feet vs. time, as recorded by a force plate while she stands still initially (until point B), then jumps off the plate. (This trial is separate from the one in the previous problem. The graph is over-simplified and idealised, compared to reality.) When her feet leave the plate, the normal force is zero.

1)What is the magnitude of the (upward) impulse generated by the normal force of Christine during the time interval of her jump off the plate?

2)What is the magnitude of the downward impulse due to gravity during this interval?

3)What is the net impulse which propels her upwards when she jumps off the plate? (Recall, the net force on her is the normal force minus the force of gravity.)

4)What is her change in speed upwards for this process?

The graph has NORMAL FORCE (N) on the y-axis and TIME (s) on the X axis.

The line is at a constant 550 N until point B (1.75 seconds) at which time it shoot up vertically to 1550 N at a time of 1.95 seconds. It peaks at this time and position then drops down to 0 N at 2.15 seconds.

Thanks in advance for any guidance that can be provided!

… **Sara**

###### Related articles

## Why are you hurt more when you punch a wall then when you punch a sponge?

Why are you hurt more when you punch a wall then when you punch a sponge?

Asked Shivangi

Answer:

This is explained on the basis of impulse momentum principle.

Impulse of a force is the product of Force and time and is equal to the change in momentum produced.

The overall effect of a force is the change in momentum produced.

When we are punching, the effect on hitting is the stopping of the fist moving initially with a particular speed.

So, if we hit on a wall, the change in momentum takes place in a small interval of time and therefore the average force involved is very large and therefore it hurts.

When we hit on a sponge, the time taken will be more and the average force will be less and therefore it hurts less.

## Momentum and Kinetic Energy

How it is possible to double the momentum of a body by increasing the kinetic energy four times?at the time what happen to mass?

Answer:

When we say, KE is increased, we mean to say by increasing speed alone unless otherwise specified.

Here mass remains constant.

Please note that the question says, KE **of a body** increased 4 times; which suggests that the body is the same and KE is increased by increasing the speed alone.

## Physics of Diving

When a high diver in a swimming event springs from the board and “tucks in”, a rapid spin **result**. Why is this?

Answer: This is the consequence of **conservation** of **angular** momentum.

The **angular** momentum of a **body** is the **product** of Moment of inertia (*A measure of rotational inertia and it depends on the mass as well as distribution of mass about the axis of rotation. Farther the masses, greater will be the rotational inertia*) and the

**angular**

**velocity**(The speed of

**rotation**)

The **angular** momentum of a **body** remains unchanged in the **absence** of any external torque.

When the diver dives, he is giving his **body** a turning and takes off with his limbs stretched. In the stretched **position**, the **moment of inertia** is more. When he “tucks in”, the **moment of inertia** decreases. But since this happens without any external torque, it would **result** in an increase in **angular** **velocity** so as to keep the **angular** momentum constant.

You must be logged in to post a comment.