Home » KINEMATICS

# Category Archives: KINEMATICS

## Time Dilation problem – Special Relativity

With what speed will a clock have to be moving in order to run at a rate that is one half the rate of a clock at rest?

Posted by Mona

Answer:

Using the formula for Relativistic Time (according to Special Theory of Relativity)

√[1 – (v²/c²)] = ½ → v² = (3/4)c²

v = [√3/2]*c = .866c

## Angle between acceleration and velocity in projectile motion

What is the angle between the directions of acceleration and velocity at the highest point of projectile?

Answer:

At the highest point of a projectile, the vertical component of velocity is zero and the velocity is entirely **horizontal.** The direction of acceleration (due to gravity) is vertically downwards throughout the motion of the projectile.

Therefore, **the angle between the direction of acceleration and velocity at the highest point of a projectile is 90 °**

###### Related articles

## The problem of a rock thrown vertically up

**A rock is thrown vertically upward from the ground with an initial speed 15m/s.**

**a. how high does it go b. how much time is required for the rock to reach its maximum height? c. what is the rock’s height at t=2.00s?**

*(Posted by Merhawi)*

**Answer:**

(a)

u=15m/s

a=-10m/s^{2}

v=0 m/s (at the max height)

S=?

v^{2}-u^{2}=2aS

S=v^{2}-u^{2}/2a

=225/20

**=11.25 m**

(b) From the above case

using v=u+at

t=v-u/a=**1.5s**

(c) Use S = ut + 1/2 at^{2}

put t=2s, u = 15m/s, a=-10m/s^{2}

S=30 – 20 = **10 m**

*(If you use g = 9.8 m/s ^{2 }The answers will be slightly different)*

## Equations of Motion – Images for easy reuse

Here you can find the equations of motion in the form of images which you can use in your documents.