# Question #beffb

##### 1 Answer

Apr 21, 2017

a)

b)

c)

#### Explanation:

We have height (or displacement) at time

# h(t) = -4.9t^2+9.8t #

Differentiating wrt

# doth(t) = -9.8t+9.8 #

**a) When is the rocket travelling at 9.8 m/s down?**

# doth(t) = -9.8 #

# :. -9.8t+9.8 = -9.8 #

# :. -9.8t = -19.6 #

# :. t = 2 \ s#

**b) When is the rocket travelling at 9.8m/s up?**

# doth(t) = 9.8 #

# :. -9.8t+9.8 = 9.8 #

# :. -9.8t = 0 #

# :. t = 0 \ s#

**c) How fast is the rocket travelling after 1 second?**

When

# doth(t) = -9.8+9.8 #

# " "= 0 \ ms^(-1)#