Well it definitely isn't any of the other ones because they're all positive.

Try x(19.99)

Try x(20)

Try x(20.01)

The values for x keep decreasing as time goes on at 20 s. So velocity must be negative at that moment.

Anyway, instantaneous velocity is the derivative of position with respect to time.

v(t) = dx/dt = 2bt - 3ct^2

At t = 20.0 s

v(20) = 2(2.4 m/s^2)(20 s) - 3(0.12 m/s^3)(20 s)^2 = -48 m/s

However, SPEED is positive 48 m/s. Because speed is just the magnitude of velocity and doesn't care about direction, so it can't be negative. Not sure if that was a typo or what. Tread carefully :)

# A car is stopped at a traffic light. It then travels along a straight road so that its distance from the light is given by?

x(t) = bt2 − ct3

, where b = 2.40 m/s2 and

c = 0.120 m/s3

. What is the instantaneous speed of the car at t = 20.0 sec ?

A) −48 m/s

B) 0

C) 48 m/s

D) 96 m/s

I know the answer is A, but can someone explain to me why

, where b = 2.40 m/s2 and

c = 0.120 m/s3

. What is the instantaneous speed of the car at t = 20.0 sec ?

A) −48 m/s

B) 0

C) 48 m/s

D) 96 m/s

I know the answer is A, but can someone explain to me why

## 1 Answers

### Trending

- If an electric skillet has 6 numbers what is 3 in degrees?
- Physics question on waves and sound?
- I am trying to understand forces and momentum, please help. The main thing is, if a e.g. bat knocks a ball, once the bat stops touching the?
- In how much time a 1000L capacity water tank fitted at 10m would take to fill by 0.5hp water pump?
- What would happen if a nuclear power plant explodes in a small town?
- Could railgun rounds lose velocity in space?
- What would happen if we launched all our nuclear weapons at the sun?
- Is it possible for a person to run at the speed of light?
- I am trying to understand forces and momentum, please help. The main thing is, if a e.g. bat knocks a ball, once the bat stops touching the?
- How can you stop temperature from effecting a resistor in a circuit (so the resistance is more constant and unaffected by temp).?