Kinematics-Review help

[Smile101]

Hey guys, I reallly don't understand how to do this question. i'd really appreciate it if you can help me..thanks :)

Homework Statement

A person applies a force of 15 N [E] on an object of weight 20 N. If the force of friction has a magnitude of 5.0 N, what is the acceleration of the object?
a) 7.5 m/s^2 [E]
b) 0.13 m/s^2 [E]
c) 0.50 m/s^2 [E]
d) 5.0 m/s^2 [E]
e) 10 m/s^2 [E]

Homework Equations

f=ma
f_net=f_app+F_f

The Attempt at a Solution

Weight= 20 N = F = F_g
F_app= 15 N
F_f=5 N

F_net= 15+5 = 20 N
I don't know what to do next ..thanks

 

[Chrisas]

Friction opposes motion. So should you be adding the 5 N to 15 N? Or substracting?

Once you have the correct Fnet, then use Fnet = m * a. You get "m" from the weight.

 

[Smile101]

Friction opposes motion. So should you be adding the 5 N to 15 N? Or substracting?

Once you have the correct Fnet, then use Fnet = m * a. You get "m" from the weight.

Yea i just realized that thanks..so it would be 15+(-5) = 10 N

But weight isn't only the mass, from what i know weight is equivalent to net force that's why it's measured in Newtons..that's where I'm getting confused!

oh btw the answer at the back of the textbook says d) 5m/s^2

 

[Smile101]

Ohhhhhh nvm! i just realized what you meant!

20/9.8= 2.0kg

f=ma
10=2a
therefore a= 5m/s^2

Thanks :D

 

[Chrisas]

Yea i just realized that thanks..so it would be 15+(-5) = 10 N

But weight isn't only the mass, from what i know weight is equivalent to net force that's why it's measured in Newtons..that's where I'm getting confused!

oh btw the answer at the back of the textbook says d) 5m/s^2

I agree with that answer. I'm assuming the applied force and friction is horizontal and that the block is moving horizontally. That means that the vertical forces are not affected by the horizontal forces. How is weight force defined? Like any other force it is F = mass * accel. However, the accel in this equation is not an acceleration due to motion but due to gravity. You should know what the acceleration of gravity is and you know what the force is. Solve for mass.

Edit: Yep you got it before I finished writing. Good Job!