Unit 4 Physics II Day 13: Calculating Force (f=ma)

Physics II Day 13: Calculating Force (f=ma)

Newton’s second law can be expressed as force = mass times acceleration!

Why?

·      If you increase the mass, then it requires more force.

·      If you want the object to go faster, it requires more force!

So now we can calculate_______________ force in Newtons using force = mass times acceleration.

Putting it together…

So if more mass means it is harder to accelerate an object, how could we find out how much an object is going to accelerate? Take force and divide_______________ it by mass.

Physicists use something called variable__________ to represent what they know or don’t know in an equation.

Variables are usually symbolized by the first letter of what is being measured:

Force = f

Mass = m

Acceleration = a

So to calculate force or acceleration, you must:

·      Identify what you knowP____________

·      Identify what you don’t know________

·      If your unknown is acceleration, you use acceleration = force divided by mass

·      If your unknown is force, you use force = mass times acceleration

·      Formulas: force = mass x acceleration; acceleration = force ÷ mass

Let’s Try as a Class

1.    Kenneth needs to accelerate a box 2 m/s². The box weighs 8 kilograms. How much force will he need to accelerate it?

2.    What if the box weighed 10 kilograms?

3.    What if the box only weighed 4 kilograms, but he wanted to accelerate it 4 m/s²?

With a Shoulder Buddy:

1.     Juan pushes his brother into a swimming pool with a force of 65 newtons. If his brother weighs 41 kilograms, at what rate will he accelerate?

Known:	Formula:
Unknown:	Solve:

2.     Which object will require more force to accelerate?

a)    2 kg book                               b) 4 kg box                c) 10 kg crate       d) 15 kg shelf

3.     You are trying to design a scooter that accelerates at a rate of 12 m/s². If the scooter weighs 22 kilograms, how much force will have to be exerted on the scooter?

Known:	Formula:
Unknown:	Solve:

4.     With the pulley, a force of 81 newtons is being exerted on the 4 kg weight. At what rate will the weight accelerate?

Known:	Formula:
Unknown:	Solve:

Now You Try On Your Own

1) A 3-kilogram pipe is falling at a rate of 10 m/s². With how much force is gravity pulling on the pipe?

Known:	Formula:
Unknown:	Solve:

2) 3) Ms. Trevino is moving the desks in her room. If she pushes on a desk with a force of 25 newtons, how much will it accelerate if it weighs 4 kilograms?

Known:	Formula:
Unknown:	Solve:

3) In order to knock down all the pins, a bowling ball must be accelerating at a rate of 1.8 m/s². If the bowling ball weighs 9 kilograms, with what force will you have to roll it?

Known:	Formula:
Unknown:	Solve:

4) You have been designing a scooter and are unhappy with how quickly it accelerates. If you can’t change the motor of the scooter and make it exert more force, what can you do to make the scooter accelerate more?

a)    Decrease the mass of the scooter

b)   Increase the mass of the scooter

c)    Give the scooter bigger wheels

d)   In this situation, there is no way to make the scooter accelerate faster.

5) Which of the following describes why big trucks usually use more gas than small cars?

a)    Big trucks have more inertia

b)   Since they are larger, trucks require more force to accelerate

c)    There is more friction between a big truck and the road

d)   All of the above

e)    None of the above