1. NAME: _______________________________________ Forces Do Now
Calculate the net force for the vectors on the left.
2. Look at the table to the left. Draw a free-body diagram of the table below. How much force is pushing up on the table? How do you know?
3. In the drawing on the left, four people are pulling on the same 200 kg box with the forces shown. Draw a free-body diagram of the situation below and calculate the net force.
4. A woman is holding two dogs on a leash.
If each dog pulls with a force of 80 newtons, how much force does the woman have to exert to keep the dogs from moving?
What would the free-body diagram of this situation look like?

2. Arrows represent size and direction
Always come from center of the object (unless more than one arrow)

3. Review-Calculate net force
1) You are asked for acceleration.
2) You are given mass and force.
3) a = F ÷ m.
4) F = -75N - 25N +45N +55N = 0 N, so a = 0.
Four people are pulling on the same 200 kg box with the forces shown.

4. Review-Calculate force
A woman is holding two dogs on a leash.
If each dog pulls with a force of 80 newtons, how much force does the woman have to exert to keep the dogs from moving?
1) You are asked for force (F).
2) You are given two 80 N forces and the fact that the dogs are not moving (a = 0).
3) Newton’s second law says the net force must be zero if the acceleration is zero.
4) The woman must exert a force equal and opposite to the sum of the forces from the two dogs.
Two times 80 N is 160 N, so the woman must hold the leash with an equal and opposite force of 160 N.

5. Friction
Friction is a force that resists the motion of objects or surfaces.
It is the force that two surfaces exert on each other when they rub against each other.

6. Friction Depends on the: Types of surfaces Force between the surfaces
When the hockey puck slides on ice, a thin layer of water between the rubber and the ice allows the puck to slide easily.

7. Identifying friction forces
Friction is a force, measured in newtons just like any other force.
Static friction keeps an object at rest from moving.

8. Identifying friction forces
Sliding friction is a force that resists the motion of an object moving across a surface.
The object is already moving in sliding friction.

10. Friction

11. Friction Friction is greater... between rough surfaces
when there’s greater force between surfaces (such as more weight)

12. Reducing the force of friction
Unless a force is constantly applied, friction will slow all motion to a stop eventually.
It is impossible to completely get rid of friction, but it can be reduced.

13. Useful friction Friction is also important to anyone driving a car.
Grooved tire treads allow space for water to be channeled away from the road-tire contac point, allowing for more friction in wet conditions.

14. Useful friction
Shoes are designed to increase the friction between their soles and the ground.
Why do you think these shoes increase friction?
Players wearing cleats can apply much greater forces against the ground to help them move and to keep from slipping.

15. Friction and energy
Friction changes energy of motion into heat energy.

16. Friction and energy
Each time two moving surfaces touch each other, tiny bits of material are broken off by friction.
Breaking off bits of material uses energy.
Friction has affected which pile of stones more?

17. Video and Practice
Answer questions 1(a, b, c, d) and 2 (a and b) with your partner.

18. Coefficient of friction
Friction Equation
Normal force (N)
Ff = m Fn
Friction force (N)
Coefficient of friction

19. What is the normal force?
A normal force is created whenever an object is in contact with a surface.
The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight.
The normal force is sometimes called the support force.

20. G: Given
U: Undefined
E: Equation
S: Substitute
S: Solve

21. A box weighs 100 N, which is the NORMAL FORCE
A box weighs 100 N, which is the NORMAL FORCE. As the box is pushed horizontally, the coefficient of sliding friction is Determine the force of friction resisting the motion.
G: Fn= 100N us=0.25
U: Ff
E: Ff = usFn
(Frictional Force = Coefficient of Friction x Normal Force)
S: Ff= 0.25 x 100N
S: Ff= 25 N

22. Calculate using friction
A steel pot with a weight of 50 N sits on a steel countertop. The coefficient of friction is 0.74.
How much force does it take to start the pot sliding?
1) You are asked for the force to overcome static friction (Ff).
2) You are given the weight (Fw) and both surfaces are steel.
3) Ff = μsFn.
4) Ff = (0.74) x (50 N) = 37 N

23. A steel pot with a weight of 50 N sits on a steel countertop
A steel pot with a weight of 50 N sits on a steel countertop. The coefficient of friction is 0.74.
How much force does it take to start the pot sliding? G: U: E: S: