11 Forces and Newton's Laws

11 Forces and Newton's Laws

• The construction and use of free-body diagrams are key to approaching problems involving forces.

• The net force is determined by the forces acting in the same direction and in opposite directions.

• The force is parallel to the surface.

• A force is a pull applied by one object to another.
• The force in the laboratory can be measured by a spring scale.
• AP physics 1 requires us to understand two aspects of forces.

• The net force acting on an object has to be connected to the object's change in velocity.

• An object can experience a force, but it cannot have a force.
• Don't say that Ball A has a bigger force than Ball B.

• A free-body diagram is a method of describing forces on an object.

• A car that is moving to the right on the freeway applies the brakes to stop.

• The Earth's force on the object is known as the object's weight.
• "gravity" is an ambiguous term, so don't call it that.
• Weight acts downward and doesn't require any contact with the Earth in order to exist.

• Chapter 15 explains how to find the weight of an object in a field.

• The contact with the car must be the cause of any other forces.
• It is touching the ground.
• The ground exerts a normal force when the car is touching it.

• The ground exerts a force when the car is sliding.
• The car skids right along the ground if the force of motion is to the left.
• We're done because the car is not in contact with anything else.

• All forces must be exerted by an object and all nongravitational and non electrical forces must be a result of contact.

• The car isn't in contact with anything that pushes it forward.

• There are no questions about what happened before the problem started.
• The car might have been pushed by the engine or pulled by a team of donkeys to start moving.
• When we watch the action, the car is moving right and slowing down.

• The force of the donkeys on the free-body would appear if they pulled the car.
• The donkeys might not have existed after they let go of the car.

• Don't make up forces.
• Unless you can clearly identify the source of the force, don't include it.

• It doesn't matter what you label the arrow as long as you define it in a list.
• Be my guest.

• The force of the ground on the car is listed as the normal force.

• Try drawing a free-body diagram for each object in the problem, or for a system with multiple objects.
• Even if you aren't asked to make one, a free-body diagram will still be useful.

• The net force on an object can be determined by treating each direction differently.
• Add forces that point in the same direction and subtract forces that point in opposite directions.

• The net force to the surface is zero.

• The normal force minus the weight is equal to the net force.
• The vertical net force on the car is zero since the car is moving along the surface.

• We can conclude that the force on the car is the same as the weight.

• The normal force can be different from the weight if more vertical forces are acting.

• The second car skids to a stop over a longer distance.

• The surfaces in contact have a coefficient of friction.
• Since a wet road is less sticky than a dry road, the coefficient of friction has gone down.
• The normal force of the surface on the car is the same as before since the second car is identical to the first.

• A larger distance to stop is caused by a smaller acceleration.

• If the object is moving, use the coefficient of static friction.

• If a book sits on a table, it doesn't need to be moved.
• The book won't slide if you apply a very small hori zontal pushing force to it.

• The weight is the same as the normal force and opposite in direction.

• The companion force can be found by looking at the description of the force in the free-body dia gram.
• The force of the ground on the car is the normal force.
• The force of the car on the ground is the third law companion force.

• The magnitude of the net force and the direction of the net force can be determined using the Pythagorean theorem.

• Net force will be zero in one or both directions in most AP problems.

• Chances are it would ask about the connection between the net force and the object's speed if it were an AP problem.
• We will revisit this example later.

• A force at an angle is drawn on a free-body diagram.
• Break the force into components when you're ready to do an analysis on the free-body diagram.

• Sometimes, this is called a third law force pair.

• There is a pair of fuzzy dice hanging from your mirror.
• You speed up.

• The dice have a mass.

• It means the force of a string.

• The car is not in contact with the dice.
• The string is applying the force to the dice.

• The net force on these dice is likely to be determined by you.

• Break the individual forces into horizontal and vertical components to find the net force.
• The weight is already vertical.

• You are ready to answer any questions.

• You won't get full credit.
• All forces should be drawn at whatever angle is appropriate.

• I'm stuck to solve for anything because the problem didn't give me an acceleration or tension.

• The problem is not likely to besolvable.
• You can't ask College Board questions on the AP exam.
• If you're absolutely certain that the exam is messed up, you can just state where you think the problem is and do your best.
• Chances are you need to find an alternative way to solve the problem.

• The dice have a mass of 0.10 kilogram.
• The weight of the dice is 1.0 N.7 since they are only moving in a horizontal direction.
• The net force is determined by the forces in opposite directions.

• That's all that's needed to calculate.

• It's tempting to use this equation in a lot of different situations.
• I'm sitting in a chair.
• The force of the Earth on me is equal to my weight.

• I'm in free fall because of the 10 m/s per second value.

• I experience more forces than the Earth.
• The chair is on me.
• The chair pushes up on me because I know my acceleration is zero.

• The AP exam doesn't like to ask for calculations.

• One kilo of mass weighs 10 N on the Earth.

• The best way to answer this type of question is to make the calculation, and then explain what part of the calculation leads to the correct answer.
• The method of answering questions that involve qualitative-quantitative translation is discussed in Chapter 8.

• If you want a bigger angle, call it 60deg from the vertical.
• The weight of the dice is still 1.0 N and the vertical acceleration is still zero, which means we can set the up force equal to the down force.

• The answer is that the tension increases.
• The weight remains the same and the vertical component of tension must stay the same since a smaller angle from the horizontal gives a bigger tension.

• If you are asked if something increases, decreases, or stays the same, you might want to make a calculation to see what happens.

• You don't use horizontal and vertical components for the forces.
• You don't look at the forces parallel to the incline and the forces parallel to the incline.

• As long as the object is moving up or down the incline, the normal force won't have to be broken into components.
• It won't have to be broken into components if the force is parallel to the incline.
• The force of the Earth will be broken into components.

• There are examples of problems and drills in Chapter 18.

• We had to measure from the horizontal according to the fact on the previous page.

• The easiest way to start is to consider both objects as a single system.
• If you need to talk about one of the two connected objects, or if you need to find the tension in the connecting rope, draw a new free-body diagram for that object.

• An alternative approach is to draw two diagrams for each object.
• The tension and acceleration are the same in each equation.

• A car goes up a plane.

• Above the horizontal is 30deg.
• The normal force must be equal to the car's speed in order for the block to work.

• The block would be allowed to slow down.

• They are talking about the second law.

• The freeway has better gas mileage.

• The car is speeding up and slowing down in town.

• Unless the rope is connected over a mass, this is not true.
• The rope can have different tensions on each side of the pulley if the pulley had mass.
• That shouldn't require any calculation because it's a rare occurrence.

• The freeway has better gas mileage.

• Better gas mileage can only be achieved by the force of the engine.

• In town, gas mileage is better.
• The speed of the car doesn't have to be accelerated because the town speed is slower than the constant speed.
• The acceleration in town is smaller because the force of the engine must be greater than the time.

• The 11 m/s per second is here.
• If the net force is 8,800 N, the motion is to the right.

• When the car changes its speed by 11 m/s, 1cos is less.

• The car experiences a greater net force than the block.

• It will work here if it's less than a bit over 1.4 N.

• Oscar has an answer about how is correct.
• The answer is way off.

• The driver would fall out of the car.

• When an object moves along a surface, it must have zero acceleration.
• The net force to the surface is zero.

• The amount and direction of the force of object A and object B are the same.
• The third law companion forces are the two forces that act on different objects.

• If the net force has both a vertical and a horizontal component, the Pythagorean theorem can be used to determine the magnitude of the net force.

• One rope has one tension.